text:
\section{\label{sec:intro}Introduction}
Among their many often extreme properties, Rydberg states of atoms and molecules can possess long radiative lifetimes. In the absence of external perturbations low-angular-momentum hydrogenic Rydberg states with principal quantum number $n>30$, and small quantum defects, typically exhibit lifetimes exceeding $1~\mu$s which scale with $n^3$. High-angular-momentum `circular' states with similar values of $n$ have lifetimes greater than $5~$ms which scale with $n^5$~\cite{gallagher94a}. Because of these long lifetimes, optical and microwave transitions involving these states exhibit narrow natural linewidths. They are therefore very well suited for precision spectroscopic measurements, including for example, the accurate determination of molecular ionization and dissociation limits~\cite{liu09a}, and studies of the role of nuclear spins in photoionization~\cite{paul09a,sassmannshausen13a}. These long lifetimes are also of importance for applications of Rydberg states in quantum information processing~\cite{gleyzes06a,saffman10a}, and in experiments with antihydrogen and positronium atoms directed toward (i) spectroscopic investigations of matter--antimatter asymmetries~\cite{bluhm99a}, and (ii) measurements of the acceleration of particles composed of antimatter in the gravitational field of the Earth~\cite{mills02a,kellerbauer08a,cassidy14a}.

In each of these areas, the long radiative lifetimes of the Rydberg states alone are generally not sufficient for measurements at the highest resolution. It is also necessary to ensure that the period of time during which the samples are interrogated, and the interaction time with the desired external fields approach, or exceed, these lifetimes, and that motional effects such as Doppler broadening are minimized. Typically these requirements can be fulfilled simultaneously through the preparation of slowly moving samples with low translational temperatures. However, there are cases where high resolution excitation to Rydberg states directly from a ground, or other low-lying electronic, state is necessary before subsequent deceleration or trapping, because of the challenges associated with the preparation of sufficiently cold ground state samples using current techniques. These cases include, for example, precision spectroscopic studies of the ionization and dissociation limits of H$_2$ and its isotopomers~\cite{liu09a}, and the preparation of long-lived excited states of positronium~\cite{cassidy12a}. In these cases, it is foreseen to minimize effects of Doppler broadening in the photoexcitation process by using counter-propagating laser beams to drive non-resonance-enhanced single-color two-photon transitions to the Rydberg states~\cite{sprecher11a,hogan13a}. The spectral resolution of the resulting Doppler-free excitation~\cite{Haensch75a} will then be limited only by the bandwidth of the laser radiation used to drive the transition, and the period of time with which it interacts with the sample.

The photoexcitation of Rydberg states using non-resonance-enhanced single-color two-photon schemes in the presence of homogeneous electric fields offers the opportunity of exploiting the large electric dipole moments associated with the resulting states for efficient acceleration, deceleration and trapping using inhomogeneous electric fields~\cite{yamakita04a,vliegen04a,hogan08a}. The magnitude of these dipole moments scale with $n^2$, and at $n=30$ can exceed 3000~D. In addition to the importance of the results presented here for Doppler-free Rydberg state photoexcitation, when driven using circularly polarized laser radiation such multiphoton excitation schemes represent a general approach to the preparation of long-lived non-core-penetrating Rydberg states of small molecules. With their small quantum defects, and long fluorescence lifetimes, these states are ideally suited for deceleration and electric trapping experiments~\cite{hogan09a,seiler11a}, directed toward studies of excited state decay processes, and low energy scattering with other gas-phase targets, or surfaces~\cite{softley04a}.

Non-resonance-enhanced single-color two-photon excitation schemes have been employed previously in high resolution spectroscopic studies of atomic Rydberg states~\cite{stoicheff79a,penent88a} and in studies of (2+1) resonance-enhanced multiphoton ionization~\cite{genevriez14a}. We report here a detailed investigation of the role of ac Stark energy shifts in single-color two-photon excitation in beams of metastable helium (He) atoms under experimental conditions which are similar to those expected in future experiments with positronium (Ps), and report the efficient excitation of selected Rydberg-Stark states.

In the following, a description of the experimental apparatus used in the work reported here is first provided. The numerical methods employed in the calculation of the spectral profiles associated with non-resonance-enhanced two-photon transitions from the 2\,$^3\text{S}_1$ state in He to high Rydberg states is then outlined, before the results of the experiments carried out (i) in the absence of externally applied electric fields, and (ii) in the presence of homogeneous electric fields oriented parallel to the direction of propagation of the laser radiation, are presented. Finally, comparisons are drawn between the single-color two-photon spectra reported here and those expected in the case of Doppler-free two-photon excitation of positronium using laser radiation with similar temporal and spectral properties.

\section{\label{expSet}Experiment}
\begin{figure}[b]
\centerline{\includegraphics[width=1.0 \linewidth]{fig1_expSetup.pdf}}
\caption{(Color online) Schematic diagram of the experimental setup (not to scale). The polarization of the laser beam is set by a Glan-Taylor polarizer (GT) and then controlled with a rotatable quarter-wave plate (WP). The coordinate system is defined such that the origin is located at the position where the laser beam axis intersects the atomic beam axis (shown as a black point)}
\label{fig:expSetup}
\end{figure}
A beam of metastable He atoms is prepared in an electric discharge within a supersonic expansion, as described in \cite{Halfmann00}. Helium is injected into the source vacuum chamber of the experimental apparatus through a pulsed valve (Series 9 General Valve). At the exit of the valve the metastable 2\,$^3$S$_1$ state is populated in an electric discharge generated between a metal anode located $\approx 1~$mm downstream from the nozzle to which an electric potential of $+240~$V is applied, and the grounded base plate of the valve. The discharge is seeded with electrons from a heated tungsten filament located $\approx 15~$mm further downstream. After expanding and cooling the metastable He beam travels with a mean forward speed of $v_z\simeq 1950~$ms$^{-1}$. Atoms in the shorter-lived 2\,$^1$S$_0$ metastable state also populated in the discharge, although present in the beam, do not play a noticeable role in the studies of the triplet states described here.

Having passed through a skimmer with a $2~$mm-diameter aperture into a second vacuum chamber, the atoms pass between  two parallel planar electrodes (labelled E$_1$ and E$_2$ in Fig.~\ref{fig:expSetup}) which each have a length of $100~$mm in the direction of propagation of the atomic beam and are separated by $18~$mm. Each electrode contains a $3~$mm aperture in the center. A single laser beam ($\sim10$~mJ per pulse at $\approx522~$nm with a pulse length of 6~ns and a bandwidth of 5~GHz, produced by a Nd:YAG-pumped pulsed dye laser) passes through these apertures. The laser beam converges as it propagates through the chamber, reaching a focus $\sim20~$mm beyond the atomic beam axis. Where the laser intersects the atomic beam axis it has a Gaussian transverse intensity profile, with a standard deviation $\sigma_y\simeq\sigma_z\simeq70~\mu$m (see Fig.~\ref{fig:expSetup} for the definition of the coordinate system), resulting in peak intensities on the axis of the atomic beam of $\sim5\times 10^{13}~$Wm$^{-2}$. By applying a potential difference between E$_1$ and E$_2$, uniform electric fields of up to $F=155~$Vcm$^{-1}$ can be produced in the excitation region, allowing the selective preparation of individual Rydberg-Stark states.

Single-color two-photon transitions driven from the 2\,$^3\text{S}_1$ state in He to Rydberg states with $n\geq 20$ converging on the $1\text{s}\,^{2}\text{S}_{1/2}$ ionization limit require radiation with wavelengths in the range from 520~nm to 524~nm (Fig.~\ref{fig:HeLevels}). With the intermediate state far from resonance with any bound state, these transitions are not resonance-enhanced.
\begin{figure}[t]
\centerline{\includegraphics[width=1.0 \linewidth]{fig2_HeLevels.pdf}}
\caption{(Color online) Energy level diagram showing possible single-color two-photon excitation pathways from the 1s2s\,$^3\text{S}_1$ state to high Rydberg states of He using circularly ($\circlearrowright$) and linearly ($\uparrow$) polarized laser radiation.}
\label{fig:HeLevels}
\end{figure}
In the experiment the laser polarization is set using a Glan-Taylor polarizer and then controlled using a $\lambda/4$-plate, permitting the production of circularly polarized radiation with a purity of $>90\%$. In the excitation region in the apparatus the electrode planes are oriented parallel to the axis of the atomic beam and perpendicular to the direction of propagation of the laser beam, such that the circularly polarized radiation drives transitions for which $\Delta$$M_J=+1$ or $\Delta$$M_J=-1$, where $M_J$ is the azimuthal quantum number associated with the total angular momentum $\vec{J}$, of the atom. In the case of the triplet states of He treated here only the interaction of the optically active electron, with total angular momentum $\vec{j}$, and azimuthal quantum number $m_j$, initially in the 2s orbital need be considered. Therefore excitation with circularly polarized radiation results in transitions for which $\Delta$$m_j=+1$ or $\Delta$$m_j=-1$. Since individual spin-orbit components are not resolved in the experiments described, in general the azimuthal quantum number, $m_{\ell}$, associated with the orbital angular momentum of the single optically active electron is referred to when discussing the effects of laser polarization in the observed spectra. With circularly polarized radiation driving single-photon transitions for which $\Delta$$m_{\ell}=+1$ or $\Delta$$m_{\ell}=-1$, two-photon transitions from the 1s2s\,$^3\text{S}_1$ state can result only in the excitation of states with $|m_{\ell}|=2$. However, linearly polarized radiation can drive transitions for which $\Delta$$m_{\ell}=\pm$1. Thus, in the configuration presented, linearly polarized radiation can drive two-photon transitions to states with $m_{\ell}=0$ and with $|m_{\ell}|=2$.

Following photoexcitation He Rydberg atoms are detected by pulsed electric field ionization 118~mm downstream from the excitation region between electrodes E$_3$ and E$_4$. The resulting ions are then collected on a micro-channel plate (MCP) detector. With E$_3$ and E$_4$ separated by 50~mm, the pulsed potential of $+4.0~$kV applied to E$_4$ for electric field ionization results in a field on the atomic beam axis of $\sim800$~Vcm$^{-1}$. This field is sufficient for complete diabatic ionization of atoms in states with principal quantum number $n\geq 35$ \cite{gallagher94a}. The signal from the He Rydberg atoms is recorded by integrating the gated output from the MCP, with each data-point the average of 30 pulses. The experiment is run at a repetition rate of 10~Hz to match that of the Nd:YAG laser pumping the dye laser.

