326 BELL SYSTEM TECHNICAL JOURNAL 



Infulence of the Nature of the Irradiation 



The niagnitiidc of the photoelectric current depends upon the angle 

 of incidence, the plane of polarization and the color or wave length 

 of the light used. This dependence is closely interlinked with the 

 choice of the photo-sensitive material and the state of its surface. 

 For the purpose of separating out the effects of the several variables 

 in the incident light, it is necessary to study the properties of optically 

 plane or specular surfaces of the photo-sensitive material. Such 

 surfaces can only be obtained with the alkali metals by raising them 

 above their melting points, by forming alloys, or by depositing ex- 

 tremely thin films on a polished underlying metal surface such as 

 platinum. Specular surfaces of the aklali metals obtained in these 

 several different ways exhibit differences in their behavior, but it will 

 be sufficient for the present purposes to disregard these secondary 

 differences and to speak merely of the photoelectric current from 

 specular surfaces when the incident light varies in wave length in cer- 

 tain typical ways, or is polarized. 



Influence of the Plane of Polarization 



When light is incident at a steep angle on a specular surface, the two 

 extreme conditions of polarization are those in which the electric vector 

 lies in the plane of incidence and that in which it lies perpendicular 

 to the plane of incidence. In the first case, the electric vector has a 

 component perpendicular to the surface. In the latter case the electric 

 vector lies parallel to the surface. It has long been known that the 

 amount of light absorbed by a metal surface is, in general, greater 

 when the electric vector is in the plane of incidence. Consequently, 

 since photoelectric emission must be due primarily to the absorption 

 of the energy from the incident light, it is to be expected that the 

 photoelectric current will be greater for light polarized with the electric 

 vector in the plane of polarization. Such is actually the case, but 

 while the ratio of absorption of light, for the two planes of polarization, 

 at say 60° incidence, never rises above a value of four for any of the 

 alkali metals, the ratio of the photoelectric currents under the same 

 conditions may mount to a very high value, such as 20 or 30 to one. 

 This effect is particularly noticeable in the liquid alloy of sodium 

 and potassium, and in the case of all the four alkali metals, sodium, 

 potassium, rubidium and caesium, when these spontaneously deposit 

 in a high vacuum upon a polished surface. It is very much less 

 marked in the case of the pure alkali metals in the molten condi- 

 tion. Typical examples of the influence of the plane of polarization 



