New Aspects of the Photoelectric Emission from Na and K

The external photoelectric effect for Na and K in the form of thick evaporated layers was investigated with photon energies h up to 6. 71 ev. When h was within 1. 5 ev of the threshold, the energy distributions of the emitted electrons were like those derived by DuBridge for a simple photoelectric effect involving an ideal metal. For larger h, there was a growing preponderance of low velocity electrons. Their appearance could not be correlated, on the basis of a simple emission process, with the assumed parabolic shapes and approximately known widths of the valence bands. The following complications were therefore considered: (a) scattering by other electrons, causing the excited electron to lose energy before escape; (b) absorption of the recoil momentum by a second electron rather than by the entire crystal, resulting in the transfer of only a portion of the photon energy to the photo-electron; (c) modifications of the simple Fermi band by electron interactions, giving the effect of initial energy states much farther below the surface barrier than had been assumed. The spectral distributions showed no evidence of a sharply defined "volume effect" such as might be due to optical transitions between zones. The form of these spectral curves was notably sensitive to variations in the structure of the evaporated films. Either matter or specular Na surfaces could be produced merely by controlling the deposition rate. There was no appreciable difference in the energy distributions for electrons from the two types of sufaces.