\section{AC Stark shifts - calculation and lineshapes}\label{acStarkSec}

The strong laser fields required to drive the non-resonance enhanced two-photon transitions reported here, give rise to ac Stark shifts (light shifts) of the associated energy levels. These energy shifts lead to broadenings and asymmetries of the corresponding spectral features. The magnitudes of these effects depend upon the frequency-dependent dynamic polarizability, $\alpha(\omega)$, of each of the states between which the two-photon transitions are driven. In a laser field with an intensity $I_{\text{laser}}$, and an angular frequency $\omega_{\text{laser}}$, the ac Stark energy shift, $E_{\text{ac}}$, experienced by an atom in an electronic state with principal quantum number $n$, total angular momentum $\vec{J}=\vec{L}+\vec{S}$ (where $\vec{L}$ is the total electronic orbital angular momentum, and $\vec{S}$ is the total electron spin) and azimuthal quantum number $M_J$, is~\cite{Deissler08a}
\begin{eqnarray}
E_{\mathrm{ac}} &=& \frac{1}{2}\alpha_{|nJM_J\rangle}(\omega)\langle F_{\text{laser}}^2\rangle
\end{eqnarray}
where $\langle F_{\text{laser}}^2\rangle=I_{\text{laser}}/(\epsilon_0 c)$ is the root-mean-square electric field strength of the laser field, with $\epsilon_0$ and $c$ the permittivity of free space, and the speed of light in vacuum, respectively.

The dynamic polarizability of an individual $|nJM_J\rangle$ state can be calculated by summing the off-resonant single-photon dipole-allowed couplings induced by the laser field, of this state to all other non-degenerate bound and continuum states. For a state $|nJM_J\rangle$ lying at an  energy $E_{|nJM_J\rangle}$~\cite{Deissler08a},
\begin{eqnarray}
\alpha_{|nJM_J\rangle}(\omega_{\text{laser}}) &=& \frac{2}{\hbar}\sum_{|n'J'M'_J\rangle\neq|nJM_J\rangle}\Bigg[\frac{\Delta\omega}{\Delta\omega^{\,2}-\omega_{\text{laser}}^2}\nonumber\\
& & \hspace*{0.8cm}\times\, |\left\langle n'J'M'_J|\hat{\mu}|nJM_J\right\rangle|^2\Bigg],
\label{eq:pol}
\end{eqnarray}
where $\hbar=h/(2\pi)$ ($h$ is Planck's constant), $\Delta\omega=(E_{|n'J'M'_J\rangle}-E_{|nJM_J\rangle})/\hbar$, and $\hat{\mu}$ is the electric dipole operator.

In electronic states with low values of $n$, such as the initial $2\,^3$S$_1$ state in the experiments reported here, the electron is localized close to the positively charged ion core and high intensity far-off-resonant laser radiation in the visible region of the electromagnetic spectrum can readily polarize the atom, giving rise to energy shifts that are proportional to the laser intensity, $I_{\text{laser}}$. Strong ac Stark shifts observed for states with low values of $n$ result from a combination of the low density of electronic states in the surrounding energetic regions, and the general propensity for bound and continuum states to lie toward higher energies.

In Rydberg states with high values of $n$, the density of states in the surrounding energetic regions are high, and far-off-resonant visible laser radiation couples states lying higher in energy, and lower in energy, with similar coupling strengths. As a result the ac Stark energy shifts of high Rydberg states in strong high frequency (visible) laser fields are weak and, as demonstrated in the data presented here, can, to a reasonable approximation, be neglected when performing experiments with pulsed laser radiation with bandwidths exceeding 1~GHz.

The determination of the ac Stark shift of a state $|nJM_J\rangle$ using Equation~(\ref{eq:pol}) requires the calculation of the dipole matrix elements $\left\langle n'J'M'_J|\hat{\mu}|nJM_J\right\rangle$. These matrix elements can be calculated using the Wigner-Eckart theorem to  express them in terms of a reduced radial matrix element $\langle n'L' ||e\hat{r}||nL \rangle$ such that~\cite{zarebook}
\begin{widetext}
\begin{eqnarray}
\left\langle n'J'M'_J|\hat{\mu}|nJM_J\right\rangle &=& (-1)^{J+J'-M'_J+L'+S+1}\sqrt{(2J'+1)(2J+1)}\nonumber\\
& &\hspace*{1.5cm}\left( \begin{array}{ccc} J' & 1 & J \\ -M'_J & \Delta M_J & M_J\end{array} \right)\left\{ \begin{array}{ccc} L' & J' & S \\ J & L & 1\end{array} \right\}\langle n'L' ||e\hat{r}||nL \rangle,
\label{eq:dipmat}
\end{eqnarray}
\end{widetext}
where the curved and curly brackets represent the Wigner 3J and 6J symbols, respectively. In the particular case of the triplet states of the He atom of relevance here, the single electron in the 1s orbital has zero orbital angular momentum. Since this orbital lies 19.8~eV below the next nearest excited level to which it can be coupled by the laser field, in our treatment of the ac Stark shift of the 2\,$^3$S$_1$ state we consider only the single optically active electron with orbital angular momentum $\ell$, which is initially in the 2s orbital. Therefore $L\equiv\ell$ and the reduced radial matrix element $\langle n'L' ||e\hat{r}||nL \rangle\equiv\langle n'\ell' ||e\hat{r}||n\ell \rangle$.

From the expression in Equation~\ref{eq:dipmat} and using the Numerov method~\cite{zimmerman79a} to calculate the reduced radial matrix elements, together with previously published values for the $n$-dependent quantum defects, $\delta_{n\ell}$, of the triplet states of He ($\delta_{30\mathrm{s}}=0.2967$, $\delta_{30\mathrm{p}}=0.0683$, $\delta_{30\mathrm{d}}=0.0029$)~\cite{martin87a}, the polarizability of the 2\,$^3$S$_1$ state, in an electromagnetic field with a frequency $\omega_{\text{laser}}/(2\pi) = 5.738\times10^{14}$~Hz ($\equiv 522.48$~nm) -- half of the transition frequency associated with the energy difference between the 2s state and the 25d state -- was determined. In doing this the off-resonant couplings to all $n$p states with values of $n$ in the range from 2 to 20 were included. For this range of states, the calculated polarizability of $\alpha_{2\,^3\mathrm{S}_1}(\omega_{\mathrm{laser}})=-3.15\times10^{-39}$~Cm$^2$V$^{-1}$ (equivalent to a frequency shift of $-8.95~$GHz in a laser field of intensity $10^{13}$~Wm$^{-2}$) converged to better than one part per thousand.

Using this value for the dynamic polarizability of the 2\,$^3$S$_1$ state, the spectral lineshapes associated with the two-photon transitions studied experimentally were calculated. This was done by determining the ac Stark shift of the 2\,$^3$S$_1$ state of a spatially distributed ensemble of atoms interacting with the laser beam. The spatial dimensions of the three-dimensional ensemble matched the known shape of the supersonic beam (uniform distributions in $y$- and $z$-dimensions with width $400~\mu$m, much greater than the size of the laser beam in these dimensions, and a Gaussian distribution in the $x$-dimension with width $\sigma_x=1.5~$mm). The measured laser beam profile and convergence at the intersection region between it and the atomic beam (a Gaussian transverse laser profile with $\sigma_y\simeq\sigma_z\simeq70\mu$m and gradient $\text{d}\sigma_{y,z}/\text{d}x=-11.5 \mu$m(mm)$^{-1}$) were then used to generate a three-dimensional intensity distribution for each laser pulse energy, from which the ac Stark shifts could be calculated. The resulting ac Stark shifted transition frequency of each atom in the ensemble was then convolved with the measured bandwidth of the laser ($\sigma_L=\sqrt{2}~5.0~$GHz at the two-photon level), and the contributions from all atoms were then summed to determine the overall lineshape. Further details of the numerical procedure used to calculate the spectral lineshapes observed experimentally are provided in the Appendix.

\section{\label{results}Results}
\subsection{Photoexcitation to high Rydberg states}
\begin{figure*}[!]
\centerline{\includegraphics[width=1 \linewidth]{fig3_longspectrumfig.pdf}}
\caption{Spectrum of triplet $n$d Rydberg states of He with $|m_{\ell}|=2$ recorded following non-resonance-enhanced single-color two-photon excitation. The calculated line positions are indicated by the vertical bars at the top of the figure. The red dashed line represents a function the amplitude of which depends on $n^{-3}$ that has been fitted to the amplitudes of the spectral features with $32\leq n \leq 70$. The offset of the zero intensity level close to the ionization limit is a result of electrical fluctuations during the measurement. Careful measurements made with and without the laser beam present confirm that there is no $\text{He}^+$ signal recorded at the ionization limit.}
\label{fig:longscan}
\end{figure*}

\begin{figure}[!]
\centerline{\includegraphics[width=1.0 \linewidth]{fig4_spectrumzoom.pdf}}
\caption{(Color online) The single-color two-photon $|m_{\ell}|=2$ spectrum encompassing transitions to states from $n=65$ up to the ionization limit. The spectral features are asymmetric, broadened to higher frequency by the ac Stark shift of the 2\,$^3{\mathrm S}_1$ state (see Sec. \ref{ACStarkExp}). The decrease in signal strength above $n=85$ is a result of electric field ionization in the fringe fields of the MCP between the photoexcitation region and detection region.}
\label{fig:specZoom}
\end{figure}
A single-color two-photon spectrum of $n$d Rydberg states of helium with $|m_{\ell}|=2$ is presented in Fig.~\ref{fig:longscan}. This spectrum was recorded by scanning the wavelength of the pulsed dye laser from 520~nm to 524~nm. This range covers two-photon transitions to Rydberg states ranging from $n=20$ up to the ionization limit. The highest Rydberg states observed are those with $n\simeq 100$, while states up to  approximately $n=75$ are clearly resolved (see Fig.~\ref{fig:specZoom}). In Figure \ref{fig:longscan} the calculated positions of the Rydberg states are indicated by the vertical bars above the spectrum and are in very good agreement with the positions of the experimentally recorded peaks. Careful inspection of Fig.~\ref{fig:longscan} reveals that each spectral feature is asymmetric, broadened to the high frequency side. This broadening is a  result of the ac Stark shift of the 2\,$^3{\mathrm S}_1$ state (see Section~\ref{acStarkSec}), and is discussed in more detail in Section~\ref{ACStarkExp}.

