Abstract
A fully quantum-mechanical theory of infrared-reflection spectroscopy from adsorbate-covered metal surfaces is presented. In contrast to our earlier semiclassical theory [B. N. J. Persson and A. I. Volokitin, Surf. Sci. 310, 314 (1994)], which is valid only for parallel vibrational modes of atomic adsorbates, the present theory is applicable for arbitrary dipole-forbidden modes. It is shown that for a metal with a broad conduction band, the asymmetry of the line shape of dipole-forbidden adsorbate vibrational modes is determined essentially by nonlocal optics effects, with negligible contribution from nonadiabaticity. The theory is in good agreement with experimental data.