Optical factors in the photoemission of thin films.
Expressions are derived for the dependence of the photoemission from thin films deposited on transparent substrates on the direction of illumination, as determined by optical factors alone. For normal incidence, the photoemission of very thin-film transparent photocathodes may be expected to be about 1.4 times as great for illumination from the glass side as for illumination from the vacuum side. Furthermore, if the index of refraction of the film is large compared with that of the substrate, the ratio, for natural light, is also almost independent of the angle of incidence for a wide angular range. For thicker films, both the optical constants of the film and the mean depth of origin of the photoelectrons play an important role. However, for strongly absorbing as well as nearly transparent photoemissive films, substantially higher photocurrent should be obtainable from a photoemissive film of optimum thickness illuminated through the substrate than for a thick film with similar optical constants illuminated from the vacuum side. An assumed simple proportionality between optical stimulus, here given by the square of the field intensity within the photosensitive film, and photoexcitation implies that the photoeffect studied is a volume effect and not dependent on the direction of the electric vector within the film.
The theory of the surface photoelectric effect in metals—II
1. In the electron theory of metals it is usually assumed that the electrons can be regarded as moving independently in a periodic potential field, and for certain properties it is sufficient to use the simpler Sommerfeld model, in which the potential inside the metal is constant. The photoelectric effect is not, however, one of these, as the electrons are effectively free, and, as is well known, the photoelectric effect cannot take place with free electrons on account of the impossibility of satisfying simultaneously the laws of conservation of energy and momentum. Because of this, Tamm and Schubin have proposed that the photoelectric emission from a metal should be regarded as arising from the superposition of two effects, the surface effect, due to the rapid changes of potential energy at the surface of the metal, and the volume effect, due to the internal variations of potential; and have further given a theory of the surface effect for an idealized case. Theories apparently of the surface effect, in that they neglect potential variations inside the metal, have also been given by Frohlich and Wentzel, and the results of all three theories are at variance. The question is examined anew in the present paper, and the conclusion is reached that none of the earlier theories can be accepted as representing the surface effect even for the very idealized case considered. The theory which is believed to be correct is given, and and certain results briefly discussed, leaving to a later paper a more detailed comparison with the empirical facts.
Photoemission and photomultipliers
A discussion is presented of photoemission and photomultipliers for the practical user of phototubes with emphasis on those parameters of importance in applications such as scintillation counting. A short description of the physics of photoemission is followed by a discussion of photosensitivity, temperature dependence of photoemission, effect of cathode resistivity, response time, effect of initial velocity on transit time dispersion and ultraviolet photo-emitters.