The overall shape of the spectrum presented in Fig.~\ref{fig:longscan} results from several competing effects. The reduction in the intensity of the spectral features from $n=30$ up to $n=85$ scales with $n^{-3}$ (see dashed curve), and is a result of the reduction in the transition dipole moments from the virtual intermediate state at the single photon level with increasing $n$. The cut-off in signal above $n=110$, below the ionization limit, is a result of electric-field ionization of the excited atoms in the fringe fields of the MCP detector before reaching the position in the detection region where they can be efficiently collected to contribute to the measured signal. The gradual increase in the intensity of the spectral features for values of $n$ from 20 to 30 arises from a combination of effects of increasing radiative lifetimes, transitions driven by blackbody radiation and the cut-off of electric field ionization  at low $n$ in the detection region of the apparatus. In the absence of blackbody radiation, the $\approx800~$Vcm$^{-1}$ pulsed electric field used for Rydberg atom ionization/detection is not strong enough to ionize atoms in states below $n=(F_0/9F)^{1/4}=29$, where $F_0=2R_{\text{He}}h c/e a_0=5.14\times10^9~$Vcm$^{-1}$, with $R_{\text{He}}$ the Rydberg constant for He. However, in the 60~$\mu$s flight-time of the atoms from the position of photoexcitation to the detection region, transitions from states with values of $n$ below 29 to those above permit detection of these lower $n$ states~\cite{farley81a}.

An expanded view of the portion of the two-photon spectrum above $n=65$ is displayed in Fig.~\ref{fig:specZoom}. From this data individual features up to approximately $n=75$ can be clearly identified. States with $n>75$ are not well resolved for two reasons. Firstly, at high $n$ the states are very closely spaced in energy, and for $n>94$ the separation of adjacent states becomes smaller than the experimental linewidth. This width,  $\simeq 20~$GHz (FHWM) as a result of ac Stark shifts (see Sec.~\ref{ACStarkExp}), is comparable to the level spacing for $n\simeq 74$. Secondly, these high-$n$ states possess very large electric dipole moments and weak stray electric fields in the excitation region will broaden the corresponding spectral features. For example, a field of only 0.1~Vcm$^{-1}$ will cause the $n=76$ state to broaden to $\sim2.5$~GHz. Finally, for states with $n\geq85$ the measured signal begins to decrease, reaching zero intensity at $n \simeq 110$. This cut-off is a result of electric field ionization in weak fringe electric fields of the MCP, between the excitation and detection regions. A stray electric field of $\sim10~$Vcm$^{-1}$ is sufficient to begin to ionize states with $n\approx85$ and strong enough to fully ionize states with $n>110$, where $F_{\text{ion}}=2F_0/(9 n^4)~$. Since the range of values of $n$ corresponds well to the extent of the cut-off region in Fig.~\ref{fig:specZoom}, we conclude that between the photoionization region and detection region atoms encounter fringe electric fields from the MCP of $\sim10~$Vcm$^{-1}$, precluding detection of states above $n\simeq 115$. This drop in signal at high $n$ also indicates that in the apparatus used here, $\text{He}^+$ ions formed by multiphoton ionization in the excitation region, or blackbody photoionization are not detected. This confirms that the recorded spectral features arise from atoms in long-lived Rydberg states and not from free ions.

\subsection{Effects of laser polarization}
\begin{figure}[t]
\centerline{\includegraphics[width=1 \linewidth]{fig5_spScan.pdf}}
\caption{(Color online) The spectral region in the vicinity of the transition to the  29d state measured with circularly (blue, dashed curve) and linearly (red, continuous curve) polarized radiation. With circular polarized radiation only the transition to the 29d state is observed while with linear polarized radiation the transition to the 29s state is also seen.}
\label{fig:spscans}
\end{figure}
The spectrum in Fig.~\ref{fig:longscan} contains transitions to $n$d Rydberg states with $|m_{\ell}|=2$ for atoms excited using circularly polarized laser radiation. From the selection rules for electric dipole transitions, excitation with linearly polarized light leads to transitions for which $\Delta m_{\ell}=\pm1$. From the virtual intermediate state with $m_{\ell}=\pm1$ the second photon can therefore drive transitions either to Rydberg states with $m_{\ell}=0$, or with $|m_{\ell}|=2$. The latter transitions will populate $n$d-states with $|m_{\ell}|=2$, whereas the former will populate $n$s- and $n$d-states with $m_{\ell}=0$ [Fig.~\ref{fig:expSetup}(b)]. To confirm the $|m_{\ell}|$-selective character of the excited states, spectra are presented in Fig.~\ref{fig:spscans}, that were recorded close to $n=29$ using linearly and circularly polarized laser radiation. In the case of circularly polarized excitation, only the transition to the 29d state is observed, while in the case of linearly polarized excitation transitions to the short-lived 29s state, and the 29d state can be seen.

The transition to the 29s state occurs at a frequency $\sim80~$GHz below the transition to the 29d state because of the larger value of the 29s quantum defect ($\delta_{29s}=0.2967$, $\delta_{29d}=0.0029$~\cite{martin87a}). The reduced intensity of the transition to the 29d state in the case of excitation with linear polarized radiation is a result of the smaller angular integral associated with the transition moments to excited $n$d-states with $m_{\ell}=0$ than to $n$d-states with $|m_{\ell}|=2$. In the case here, the spectral intensity associated with the transition to the 29d states with $|m_{\ell}|=0$ and 2 deiven using linearly polarized radiation is expected to be 0.61 times that associated with the excitation of the 29d state with $|m_{\ell}|=2$ using circularly polarized radiation of the same intensity. As can be seen from the data in Fig.~\ref{fig:spscans}, this intensity ratio is consistent with that observed experimentally. The signal associated with the transition to the 29s state is weaker because of a smaller transition dipole moment than that to the 29d state and because of its shorter fluorescence lifetime. This reduced lifetime makes it difficult to compare the observed relative intensity of this transition to that expected theoretically.

\subsection{ac Stark shift}\label{ACStarkExp}
As discussed in Sec \ref{acStarkSec} the intense laser radiation employed to drive the non-resonance-enhanced single-color two-photon transitions presented here causes the $2\,^3\mathrm{S}_1$ state to shift to lower energy with increasing intensity, this shifts the 2s-$n$d transitions to higher frequency. The spatial spread of the atoms in the supersonic beam,which is much larger than the laser beam waist, leads to an asymmetric broadening of the observed transitions, since the atom cloud covers a volume over which there is a considerable gradient in the laser intensity. Atoms that are in the center of the laser beam, where the intensity is greatest,  experience a large ac Stark shift, while those at the edge of the beam experience only a small shift. Since the atoms that are subjected to the greatest intensities are also the most likely to be excited to a Rydberg state, this would indicate that for a collimated laser beam the spectral features would be shifted towards the frequency associated with the greatest ac Stark shift. However, the recorded spectra show features with maxima close to the unperturbed line-position, with a long tail towards higher frequency. This line shape is a result of the intensity gradient of the converging laser beam, which reaches a focus beyond the atomic beam axis. Atoms that interact with the laser beam at a position close to the focus can experience strong ac Stark shifts, while atoms located further from the focus experience weaker shifts. The ensemble of atoms located further from the focus experiences a smaller range of ac Stark shifts, causing the overall signal from these atoms to `bunch up' close to the unperturbed transition frequency, whereas closer to the focus atoms experience a wider range of intensities, causing the signal from these atoms to be spread out in frequency.
\begin{figure}[t]
\centerline{\includegraphics[width=1 \linewidth]{fig6_n26acStark.pdf}}
\caption{(Color online) Experimentally measured spectrum of the transition to the 26d state, recorded with a laser energy of 9.5~mJ per pulse focussed to a beam width of $\sigma_x\simeq\sigma_y\simeq70~\mu$m resulting in a spatially averaged peak laser intensity of $I_{\mathrm{laser}}=2.3\times10^{13}~$Wm$^{-2}$ (blue, continuous curve), and the calculated line shape (red, dashed curve)}
\label{fig:acStarkN26}
\end{figure}

The cumulative effect of the three-dimensional ensemble of atoms interacting with the converging laser beam is a spectral line with a maximum close to the unperturbed transition frequency, and a long tail towards higher frequencies. An example of this is shown in Fig.~\ref{fig:acStarkN26} where the spectral feature corresponding to the transition to the $26$d state is shown. In this figure the frequency is displayed relative to the unperturbed $26$d transition frequency. The transition was measured with a spatially averaged peak laser intensity on the atomic beam axis of $2.3\times10^{13}~$Wm$^{-2}$, corresponding to a maximal ac Stark shift of $21~$GHz. The data show a strong peak located $\sim12~$GHz above the unperturbed frequency, with a FWHM of 19~GHz and a long tail extending to $\sim60~$GHz. This long tail results from the contribution of atoms passing through the beam closer to the focus, where the local peak intensity is $7.3\times10^{13}~$Wm$^{-2}$, and the maximum ac Stark shift is 65~GHz. Also shown is the result of a numerical calculation that accounts for the laser beam profile and the shape of the atomic beam, and is in excellent agreement with the experimental data.

\begin{figure}[t]
\centerline{\includegraphics[width=1 \linewidth]{fig7_sigVsEnergyFig.pdf}}
\caption{(Color online) (a) Measured and (b) calculated spectra of the transition to the 29d state for peak laser intensity $I_{\text{laser}}=4.6\times10^{13}~$Wm$^{-2}$ (red, continuous curve), $3.2\times10^{13}~$Wm$^{-2}$ (orange, dashed-dotted curve), $1.9\times10^{13}~$Wm$^{-2}$ (green, long-dashed curve), and $1.1\times10^{13}~$Wm$^{-2}$ (blue, short-dashed curve).}
\label{fig:sigVSenergy}
\end{figure}

Figure \ref{fig:sigVSenergy}(a) shows spectra close to the transition to the 29d state recorded with a range of spatially averaged peak laser intensities. With a low intensity  on the atomic beam axis of $I_{\text{laser}}=1.1\times10^{13}~$Wm$^{-2}$ the spectral profile is slightly asymmetric, and is centered close to the unperturbed transition frequency. As the intensity is increased the profile becomes gradually more asymmetric, with the tail shifting out to higher frequencies. The maximum also moves gradually to higher frequencies, but remains close to the unperturbed transition frequency. Figure \ref{fig:sigVSenergy}(b) shows calculated line profiles which are in very good agreement with the experimental data, confirming the accuracy of the numerical procedure used to model the excitation process. In the spectra recorded at high laser intensities, the fluctuations in the He${^+}$ ion signal exceeds the shot-to-shot fluctuations in the experiment. The observed fluctuations (most clearly displayed in Fig.~\ref{fig:sigVSenergy}(a) in the data recorded with $I_{\text{laser}}=4.6\times 10^{13}~$Wm$^{-2}$) appear to result from interference between different pathways in the excitation process, and will be investigated in more detail in the future.

