Element-specific X-ray methods play a key role in the study of the atomic structure and composition of functional materials. X-ray spectroscopy can be used to determine oxidation states, distances, coordination numbers and the nature of the nearest neighbors of the selected element. With a wide variety of X-ray spectroscopic methods, many gaseous, liquid and solid samples or molecular systems at interfaces have been studied. Stationary and time-dependent material properties were determined predominantly at synchrotron radiation sources and more recently at X-ray free-electron lasers.
The investigation of liquid samples with absorption spectroscopy in the soft X-ray range (in the energy range of about 0.2 to 1.5 keV) presents a particular challenge. First, the experiments must be carried out under ultrahigh-vacuum conditions, in an environment that seems to be incompatible with the high vapor pressure of water. In addition, due to the large absorption cross sections in the soft X-ray range, measurement of the transmission requires difficult-to-implement sample thicknesses in the range of one micron and less (1 micron = 10-6 m = one millionth of a meter). In contrast, absorption spectrum measurements based on detection of secondary decay signals, such as X-ray fluorescence, are limited to relatively high concentration samples.