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Paper text:
pentacene was deposited on a muscovite mica surface by physical 
 vapor deposition from a knudsen cell in an ultrahigh vacuum chamber . 

the cell temperatures were adjusted between 450 and 490 k , in order 
 to realize deposition rates between 0.01 ml / min and 4.5 ml / min , respectively . 

the mica samples ( 10  10  0.01 mm ) were attached 
 to a steel plate via tantalum wires .
the steel plate was heated resistively , 
 and its temperature was controlled by a ni 
, the temperature 
 of the mica sample could be varied between 100 and 1000 k. typically , 
 the sample holder was cooled during the experiments for a better residual 
 gas pressure , but the sample was hold at 300 k during deposition by 
 proper sample heating . for a quantitative determination of the deposited 
 material a quartz microbalance was used , which was located next to 
 the mica substrate .
the mica substrate was cleaved with adhesive tape 
 prior to installation into the vacuum chamber and subsequently gently 
 sputtered by argon ions .
ten minutes of sputtering with 500 ev ar+ 
 ions at an argon pressure of 5  10 mbar 
 were sufficient to change the 5a film morphology from needle like 
 islands , composed of lying molecules , to compact islands composed 
 of standing molecules .
the surface chemical composition was analyzed 
 by auger electron spectroscopy and x - ray photoelectron spectroscopy . 

thermal desorption spectroscopy was applied to determine the thermal 
 stability of the pentacene film and the sticking coefficient .
ex - situ 
 atomic force microscopy ( nanosurf , easyscan2 ) was used to analyze 
 the film morphology .
it 
 is known in thin - film deposition that the density of nucleated 
 clusters n varies with the deposition rate f as a power law , 
 n  f. the exponent  is a function of the critical nucleus size i in a way that changes with the aggregation limiting process . 
 
 we extend here the derivation of the analytical capture - zone distribution 
 function p(s ) 
 = asexp(-bs2 ) of pimpinelli and einstein to generic aggregation - limiting processes . 
 
 we show that the parameter  is generally related to the critical 
 nucleus size i and to the exponent  by the 
 equality  = i , in the case of 
 compact islands . 
 this remarkable result allows one to measure i with no a priori knowledge of the actual 
 aggregation mechanism . 
 we apply this equality to measuring the critical 
 nucleus size for pentacene deposition on mica . 
 this system shows a 
 crossover from diffusion - limited to attachment - limited aggregation 
 with increasing deposition rates .