The spectral intensity of the non-resonance-enhanced two-photon transitions studied here are expected to be proportional to $I_{\mathrm{laser}}^2$ because of the intensity-dependence of the two-photon process. However, the features in the recorded spectra are broadened by the intensity-dependent ac Stark shift of the $2\,^3\mathrm{S}_1$ state. Therefore the integral of the spectral intensity is the quantity dependent upon $I_{\mathrm{laser}}^2$, and not simply the spectral amplitude. This can be seen in Fig.~\ref{fig:intVSenergy} where the integral of the spectral intensity over the frequency range encompassed in Fig.~\ref{fig:sigVSenergy} is displayed as a function of the spatially averaged peak laser intensity on the atomic beam axis, together with a quadratic function which confirms the $I_{\text{laser}}^2$  dependence of the integrated signal.

\begin{figure}[t]
\centerline{\includegraphics[width=1 \linewidth]{fig8_intSigVsEnergyFig.pdf}}
\caption{(Color online) Integrated spectral signal for the measured transitions to the 29d state shown in Fig.~\ref{fig:sigVSenergy} as a function of the peak laser intensity along the atomic beam axis, $I_{\mathrm{laser}}$. Also shown is a quadratic function that has been fit to the experimental data (purple continuous curve).}
\label{fig:intVSenergy}
\end{figure}

Because the ac Stark shift is a feature of the interaction of the 2\,$^3$S$_1$ state with the laser field, and does not affect the Rydberg states, similar asymmetric lineshapes are seen for all recorded two-photon transitions. This is also the reason why all the spectral features in Fig.~\ref{fig:longscan} are shifted slightly to higher frequency from the calculated line positions. The peak widths are derived from the overlap of the atoms with the laser beam (as described in the Appendix), and are thus independent of the principal quantum number of the Rydberg states. This is illustrated in Fig.~\ref{fig:peakWidthsN} which shows the $n = 23$, 40 and 50 spectral features (scaled to equal amplitude for ease of comparison) overlaid, clearly demonstrating the $n$-independence of the line shape.
\begin{figure}[t]
\centerline{\includegraphics[width=1 \linewidth]{fig9_lineWidthsFig.pdf}}
\caption{(Color online) Measured spectral features associated with the $n=23$ (red, continuous curve), 40 (blue, dashed-dotted curve) and 50 (green, dashed curve) Rydberg states, scaled to equal amplitude for ease of comparison. The spectral features were recorded with the same laser intensity, and clearly demonstrate the $n$-independence of the spectral line profile.}
\label{fig:peakWidthsN}
\end{figure}

\subsection{Electric field}
The application of a homogeneous dc electric field in the excitation region mixes Rydberg states with the same value of $m_{\ell}$ but with values of $l$ differing by $\pm1$.
\begin{figure*}[t]
\centerline{\includegraphics[width=.6 \linewidth]{fig10_starkStatesPlot2.pdf}}
\caption{(a) Spectra of $|m_{\ell}|=2$ Rydberg states of He with $n=25$--27, recorded with a spatially averaged peak laser intensity of $I_{\text{laser}}=2.3\times10^{13}$Wm$^{-2}$ in the presence of dc electric fields of (i) 0~Vcm$^{-1}$, (ii) 28~Vcm$^{-1}$, (iii) 56~Vcm$^{-1}$, (iv) 111~Vcm$^{-1}$ and (v) 154~Vcm$^{-1}$.(b) Calculated spectra for the same conditions as those of the experimental data in (a).}
\label{fig:StarkPlotN}
\end{figure*}
As the applied electric field is increased the resulting Stark states in a given $n$-manifold separate in energy. Figure \ref{fig:StarkPlotN}(a) shows measured spectra of the transitions to the $n=25$, 26 and 27 states in a range of dc electric fields applied in the excitation region. In zero field the lines are asymmetric, with widths of $\sim20$~GHz (FWHM) resulting from the ac Stark shift (Sec. \ref{ACStarkExp}). As the field is increased to 28~Vcm$^{-1}$ the lines start to broaden symmetrically as the Stark states begin to separate in energy. Increasing the field further to 56~Vcm$^{-1}$ the energy splittings of the Stark states further increase, with the features becoming symmetric as the dc Stark shifts in the Rydberg states dominate the ac Stark shifts of the 2\,$^3$S$_1$ state. With this applied field strength the Stark states begin to be resolved.

As the electric field is increased further the dc Stark shifts continue to increase and the spectral features become more symmetric. In a field of 111~Vcm$^{-1}$, the Stark state resolution improves and the structure is clearer. With the maximum applied field of 154~Vcm$^{-1}$ the Stark states are clearly separated with each individual state identifiable. Note that in this field the $n=26$ and $n=27$ Stark manifolds are sufficiently broad that they overlap. This field is greater than the Inglis-Teller field of $\sim144~$Vcm$^{-1}$ for $n=26$~\cite{gallagher94a}. Fig.~\ref{fig:StarkPlotN}(b) shows calculations corresponding to the experimental data in Fig.~\ref{fig:StarkPlotN}(a). These spectra were calculated by convolving the calculated line profile arising from the ac Stark shift of the $2\,^3\mathrm{S}_1$ state with the spectral intensities of the transitions to each individual Stark state determined by diagonalization of the Hamiltonian matrix~\cite{zimmerman79a}. Fig.~\ref{fig:StarkPlotZoom} shows the spectrum at $n=26$ in a field of 154~Vcm$^{-1}$ in more detail. In this spectrum the 24 Stark states can be clearly identified. Also shown is the calculated spectrum which agrees well with the experimental data. The outer Stark states are not completely resolved because they are more susceptible to electric field inhomogeneities in the excitation region. Since the transition strengths to these states is weakest their spectral appearance is also more susceptible to shot-to-shot laser intensity variations, which slightly wash out their structure.

\begin{figure}[t]
\centerline{\includegraphics[width=1 \linewidth]{fig11_n26StarkZoom.pdf}}
\caption{(Color online) Spectrum of the $n=26$ Stark manifold of He in a dc electric field of 154~Vcm$^{-1}$ (blue continuous curve), shown with the calculated spectrum (red, dashed curve). Each of the 24 Stark states are labelled with the index $k$ which ranges from $-(n-|m_{\ell}|-1)$ to $+(n-|m_{\ell}|-1)$ in steps of 2. The lowest-energy Stark states of the $n=27$ manifold are visible toward the high-frequency end of the spectrum.}
\label{fig:StarkPlotZoom}
\end{figure}

These measurements show that by applying a dc electric field in the excitation region it is possible to split the Stark states sufficiently to allow selective excitation using this non-resonance-enhanced two-photon scheme. The limiting factor in the resolution is the ac Stark shift of the initial state.

\section{Application to Doppler-free photoexcitation of Rydberg states of positronium}

In addition to being of importance for precision spectroscopic studies and the preparation of long-lived high-angular-momentum Rydberg states, non-resonance enhanced Doppler-free two-photon excitation schemes are expected to be particularly well suited to the photoexcitation of Rydberg states of positronium (Ps) with high spectral resolution.

Positronium atoms are most efficiently created by implanting positrons into solid state materials. Various mechanisms for Ps formation exist, which are either thermal in nature~\cite{Mills79}, or rely on other energy transfer processes~\cite{cassidy2010}, and typically produce atoms with velocities on the order of $10^5$~ms$^{-1}$. This leads to very large Doppler broadenings of single photon transitions (e.g., increasing the 50~MHz natural width of the 1s--2p transition to $\sim 500~$GHz~\cite{cassidy12a}). Such large Doppler widths preclude the selective excitation of individual Ps Rydberg-Stark states using previously demonstrated two-color two-photon excitation schemes~\cite{Jones14}. However, in order to overcome some of the challenges associated with performing gravitational~\cite{mills02a} or spectroscopic~\cite{Fee93} measurements on Ps it is desirable to photoexcite them to well-defined Rydberg-Stark states with long radiative lifetimes, and large electric dipole moments, so that they can be guided, decelerated and manipulated using inhomogeneous electric fields~\cite{vliegen04a,vliegen2006,hogan08a,hogan11a}. By implementing two-photon excitation schemes of the kind demonstrated here, but using two counter-propagating laser beams in a Doppler-free configuration~\cite{Haensch75a}, a spectral resolution limited by the combination of the laser bandwidth and the ground state ac Stark shift will be achievable. This approach would provide sufficient resolution to selectively prepare individual Rydberg-Stark states of positronium (as demonstrated for He in Fig.~\ref{fig:StarkPlotZoom}).

In the experiments reported here, using circularly polarized laser radiation at a wavelength of $\sim520~$nm, with a spatially averaged peak intensity of $\sim5\times10^{13}~$Wm$^{-2}$, approximately 10\% (determined by comparison with the measured He$^+$ ion signal recorded by single photon excitation at saturation) of the metastable helium atoms that interact with the laser beam were excited from the 2\,$^3\text{S}_1$ state to $n$d Rydberg states, close to $n=25$. In these non-resonance-enhanced single-color two-photon excitation processes the transition rates depend on the sum of the off-resonant (electric dipole allowed) couplings at the single-photon level, between the initial state and all available non-degenerate bound and continuum intermediate states, and the further off-resonant couplings from these intermediate states to the excited Rydberg state~\cite{Haas2006}. By summing the contributions of all far off-resonant intermediate states up to $n=50$ we have determined the approximate transition rate from the 2\,$^3\text{S}_1$ state to the triplet 25d state in He to be $2\times10^{-5}~$Hz/(Wm$^{-2}$).

