Alkali Metal Films Research Articles

The Photoelectric Properties of Alkali Metal Films as a Function of their Thickness

Previous workers have given very little consideration to the thickness of the alkali metal films under investigation. The purpose of the present work was to study the photoelectric properties of the films as a function of their thickness. A molecular beam was used to deposit a computed number of alkali metal atoms on a silver surface cooled with liquid air. In this way, films of various thicknesses were formed and their corresponding spectral response curves taken. The recorded thresholds were arbitrarily chosen as the points, where the currents became definitely measureable. For potassium, the maximum threshold, approximately 5800A, occurred at a film thickness of 3.0 molecular layers while the maximum total photoelectric emission occurred at 12.4 molecular layers, and the photoelectric properties remained constant after 19.0 molecular layers. The atomic spacing in the monomolecular layer is assumed to be the same as that in a solid mass of alkali metal. The threshold for the thick film was approximately 5500A. For rubidium, the maximum threshold, approximately 6200A, occurred at a film thickness of 1.5 molecular layers, while the maximum total photoelectric emission occurred at 5.0 molecular layers, and the photoelectric properties remained constant for film thicknesses beyond 12 molecular layers. The threshold for the thick film was approximately 5900A. For caesium, the maximum threshold, approximately 6600A, occurred at a film thickness of 1.5 molecular layers, while the maximum total photoelectric emission occurred at 5.0 molecular layers, and the photoelectric properties remained constant for film thicknesses beyond 10.0 molecular layers. The threshold for the thick film was approximately 6300A. Some anomalous effects are reported in the case of sodium. The maximum extension of the threshold was found to be much less than that reported by other investigators. A theory is advanced to account for the existence of a maximum in the excursion of the threshold.

The Depth of Origin of Photoelectrons

Previous work has shown that the photoelectrons from a silver plate covered with an equilibrium film of alkali metal follow the wave-length distribution of energy just above the silver surface, i.e., in the alkali metal. This question has been further investigated with particular references to alkali metal films in their early stages of development, where their average depth is less than one atom. Computations made on the absorption of light just within the silver surface show that there should be very definite and striking differences in the wave-length distribution of photoemission if emission occurs due to light absorption in the silver, as contrasted with emission from a film on the silver. Experimental tests made with sodium and caesium films show that in the earliest measurable state the emission exhibits characteristics peculiar to the light absorption in silver, and that as the films build up the emission becomes characteristic of the energy above the silver. It is concluded that the photoelectrons originate partly in the underlying metal and partly in the alkali metal film, the relative proportions varying with the film thickness.

The Photoelectric Effect from Thin Films of Alkali Metal on Silver

The thin films of alkali metals which spontaneously deposit in vacuo on other metals have long been known to exhibit photoelectric effects which vary in amount and character, depending on the underlying material, but the exact nature of this dependence has been obscure. Silver, because of its region of exceedingly low reflecting power in the ultraviolet and the accompanying variation of optical constants, is exceptionally well suited for studying the influence of the underlying metal. It is found that the region of low reflecting power profoundly affects the photoemission, but in a manner not to be explained simply by reduction of light reflected back through the alkali metal film or by the absorption of light by the silver. The results obtained are very satisfactorily explained upon computing, from the optical constants, the intensity at the surface, of the interference pattern formed by reflection just above the silver surface. The positions of the maxima and minima of photoemission, and their variations with angle of illumination and plane of polarization are accurately indicated.

Photo-Electric Properties of Thin Films of Alkali Metals

Photo-Electric Properties of Thin Films of Alkali Metals