234 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



where ecp is the energy required for the electron to overcome the 

 binding forces of the metal, e is the charge of the electron, and cp is 

 the work function of the material of the cathode. Of all elements, 

 the smallest values for cp are found for the alkali metals, in particular 



° 6x10 4 



a 



Normal 

 photoelectric 

 effect on 

 p/otinum 

 [ordinate left) 



b 



Selective 

 photoelectric 

 effect on 

 potassium 

 ordinate 

 right) - 



2x10' 



.5 



1.0 



-0.5 



5 4 



Energy, ev 



J_ 



J 



250 300 



Wavelength, mju 



400 500 600 



Fig. (5-1)2. Quantum yield for photoelectric emission: (a) normal 

 photoelectric effect on a platinum surface; (6) selective photoelectric 

 effect on a potassium-oxygen surface (after R. W. Pohl, Optik, Springer 

 Verlag, Berlin, 1940; by permission). 



for cesium; compounds of the alkali metals can have even smaller 

 work functions. 



The energy with which the photoelectrically liberated electron 

 leaves the metal becomes zero for 



A =- = 



where 1 Q is the threshold wavelength (in millimicrons) for a metal 

 surface with the work function cp. Any radiation with a wavelength 

 greater than A will not cause photoe mission from this surface. The 

 threshold wavelength of commercial photoelectric surfaces is, in 

 general, between 600 and 1,200 m^. 



The quantum yield, i.e., the number of electrons emitted for a 

 given number of absorbed light quanta, increases with decreasing 