Using a Doppler-free single-color two-photon excitation scheme to drive transitions from the 1s ground state of ortho-Ps to $n$d Rydberg states will require circularly polarized laser radiation at $\sim 364~$nm. From the exact calculations for two-photon transitions in hydrogenic systems by Haas~et~al.~\cite{Haas2006} the corresponding transition rate for the 1s$\rightarrow$25d transition in Ps is also $\sim 2\times10^{-5}~$Hz/(Wm$^{-2}$). The similarity of this number to the corresponding rate for the two-photon transitions in He reported here indicates that similar excitation efficiencies can be expected in Ps. In addition, the frequency-dependent ground state polarizability of Ps, calculated using the methods outlined in Sec. \ref{acStarkSec} for laser radiation with a photon energy appropriate for driving the Doppler-free two-photon 1s$\rightarrow$25d transition is $+1.4\times10^{-39}~$Cm$^2$V$^{-1}$ (equivalent to a frequency shift of +4~GHz in a laser field of intensity $10^{13}~$Wm$^{-2}$), with a magnitude of approximately half that of the $2\,^3\mathrm{S}_1$ state in He. It is expected that this lower dipole polarizability and hence reduced ac Stark energy shift at a given laser intensity will also enhance the excitation efficiency in Ps.

The laser intensities used to drive single-color two-photon transitions in He were achieved by focussing the output of a pulsed dye laser to a width of $\sigma\sim70~\mu$m. While it is possible to employ greater laser powers using commercially-available dye laser systems, a practical upper limit to the dimension of the beam size required to obtain the appropriate intensity is expected to be $\sigma\sim200~\mu$m, corresponding to a FWHM of $\sim0.5~$mm. The production of a $\sim1~$mm diameter positron (and hence also Ps) pulse from an accumulator can be achieved with moderate magnetic fields of 500~G~\cite{Cassidy2006}. For experiments in which such fields are acceptable this means that approximately half of the available atoms could be addressed, and $\sim5\%$ excited. However, because Ps velocities are generally high, ionization of Rydberg states  by motionally induced electric fields can lead to significant losses~\cite{cassidy12a,hogan13a}. For magnetic fields of 50~G, such losses are largely mitigated, but then the positron beam size will be $\sim 3~$mm, and the overlap with a 0.5~mm FWHM laser beam would be about 15\%, leading to an overall excitation efficiency of 1.5\%. A typical Ps pulse in an apparatus operating at 1~Hz would contain $\sim10^6$ atoms, so that more than $10^4$ Rydberg atoms could be produced per second.

An alternative approach is to remoderate the positron beam~\cite{Mills1980}. In this arrangement the positrons would be extracted from the trap magnetic field completely, and focussed into a solid material (the remoderator). Non-conservative interactions in the remoderator make it possible to circumvent phase-space  limitations, and the re-emitted positron beam brightness may be considerably enhanced. For typical trap parameters this would result in a loss of $\sim 90\%$ of the particles, but with a spot size of less than $100~\mu$m. In this case the initial $10^6$ positrons would again result in $\sim10^4$ Rydberg atoms, but this time in zero magnetic field, and with a spatial spread smaller than the laser beam size, so that significantly smaller laser pulse energies ($\sim$1~mJ) would be required.

A further reduction in the required laser energy can be achieved by using a laser with a reduced bandwidth. Seeding a pulsed dye laser amplifier, with, for example a narrow-band continuous wave (cw) diode laser tuned to the desired fundamental wavelength, will result in a reduction in the laser bandwidth by a factor of $\sim20$. Using such a transform-limited pulse will reduce the energy required for a similar excitation efficiency by an order of magnitude. The use of transform-limited laser radiation and a brightness-enhanced re-moderated positron source could reduce the energy required to drive two-photon transitions in Ps to $\sim 100~\mu$J per pulse.

\section{\label{concl}Conclusions}
Non-resonance-enhanced single-color two-photon excitation to long-lived Rydberg states in He has been demonstrated by driving transitions from the metastable $2\,^3\text{S}_{1}$ state to Rydberg states in the range from $n=20$ up to $n\simeq 100$. The effect of the intensity and polarization of the laser radiation on the excitation efficiency and spectral profiles has been investigated in detail. The spectral features in the recorded spectra were observed to be broader than the bandwidth of the laser, and asymmetric with a long tail to high frequency. It has been shown by comparison of the experimental data with the results of numerical calculations that this is a result of the ac Stark shift of the $2\,^3\text{S}_{1}$ state in the laser field, which non-resonantly couples the $2\,^3\text{S}_{1}$ state to other bound states. The polarizability of the $2\,^3\text{S}_{1}$ state was calculated by summing all off-resonant couplings to $n$p states in the range from $n=2$ up to 20. With detailed measurements of the laser beam profile this allowed the calculation of the spectral lineshapes, which were found to be in excellent agreement with those recorded experimentally.

Photoexcitation in dc electric fields was also performed, and with a field strength of 154~Vcm$^{-1}$ individual Rydberg-Stark states were clearly identified. This selective population of individual Rydberg-Stark states in a non-resonance-enhanced multi-photon photoexcitation scheme as demonstrated is important for the preparation of atoms and molecules in long-lived high-angular-momentum Rydberg states for deceleration and trapping experiments using inhomogeneous electric fields.

In particular, the application of similar schemes to Ps has been discussed. Based on the measured excitation rates in He under experimental conditions similar to those expected in experiments with Ps, we conclude that it is feasible to excite ground state Ps atoms to Rydberg states with $n\simeq25$ in Doppler-free two-photon transitions with sufficient resolution to address individual Stark states. This will enable new experiments designed to control Rydberg Ps atoms with inhomogeneous electric fields via their large electric dipole moments.

\begin{acknowledgments}
This work has been supported financially by the Department of Physics and Astronomy, and the Faculty of Mathematical and Physical Sciences at University College London, and by the Engineering and Physical Sciences Research Council under grant number EP/K028774/1. We are grateful to the Engineering and Physical Sciences Research Council Laser Loan Pool (loan number 13250005) for the loan of the laser system used in this research.
\end{acknowledgments}

redpajama_set_name: RedPajamaArXiv


At Kelso Arms Company we pride ourselves on customer service. We hope that all of our customers are 100% satisfied. That said, we realize there may be times we don't meet someone's expectations. We are firm believers we can't fix what we do not know is broken. With that in mind, we ask all our customers to take a few minutes to give us feedback, good, bad, or otherwise.


text:
A W74 foi uma arma nuclear experimental dos Estados Unidos da América destinada para ser usada na artilharia nuclear.

Respondendo para um requerimento do exército dos Estados Unidos, datado do ano 1969 para a substituição do projétil de artilharia nuclear W48, com 155 mm de diâmetro, o Laboratório Nacional de Los Alamos começou o desenvolvimento da W74. Contudo, pela metade do ano de 1973 o programa foi descontinuado sem produzir qualquer arma para o uso. As razões primárias para a cancelação da W74 foi a tecnologia obsoleta e o alto custo.A arma somente utilizou a fissão nuclear, não tinha melhoramentos na precisão e não tinha capacidade de desenvolvimentos futuros como uma bomba de nêutrons; o resultado seria uma arma obsoleta e sem capacidade de evolução que custaria 425 000 dólares(1973) por unidade.

A W74 teve um desenvolvimento em que apenas se alterava apenas o seu diâmetro, a W75, também foi cancelada.

 Hansen, Chuck; Swords of Armageddon, Sunnyvale, California, Chucklea Publications, 1995

Ver também
 Lista de armas nucleares
 Artilharia nuclear
 W9
 W19
 W23
 W75

Ogivas nucleares dos Estados Unidos
Armas nucleares de implosão linear

redpajama_set_name: RedPajamaWikipedia


Heavenly Father, we are to be counted among the wayward and lost, ones who wander and squander. We confess to you specific sins of omission and commission, seeking your forgiveness through Jesus Christ our Lord. Amen.


Do I need a gun safe for car? Why?
The most important possession that imposes value to a person is his home. It is where the people take a rest and feels safe and protected. Therefore guns become an important accessory for homes, after the homes, the next thing that can be on the list is the car you own. Thus car security is considered to be similar to that of your home security.
Without the security, your car would not remain safe neither the belongings you have kept inside it. The car security is quite important and while you are travelling, it is your responsibility to maintain your own safety. This is because, during your travel, it may be quite common that you get trapped in the hands of thieves and burglars. In that case, if you have a gun with yourself, you can surely protect yourself or your family and peers, everyone. This is mostly important to keep safety along with you as you will be finding no one to defend you while you are on the roads and is travelling. The entire story will finish off much before you reach the location. There is no reason to think that these thieves or burglars are anyway lesser than you and therefore being fewer alert owners, you will easily get into the traps of those villains and they shall rob away your valuables present in the car or in some worse situation they may even rob the car.
However keeping guns can be good at times but it is your prior concern to check that the gun does not get into any wrong hands. Make sure that the people who travel with you may not take away your gun as you gun is your personal responsibility and if any disrespectful events occur, the entire blame shall be imposed on you. If you are travelling with your family, it is good to carry the car, but take care that your children do not take them into their hands. Therefore along with the gun, make sure that you carry a gun safe in your car. There are basically two types of car securities or rather the gun safe that represent the mechanical security or the electronic security. You may also acquire a combination.
herefore you shall be more prone towards causing damage to your gun and that may also result to different injuries. While buying a gun safe there are certain things that you should consider at first. Here are the security features and the features act as the most important factor rather than the price you is spending. There should be no point of scrimping of affording a quality gun safe. If you were able to buy the gun, you shall surely be able to buy the gun safe.
Having a gun along with the gun safe in your car will surely help you to deter your potential rabbet and you will have a safe travel. In this way, you shall also be able to attract the attention of your thief. Thus, you will also be able to use your time and make out possibilities to help yourself. The most basic kind of gun safe that can help you to protect yourself is the combination locks. These locks are usually not enough in the present days. It is good to maintain the security and keep it away from the hands of others but at times, especially during the emergency circumstances, people usually panic so much that they do not keep the patience to wait still and open the lock. Nowadays, there are quite a number of gun safes that use biometric methodologies or voice activation capabilities. They are easily accessible and safe.


