2-PHOTON-ELECTRON EMISSION FROM SMOOTH AND ROUGH METAL-FILMS IN THE THRESHOLD REGION

Two-photon photoemission spectra are reported for silver films deposit ed under ultrahigh vacuum onto a room-temperature (smooth film) or a c old substrate (rough film). A theory is developed which successfully a ccounts for the qualitative features of the observed spectra in the ne ar-threshold region when the Ag is excited by the absorption of one or two photons. Photoemission from smooth films can be successfully desc ribed in terms of direct electron transitions. The traditional one-pho ton photoemission theory is generalized to include two-photon excitati on. Photoemission from rough Ag surfaces is interpreted in terms of th e excitation of localized surface plasmons in the roughness features w hich are assumed to be small polarizable elements of a self-affine fra ctal. The rapid spatial rate of change in the local electric field sur rounding these elements breaks translational invariance, thereby allow ing indirect electron transitions to take place, ultimately leading to a predicted (10(3)-10(6))-fold enhancement of the two-photon photoemi ssion yield. Approximate analytic expressions are derived for the tota l photoemission yields following one- or two-photon excitation of smoo th silver and two-photon excitation of rough silver.