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 $h\nu$ up to 6.71 ev. When $h\nu$ 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 $h\nu$, 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.