This is the third post in a series called "Cretaceous Cold Cases" in which the science of taphonomy, or prehistoric forensics, is explained by fascinating cases from the files of Terry "Bucky" Gates, a research scientist with a joint appointment at NC State and theNorth Carolina Museum of Natural Sciences.
The Utah team is in an area of the state called Mussentuchit Flats looking for dinosaurs about 98 million years old. There is nothing at all out there except lots of rock with dinosaurs!! Cell phones are useless, which is one of the great things about paleontology field work every now and then…to go somewhere no one can reach you. Communing with prehistory and yourself actually is healthy. But the lack of showers gets old after about 10 days.
So while we all wait for the next field update I wanted to fill in a little more information about our trip to Botswana.
At this point their face would crinkle into a funny contortion of disgust at the thought of a dead animal, and smoke would arise from their ears as they tried really really hard to figure out why in the world anybody would actually want to find a dead animal, let alone as many as humanly possible.
The answer is really quite simple. That is…because everything in the fossil record is dead.
Well DUH!! But really think about that. If everything that we are studying as fossils are dead, all of the experiences and history that each of those animals led is barely transferred to their fossilized remains.
So the best way to tease out every ounce of information about ancient life is to understand exactly what types of information can be preserved by studying modern dead animals.
The main purpose of this trip was to first find, map, and detect any patterns in what bone sites looked like. We did not have armed guards with us on this trip, and the one short walk that was taken through a savannah felt like a literal death march because our guide spotted tracks from a pride of lions that passed through only a few hours earlier. Therefore, most of our carcass prospecting was from the vehicle. Nonetheless, we found over 30 carcasses, some over 10 years old.
And our preliminary findings are positive. It seems that giraffe carcasses are found where the giraffes live, as well as the water buffalo (these animals don't live in the same habitat). Elephants are just everywhere.
Our next goal is to walk miles across the Okavango Delta to find even more carcasses. Ultimately, our data can be used to assess conservation efforts, changes in populations, and predator behaviors through time.


text:
Q: Rank of a matrix with diagonal elements zero, all other elements either 1 or -1 and row-sum of eash row is 0. Let A be a $n \times n$ matrix such that:
1) all the diagonal elements are $0$,
2) all other elements are either $1$ or $-1$
3) number of $1$'s in a row are equal to number of $-1$'s in the row, i.e. the row-sum is $0$ for all rows.
It is obvious that $n$ is odd here. I have done some experiments and found that the rank of such an $A$ is always $n-1$. Can someone help me with a proof? I can only see that
\begin{bmatrix}
1 \\
1\\
\vdots\\
1\\
1\\
 \end{bmatrix}
is an eigen vector with eigen value =0.

A: Look at the leading principal $(n-1)\times(n-1)$ submatrix $A_{n-1}$. In modulo-2 arithmetic, $A_{n-1}=ee^T-I_{n-1}$, where $e^T=(1,\ldots,1)$. Its eigenvalues are $n-2$ and $-1$ modulo $2$, which are nonzero because $n$ is odd. So $\det A_{n-1}\ne0$ in modulo-2 arithmetic and $\det A_{n-1}\ne0$ over $\mathbb R$ too. Hence $A_{n-1}$ is nonsingular and $A$ has rank $n-1$.

A: I'll outline a proof ...

Let $A$ be an $n$ x $n$ matrix of the specified type.

As noted, $n$ must be odd.

Let $B$ be the $(n-1)$ x $(n-1)$ upper left-hand submatrix of $A$.

The determinant of $B$ is the sum of the products of the generalized diagonals.

Of the $(n-1)!$ such products, the nonzero products are odd (equal to either $+1$ or $-1$).

The number of diagonals whose product is nonzero is the number of derangements of $\{1,...,n-1\}.$

Since $n-1$ is even, the number of derangements is odd.

Thus, $\text{det}(B)$ is the sum of an odd number of odd numbers, so is odd, and hence is nonzero.

It follows that $\text{rank}(A) \ge n-1$.

Since the columns of $A$ sum to zero, $\text{rank}(A) \le n-1$.

Therefore $\text{rank}(A) = n-1$.

redpajama_set_name: RedPajamaStackExchange


text:
Sojoez TMA-7 (Russisch: Союз ТМА-7) was een Sojoez-missie naar het International Space Station.

Bemanning

Gelanceerd
 Valeri Tokarev (2) bevelhebber -
 William McArthur (4) boordingenieur-
 Gregory Olsen (1) ruimtetoerist-

Geland
 Valeri Tokarev (2) bevelhebber -
 William McArthur (4) boordingenieur -
 Marcos Pontes (1) vluchtingenieur -

Missie parameters
 Massa: 7.200 kg
 Perigeum: 200 km
 Apogeum: 252 km
 Glooiingshoek: ~51.7°
 omlooptijd: 88.7 min

Gekoppeld aan het ISS
Gekoppeld aan het ISS: 3 oktober, 2005,05:27 UTC (aan de pirs module)
Afgekoppeld van het ISS: 18 november, 2005, 08:46 UTC (van de pirs module)
Gekoppeld aan het ISS: 18 november, 2005, 09:05 UTC (aan de Zarya module)
Afgekoppeld van het ISS: 20 maart, 2006, 06:49 UTC (van de Zarya module)
Gekoppeld aan het ISS: 20 maart, 2006,07:11 UTC (aan de Zvezda module)
Afgekoppeld van het ISS: 8 april, 2006, 20:28 UTC van de Zvezda module)

Zie ook
 ISS Expeditie 12

TMA-07
Bemand ruimtevaartuig


All the original, high grade speakers where still here, but dry-rotted and every single one of them had the wires cut. Who in their right mind would take each speaker down and cut the wires and reattach the speaker? Maybe he did this because he put in even fancier speakers with their own wires?
And thanks Rick, I will look above the passenger seat. I do remember seeing an amp up there. all the horn and speakers were all attached to the digitel system....which Tony carefully removed. He hasn't put in the new outdoor speakers yet cause he doesn't know where to attach the wires...yet.
Oh, lower dash...haven't looked there...yet. Did find a large dead nest of wasps behind that headlight when I was working on the faded paint the other day. didn't open the front panel though.
Run new speaker wires,you will be better off.
How did the fuel filter change go?
Inside! Remove the wooden lower panel in front of the passenger.
The bus is up on wood and we have a plan to get the generator bay pulled open...but it's going to take both of us to get it open and we can't seem to get our planets to align.
What was that part you suggested we change the air ram to, when we were at your place?
I see, said the blind man. I have never removed that panel. Hopefully it's still daylight when I get home from work and I will check it out ASAP! Thanks!
It is there with the blue flat wire still attached. Now at least we know it IS possible to get it working.


text:
La gare de Krasnoïarsk-Passajirski (en ) est une gare ferroviaire russe, située dans la ville de Krasnoïarsk, dans l'kraï de Krasnoïarsk, en Russie.

Situation ferroviaire

La gare est exploitée par Krasnoïarskaïa jeleznaïa doroga, partie de Chemins de fer russes.

Histoire
Elle a été ouverte en 1895. Elle fut électrifiée en 1959.

Service des voyageurs

Accueil

Desserte

Intermodalité

Notes et références

Voir aussi

Articles connexes
 Liste de gares en Russie
 Transport ferroviaire en Russie

Liens externes

Gare en Russie
Gare mise en service en 1895


It's a year ago that Bart and I travelled to Taiwan for 2 weeks, and after that I continued alone to visit Macau, Malaysia and Taiwan for 4 weeks.
Recently, I decided to start posting the pictures of this journey, then I found that I didn't match the GPS coordinates of my photos yet. Hence, it took me few days to finish the task. Luckily, I can skip this step in the future because my current Canon 6D can log the track and write the GPS coordinates on picture directly. After that, I have to resize the pictures and make selections for blog. Then, upload and edit them in WP. This post shares the pictures of our tour from Holland to Taiwan, and the first day in Hsinchu.
Bart's only visiting Taiwan for 2 weeks and decided to park his car at Schiphol.
Heading to long-term parking area.
There're shuttle busses between parking area and terminal, quite convenient.
We took the direct flight of China Airlines to Taiwan.
Spotted KLM's orange+blue plane again.
A lot of greenhouses in this area.
Get the high speed train to Hsinchu.
The trip from Taoyuan HSR to Hsinchu HSR was about 13 minutes. We decided to stand for a while instead of rushing to find a seat in crowded train.
Having lunch at 11th Street Noodle shop.
Pizza and fried chicken as dinner.


text:
<?php

namespace Cocorico\TimeBundle\Model;

use DateInterval;
use DatePeriod;
use DateTime;
use DateTimeZone;

/**
 * Class DateTimeRange
 *
 * Object representing a date range and its relative time ranges
 *
 * Its main goal is to return DayTimeRange array containing all time ranges for each days of date range
 */
class DateTimeRange
{
    /**
     * @var DateRange
     */
    public $dateRange;

    /**
     * @var TimeRange[]
     */
    public $timeRanges;

    /**
     * DateTimeRange constructor.
     * @param DateRange|null $dateRange
     * @param TimeRange[]    $timeRanges
     */
    public function __construct(DateRange $dateRange = null, $timeRanges = array())
    {
        $this->setDateRange($dateRange);
        $this->setTimeRanges($timeRanges);
    }

    /**
     * @param DateTime|null $start
     * @param DateTime|null $end
     * @param DateTime|null $startTime
     * @param DateTime|null $endTime
     * @return static
     */
    public static function createFromDateTimes(
        DateTime $start = null,
        DateTime $end = null,
        DateTime $startTime = null,
        DateTime $endTime = null
    ) {
        $dateRange = new DateRange($start, $end);
        $timeRanges = array(new TimeRange($startTime, $endTime, $start));

        return new static($dateRange, $timeRanges);
    }

    /**
     * @return DateRange
     */
    public function getDateRange()
    {
        return $this->dateRange;
    }

    /**
     * @param DateRange $dateRange
     */
    public function setDateRange($dateRange)
    {
        $this->dateRange = $dateRange;
    }

    /**
     * @return TimeRange[]
     */
    public function getTimeRanges()
    {
        return $this->timeRanges;
    }

    /**
     * @param TimeRange[] $timeRanges
     */
    public function setTimeRanges($timeRanges)
    {
        if ($timeRanges && count($timeRanges)) {
            //Order descending
            usort(
                $timeRanges,
                function (TimeRange $first, TimeRange $second) {
                    return strcmp(
                        $first->getStart()->format('YmdHi') . '-' . $first->getEnd()->format('YmdHi'),
                        $second->start->format('YmdHi') . '-' . $second->getEnd()->format('YmdHi')
                    );
                }
            );
        }

        $this->timeRanges = $timeRanges;
    }

    /**
     * @return TimeRange|null
     */
    public function getFirstTimeRange()
    {
        return count($this->timeRanges) ? $this->timeRanges[0] : null;
    }

