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.
New values of α/p0 in pure neon obtained with the luminous-flux method
Abstract In this contribution we report on the “luminous-flux method” for the determination of Towns-end's primary ionization coefficient α. This method gives α directly without the interference of the secondary coefficient γ that may be, due to various effects, a function of the electrode distance. The method mentioned makes questionable the results obtained in pure neon with the classical current-growth method. We discuss the difference in the values of α obtained by these methods. Finally, by table and graph, we give the classical as well as the new values of α, i.e. the well known results obtained by Kruithof and Penning and by Chanin and Rork, and 46 average values of α obtained with the luminous-flux method in the range of the reduced field strength E / p 0 from 5.5 up to 484 V cm -1 torr -1 .
Multiple beam interference in opaque photocathodes.
The recently published paper by Sizelove and Love presents an approach of doubtful validity in determining the light intensity as a function of position within the cathode. The assumption is made by the authors in Eq. (3), stating that the intensity at a specific point within the layer may be determined by the sum­ mation of the intensities due to each of the multiply reflected beams. Certainly for a typical photocathode with a thickness less than 1000 A, the distances involved for the first several reflec­ tions are well within the coherence length of most common "wide band, noncoherent" sources. It is therefore more appro­ priate to sum the multiply reflected beams in terms of their amplitudes and phases. Multiplication of the resultant ampli­ tude function by its complex conjugate will produce the desired intensity function. The authors attribute their Eq. (3) to a paper by Deutscher. We do not find such an equation in the Deutscher reference. In fact, Deutscher clearly states "Zur Berechnung der elektrischen Feldstarke im Punkt P nach der Methode der Partialwellen hat