ON THE COEFFICIENT OF ABSORPTION OF PHOTO- 

 ELECTRONS IN SILVER AND PLATINUM. 



OTTO STUHLMAN, JR. 



The coefficient of absorption can be defined by the constant « 

 in the exponential relation N=Noe-rt'. The precise physical 

 interpretation of such an exponential law when applied to a 

 beam of electrons moving through a metal plate of thickness " t " 

 is not so simple as its mathematical expresision would lead one to 

 suppose. It may, however, be interpreted if we assume that 

 there exists no "scattering" or energy transformation as the 

 beam of electrons passes through the metal. If «x is taken as 

 that fraction of the number which is absorbed when the beam of 

 electrons passes normally through a very thin screen of thickness 

 X (cm), then for a plate of thickness t (cm), X^XyC-a', in which 

 No is the intensity of the beam when it enters, and N that of the 

 beam when it emerges from the slab of material in which the 

 absorption has taken place. 



A more convenient form for experimental use is, however, the 

 following. a= -(•- (log X„-log N) where the logarithms are 

 taken to the base 10. Hence given a set of observations with 

 homogeneous (i. e. electrons of the same initial velocity) rays, 

 if log N is plotted as ordinate against t, the thickness of the slab 

 through which the electrons pass, the graph is then a straight 

 line and « is 2.3 times the slope of this line. If large values of « 

 are obtained, this is an indication of easily absorbed electrons 

 (velocity small) while if « is found to be small it indicates the 

 presence of penetrating rays or fast moving electrons. 



In some cases it is, however, easier to think of the absorption 

 coefficient in terms of the thickness t which reduces the number 

 of electrons to half value. Since ^ represents the distance to 

 which the electrons penetrate before their number is reduced ^ 

 of the original value, thien a may be written in the fonn, ^ = --^ 

 where e"°-''^^=O.500. In the followdng obsier^^ations this ex- 

 pression was used in determining the coefficient of absorption 

 of electrons liberated in a metal by monochromatic ultra violet 

 light. 



A Cooper-Hewitt, quartz, 110 volt, mercury vapour lamp 

 served as a source of ultra violet light. This passed through a 

 Hilger monochromatic illuminator with a quartz optical system. 