    /**
     * Get time ranges for each day of this date range.
     * For each date if a time range spans day (22h -> 02h) the time range is splitted in two time ranges on the two spanning days:
     *  - first time range is from start time to midnight (2017-10-13 22h -> 2017-10-14 00h)
     *  - second one is from midnight to end time (2017-10-14 00h -> 2017-10-14 02h)
     *
     * @param bool   $endDayIncluded
     * @param array  $weekDays
     * @param string $timezone
     *
     * @return DayTimeRange[]
     */
    public function getDaysTimeRanges($endDayIncluded, $weekDays = array(), $timezone = 'UTC')
    {
        $start = clone $this->getDateRange()->getStart();
        $start->setTime(0, 0, 0);
        $end = clone $this->getDateRange()->getEnd();
        $end->setTime(0, 0, 0);

        if ($endDayIncluded) {
            $end->modify('+1 day');
        }

        $firstDayTimeRange = array();
        if (count($this->getTimeRanges()) && $this->getFirstTimeRange()->getStart()) {
            $this->adjustStartAndEndDay($start, $end);
            $firstDayTimeRange = $this->getInitialTimeRanges($start);
        }

        $daysTimeRanges = $this->iterateInitialTimeRangesOnDays($start, $end, $firstDayTimeRange, $timezone);

        if (count($weekDays)) {
            $hasTimeRanges = count($firstDayTimeRange);
            $daysTimeRanges = $this->filterByWeekDays($daysTimeRanges, $weekDays, $timezone, $hasTimeRanges);
        }

        return $daysTimeRanges;

    }

    /**
     *  Adjustment of first and last day of date range depending on first smaller day of time ranges
     *
     * @param DateTime $start
     * @param DateTime $end
     */
    private function adjustStartAndEndDay(DateTime &$start, DateTime &$end)
    {
        //First day is equal to the first smaller day of time ranges
        $firstDay = clone $this->getFirstTimeRange()->getStart();
        $firstDay->setTime(0, 0, 0);

        //If the first smaller day of the time range is less than start date range day then end date range day is decremented by one.
        //This case occurs when first time range day is before DateRange start in UTC timezone
        if ($firstDay->format('Y-m-d H:i') < $start->format('Y-m-d H:i')) {
            $end->modify('-1 day');
        }

        $start = $firstDay;
    }

    /**
     * Return initial time ranges by day since first day
     *
     * @param DateTime $firstDay
     * @return array [offsetDay][nbMinutes] => array( 0 => firstStartMinute, 1 => secondStartMinute, ...)
     *               - offsetDay: number of day since $firstDay (ex: 1 => time range day is equal to $firstDay + 1 day)
     *               - nbMinutes: time range duration in minutes (ex: 60 => time range duration is 60 minutes)
     *               - firstStartMinute, ...: time range start at this minute in the day (ex: 120 => 02h00 AM)
     */
    private function getInitialTimeRanges(DateTime $firstDay)
    {
        $ranges = array();
        $nextDayTimeRange = null;
        foreach ($this->getTimeRanges() as $i => $timeRange) {
            $startTime = $timeRange->getStart();
            $endTime = $timeRange->getEnd();

            $day = clone $startTime;
            $day->setTime(0, 0, 0);
            $diffDay = $firstDay->diff($day)->days;//Number of days between first day and time range start time

            $startDay = clone $startTime;
            $startDay->setTime(0, 0, 0);
            if ($timeRange->overlapDays()) {
                $midnight = clone $startTime;
                $midnight->modify('+1 day');
                $midnight->setTime(0, 0, 0);

                //Time range start and duration
                $nbMinutes = ($midnight->getTimestamp() - $startTime->getTimestamp()) / 60;
                $startMinute = ($startTime->getTimestamp() - $startDay->getTimestamp()) / 60;
                $ranges[$diffDay][$nbMinutes][] = $startMinute;

                //Next day time range start and duration
                $nbMinutes = ($endTime->getTimestamp() - $midnight->getTimestamp()) / 60;
                $ranges[$diffDay + 1][$nbMinutes][] = 0;
            } else {
                //Time range start and duration
                $nbMinutes = ($endTime->getTimestamp() - $startTime->getTimestamp()) / 60;
                $startMinute = ($startTime->getTimestamp() - $startDay->getTimestamp()) / 60;
                $ranges[$diffDay][$nbMinutes][] = $startMinute;
            }
        }

        return $ranges;
    }

    /**
     * Return DayTimeRange objects for all time ranges of all days of this date range and initial time range
     *
     * @param DateTime $start
     * @param DateTime $end
     * @param  array   $initialTimeRanges see getInitialTimeRanges
     * @param  string  $timezone
     *
     * @return array with date string as index
     */
    private function iterateInitialTimeRangesOnDays($start, $end, $initialTimeRanges, $timezone)
    {
//        echo '
';
        $hasTimeRanges = count($initialTimeRanges);
        $dstTransitions = $this->getDSTTransitions($start, $end, $timezone);

        $interval = new DateInterval('P1D');
        $days = new DatePeriod($start, $interval, $end);
        $nbDays = max(iterator_count($days), 1);

        /** @var DayTimeRange[] $daysTimeRanges */
        $daysTimeRanges = array();
        $newOffset = 0;
        for ($d = 0; $d < $nbDays; $d++) {
            $day = clone $start;
            $day->modify('+' . $d . ' day');
            $day->setTime(0, 0, 0);

            if ($hasTimeRanges) {
                $this->iterateInitialTimeRangesOnDay(
                    $day,
                    $initialTimeRanges,
                    $dstTransitions,
                    $newOffset,
                    $daysTimeRanges
                );
            } else {
                $newDay = clone $day;
                $startDayString = $day->format('Y-m-d');
                $daysTimeRanges[$startDayString] = new DayTimeRange($newDay, array());
            }
        }
//        print_r($daysTimeRanges);
//        die();

        return $daysTimeRanges;
    }

    /**
     * Iterate initial time range on a day
     *
     * @param DateTime              $day
     * @param                       $initialTimeRanges
     * @param  array                $dstTransitions
     * @param  int                  $newOffset      DST offset in minutes
     * @param  DayTimeRange[]       $daysTimeRanges result
     */
    private function iterateInitialTimeRangesOnDay(
        DateTime $day,
        $initialTimeRanges,
        &$dstTransitions,
        &$newOffset,
        &$daysTimeRanges
    ) {
        foreach ($initialTimeRanges as $diffDay => $timeRanges) {
            /** @var TimeRange $timeRange */
            foreach ($timeRanges as $nbMinutes => $startMinutes) {
                foreach ($startMinutes as $startMinute) {
//                        echo $newOffset . '
';
                    $startMinute += $newOffset;//Add/remove DST offset if any DST transition occurs in time ranges
                    $startMinute = $startMinute >= 0 ? '+ ' . $startMinute . ' minute' : $startMinute . ' minute';
                    //Start time
                    $startTime = clone $day;
                    $startTime->modify('+' . $diffDay . ' day');
                    $startTime->modify($startMinute);
                    $offset = $this->getDSTOffset($startTime, $dstTransitions);
                    if ($offset) {
                        $offsetString = $offset >= 0 ? '+ ' . $offset . ' minute' : $offset . ' minute';
                        $startTime->modify($offsetString);
                        $newOffset = $newOffset + $offset;
                    }

                    //End time
                    $endTime = clone $startTime;
                    $endTime->modify('+ ' . $nbMinutes . ' minute');
                    $offset = $this->getDSTOffset($endTime, $dstTransitions);
                    if ($offset) {
                        $offsetString = $offset >= 0 ? '+ ' . $offset . ' minute' : $offset . ' minute';
                        $endTime->modify($offsetString);
                        $newOffset = $newOffset + $offset;
                        //todo: check multiple DST transitions in the date range
                    }

                    $this->addDayTimeRange($startTime, $endTime, $daysTimeRanges);
//                        echo $startTime->format('Y-m-d H:i') . ' / ' . $endTime->format('Y-m-d H:i') . '
';
                }
            }
        }
    }

    /**
     * @param DateTime        $startTime
     * @param DateTime        $endTime
     * @param  DayTimeRange[] $daysTimeRanges result
     */
    private function addDayTimeRange(DateTime $startTime, DateTime $endTime, &$daysTimeRanges)
    {
        $startDayString = $startTime->format('Y-m-d');
        $endDayString = $endTime->format('Y-m-d');
        /** @var DateTime[][] $newTimeRanges */
        $newTimeRanges = array($startDayString => array($startTime, $endTime));
        //If endTime is tomorrow then time range is splitted on spanning days
        if ($endDayString != $startDayString && $endTime->format('H:i') != '00:00') {
            $midnight = clone $endTime;
            $midnight->setTime(0, 0, 0);

            $newTimeRanges[$startDayString][1] = $midnight;
            $newTimeRanges[$endDayString] = array($midnight, $endTime);
        }

        //Fill $daysTimeRanges day by day and time range by time range
        foreach ($newTimeRanges as $dayString => $newTimeRange) {
            if (isset($daysTimeRanges[$dayString])) {
                $dayTimeRange = $daysTimeRanges[$dayString];
            } else {
                $newDay = clone $newTimeRange[0];
                $newDay->setTime(0, 0, 0);
                $dayTimeRange = new DayTimeRange($newDay, array());
            }

//                            echo $newTimeRange[0]->format('Y-m-d H:i') . ' / ' . $newTimeRange[1]->format('Y-m-d H:i') . '
';

            $dayTimeRange->timeRanges[] = new TimeRange($newTimeRange[0], $newTimeRange[1]);
            $daysTimeRanges[$dayString] = $dayTimeRange;
        }
    }

    /**
     * Return DST offset if DST transition occurs before $time.
     * If DST transition occurs then it is removed from DSTs to apply.
     * As time ranges are checked time after time DST offset has to be apply only one time.
     *
     * Ex: With DST change at 2017-10-29 01:00 in Paris timezone (UTC +2)
     *
     * @param DateTime $time
     * @param array    $dstTransitions decreasing ordered
     * @return int DST offset in minutes
     */
    private function getDSTOffset($time, &$dstTransitions)
    {
        $dstOffset = 0;
        foreach ($dstTransitions as $dstDateString => $offset) {
            if ($time->format('Y-m-d H:i') >= $dstDateString) {// First start range impacted by DST
                $dstOffset = $offset * 60;
                unset($dstTransitions[$dstDateString]);
                break;
            }
        }

        return $dstOffset;
    }

    /**
     * Get date and offset in hour for DST transitions between start and end date
     *
     * @param DateTime $start
     * @param DateTime $end
     * @param  string  $timezone
     *
     * @return array
     */
    private function getDSTTransitions(DateTime $start, DateTime $end, $timezone)
    {
        //We only need DST transitions from first lowest date time so if fist time range exists
        // then start is equal to start time
        $start = clone $start;
        $start = $this->getFirstTimeRange() && $this->getFirstTimeRange()->getStart() ?
            $this->getFirstTimeRange()->getStart() : $start;

