Photofield emission spectroscopy of surface electronic states of tungsten

A novel photofield emission spectrometer has been used to observe departures from free electron behaviour in the total energy distributions of photofield emission from the (100), (110), (111), (211), and (510) facets of a tungsten field emitter. Measurements with p-polarized light at grazing incidence at seven photon energies in the visible and near ultraviolet show departures from free electron behaviour having the same form as those observed in field emission distributions, but shifted to higher energy by one photon energy. The shapes and polarization dependence of the distributions are precisely those expected for surface photoexcitation from electron states near the surface to vacuum tunnelling final states. These observations provide strong evidence that the surface photoeffect is the dominant mechanism of excitation by p-polarized light in photofield emission from tungsten.