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Two-photon photoemission spectra are reported for silver films deposited under ultrahigh vacuum onto a room-temperature (smooth film) or a cold substrate (rough film). A theory is developed which successfully accounts for the qualitative features of the observed spectra in the near-threshold region when the Ag is excited by the absorption of one or two photons. Photoemission from smooth films can be successfully described in terms of direct electron transitions. The traditional one-photon photoemission theory is generalized to include two-photon excitation. Photoemission from rough Ag surfaces is interpreted in terms of the excitation of localized surface plasmons in the roughness features which are assumed to be small polarizable elements of a self-affine fractal. The rapid spatial rate of change in the local electric field surrounding these elements breaks translational invariance, thereby allowing indirect electron transitions to take place, ultimately leading to a predicted (10^3--10^6) -fold enhancement of the two-photon photoemission yield. Approximate analytic expressions are derived for the total photoemission yields following one- or two-photon excitation of smooth silver and two-photon excitation of rough silver. 1996 The American Physical Society.
Shalaev et al. (Mon,) studied this question.