        $tz = new DateTimeZone($timezone);
        $transitions = $tz->getTransitions($start->getTimestamp(), $end->getTimestamp());

        $dstTransitions = array();
        if (count($transitions) > 1) {
            foreach ($transitions as $i => $transition) {
                if ($i == 0) {
                    continue;
                }
                $date = new DateTime();
                $date->setTimestamp($transition['ts']);
                $offset = round(($transitions[$i - 1]['offset'] - $transition['offset']) / 3600);
                $dstTransitions[$date->format('Y-m-d H:i')] = $offset;
            }
        }

        arsort($dstTransitions);

//        print_r($dstTransitions);
//        die();

        return $dstTransitions;
    }

    /**
     * Filter DayTimeRanges by week days
     *
     * @param DayTimeRange[] $daysTimeRanges $daysTimeRanges
     * @param array          $weekDays
     * @param string         $timezone
     * @param bool           $hasTimeRanges
     *
     * @return DayTimeRange[]
     */
    private function filterByWeekDays($daysTimeRanges, $weekDays, $timezone, $hasTimeRanges)
    {
        $prevEnd = $added = false;
        foreach ($daysTimeRanges as $day => $dayTimeRanges) {
            if (!$hasTimeRanges) {
                $isInWeekDays = in_array($dayTimeRanges->getDay()->format('N'), $weekDays);
                if (!$isInWeekDays) {
                    unset($daysTimeRanges[$day]);
                }
            } else {
                foreach ($dayTimeRanges->getTimeRanges() as $index => $timeRange) {
                    $start = clone $timeRange->getStart();
                    $end = clone $timeRange->getEnd();
                    $start->setTimezone(new DateTimeZone($timezone));
                    $end->setTimezone(new DateTimeZone($timezone));

                    $isInWeekDays = in_array($start->format('N'), $weekDays) &&
                        (in_array($end->format('N'), $weekDays) || $end->format('H:i') == '00:00');
                    if ((!$prevEnd && $isInWeekDays) ||
                        ($prevEnd && $isInWeekDays && ($start != $prevEnd || $start->format(
                                    'H:i'
                                ) == '00:00' || $added))) {
//                        echo 'added';
                        $added = true;
                    } else {
                        unset($daysTimeRanges[$day]->timeRanges[$index]);
//                        echo 'not added';
                        $added = false;
                    }
                    $prevEnd = $end;
                }

                if (!count($daysTimeRanges[$day]->getTimeRanges())) {
                    unset($daysTimeRanges[$day]);
                }
            }

        }

        return $daysTimeRanges;
    }

    /**
     * Add time range times to date range dates
     *
     * Start date have time equal to start time
     * End date have time equal to end time
     *
     * @param DateRange $dateRange
     * @param TimeRange $timeRange
     *
     * @return DateTimeRange
     */
    public static function addTimesToDates(DateRange $dateRange = null, TimeRange $timeRange = null)
    {
        if ($dateRange && $timeRange) {
            if ($timeRange->getStart() && $timeRange->getEnd()) {
                $nbDays = $dateRange->getStart()->diff($dateRange->getEnd())->days;

                $dateRange->setStart(clone $timeRange->getStart());
                $dateRange->setEnd(clone $timeRange->getEnd());
                $dateRange->getEnd()->modify('+' . $nbDays . ' day');
            }
        }

        return new static($dateRange, array($timeRange));
    }

}

redpajama_set_name: RedPajamaGithub


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Mark is a software leader with extensive experience in taking complex technologies and making them easy to use. An experienced builder of high-performance software development teams, Mark was a winner at the UK Agile Awards in 2012.
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text:
Bank tightening could result in 'credit crunch'
By Annie Kane
Researchers at an investment bank have warned that banks could tighten their income and expenses assessments for mortgage borrowers off the back of the royal commission, which could result in "a sharp reduction in credit availability" and could negatively impact the economy.
UBS analysts Jonathan Mott and Rachel Bentvelzen (alongside economists George Tharenou and Carlos Cacho) have released their latest Australian Banking Sector Update, which outlines that they believe the banks will soon "move further to tighten underwriting standards to ensure they fully comply with the National Credit Act regarding responsible lending".
The assessment comes off the back of hearings held by the Royal Commission into Misconduct in the Banking, Superannuation and Financial Services Industry, which criticised several lenders for failing to "take positive steps to verify the customer's expenses".
During one of the hearings earlier this month, Rowena Orr, the senior counsel assisting the commission, highlighted a guide by the Australian Securities and Investments Commission (ASIC) that tells banks they are "obliged to take reasonable steps to verify consumers financial situation".
When it was revealed that ANZ did not undertake verification of expenses in some instanes, Ms Orr said: " "I want to put to you that your processes, or your lack of processes in relation to the verification of a customer's expenses, are non-compliant with the National Credit Act, responsible lending obligations and with this regulatory guide issued by ASIC."
While the interim findings of the royal commission will not be released until September (with a final report next year), UBS has said it believes the banks will "continue to act to address irresponsible lending".
It highlighted that ANZ has already said it will suspend new asset finance loans for retail customers, while CBA has suspended credit card and personal loan insurance, adding that it believes the banks' boards and management will "move further to tighten underwriting standards to ensure they fully comply with the National Credit Act regarding responsible lending".
Suggesting that mortgage underwriting standards seem to have been "lax for some period of time", UBS said that the evidence given at the royal commission hearing leads its analysts to believe that there is "a material way to go" to ensure adherence to NCCP.
"While a tightening of mortgage underwriting standards is prudent, especially as the banks move to fully complying with responsible lending, it has a material impact on the economy," the update reads.
"It must be remembered that house prices are determined by the demand and supply of credit (not the demand for and supply of housing)."
As a result, UBS' analysts said they believe there are two potential scenarios:
(1) As the banks tighten underwriting standards assessed household income is reduced. The banks will need to undertake more detailed analysis of customers' individual living costs and the Household Expenditure Measure (HEM) benchmark is increased slowly over time. "As the banks gradually move towards fully complying with the responsible lending laws the flow of credit is steadily constrained and the housing market slowly deflates"; or
(2) The Royal Commission sets a strict definition of the "reasonable inquiries" and lenders are required to immediately comply with Responsible Lending obligations (eliminating 'predatory lending'). Income assessment is tightened, the banks are required to undertake a full review of each borrower's living costs. The back-up HEM benchmark is increased to realistic levels which borrowers could be expected to live off during the full life of the loan (including 'lumpy' items). "This leads to a sharp reduction in the Net Income Surplus and false applications are largely eliminated. However, in this scenario fully complying with responsible lending laws would result in a sharp reduction in credit availability ie a 'Credit Crunch' especially for lower income households. RBA rate cuts would not help credit availability as borrowers are assessed using the 7.25 per cent interest rate floor. This could potentially result in a significant economic downturn."
While the investment bank suggests that its base case economic outlook "assumes only a slow and modest tightening of credit conditions", it is "more concerned about the possibility of a 'tighter credit' scenario".
The latter could lead to a "credit crunch scenario", the analysts warn, which could lead to potential economic downturn.
"Under a credit crunch scenario, house prices would likely fall over a prolonged period across a few years," the update reads. "This is because the price of money (i.e. ~record low interest rates) becomes less relevant compared with the supply of money (i.e. credit availability) or demand for money (i.e. employment, income, population) – particularly for the marginal new borrower (especially on lower incomes) which sets the price of the stock of existing housing.
"There is a great deal of uncertainty over how 'bad' this could become, simply because Australia has never had a fall in house prices of 10 per cent + (even during the GFC prices 'only' fell to -8 per cent year-on-year, but the RBA slashed rates by 425bp and reflated the housing market), and neither have we had a domestic recession in almost three decades," the analysts write.
However, UBS concludes that should a credit crunch feed through to the broader economy and result in rising unemployment, this could see the RBA consider cutting the cash rate further should they become concerned the slow-down was "turning into a recession".
UBS states: "In this scenario there is a risk of a pick-up in arrears as existing borrowers become financially stressed, and could precipitate a broad-based credit event."
[Related: Westpac to collect customer repayment history next month]
Annie Kane
Last Updated: 26 March 2018 Published: 27 March 2018
Annie Kane is the editor of The Adviser and Mortgage Business.
As well as writing about the Australian broking industry, the mortgage market, financial regulation, fintechs and the wider lending landscape – Annie is also the host of the Elite Broker and In Focus podcasts and The Adviser Live webcasts.
Contact Annie at: This email address is being protected from spambots. You need JavaScript enabled to view it.

redpajama_set_name: RedPajamaCommonCrawl


I want to get more for less.
are self-focused, socially withdrawn, antagonistic.
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→ To achieve a 360 success in all areas of → your life and → business , you must strive to create a greater – and unique! – value for people around you. Just creating some value is not enough. You must → be different and very special in their lives.
People are → motivated for their reasons, not yours. Satisfy the deepest needs of people around you. The deepest need that each person has is for self-esteem, a sense of being important, and valuable. Everything that you do in your interactions with other people affects their self-esteem in some way.
Continually look for ways to make people feel more valuable and important.
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One of the golden rules of the therapy profession is that everyone needs at least one person with whom they can openly and unashamedly discuss every little detail – happenings, desires, fears – of their life, whether it is from the past, present, or future. There are other psychological needs – to be accepted unconditionally, appreciated, recognized, respected, desired, valued, approved of, or complimented – that affect the way we communicate with each other.
"In our → new economy ‒ where ideas rule and so many people are looking for new strategies to get ahead ‒ the most successful → entrepreneurs will be content creators, trainers, coaches, and online information marketers who help others survive and thrive. People who use their life's story and strategies for success to help other people improve their lives or → businesses will have more income, influence and impact."
You grow when you help others. In order to help others to grow, you need to be a good example for them. You must → stretch yourself. To teach and inspire others, you must learn a lot and be inspired yourself. You must learn how to → motivate yourself and others.
You must learn to be a great leader. Leadership is essentially about helping people to achieve a better life and extraordinary results. Great → leaders help other people lead themselves, energize them and help them achieve their stretch goals.
Jesus said that in order to be a leader you had to first learn how to serve others.
Kind word can be short and easy to speak, but their echoes are truly endless.
but I'd love to improve people's lives, especially socially.
can warm three winter months.