Photoemission spectroscopy—Correspondence between quantum theory and experimental phenomenology

We describe the relation between the observed behavior of photoemission energy distributions vs frequency, angle, etc. , and the quantum theory of photoemission as recently set forth formally by Caroli et al. We derive a Fermi's Golden-Rule formula for the angle- and energy-resolved photocurrent from an independent electron solid, and show in detail the approximations which render this formula equivalent to that of the familiar three-step model of bulk photoemission. In terms of the Golden-Rule formula, we account for the direct-transition band-structure regime generally observed at low photon energies (h20-30 eV) and the "photoemission density of states", or x-ray-photoemission-spectroscopy regime, observed at higher energies. We also propose an explanation of Feuerbacher et al. 's observation of "direct photoemission into the vacuum" from single-crystal tungsten surfaces. Finally, we discuss the criteria which determine the relative magnitudes of photocurrents from a surface adsorbate layer and an underlying substrate.