INFRARED REFLECTION-ABSORPTION SPECTROSCOPY OF DIPOLE FORBIDDEN ADSORBATE VIBRATIONS

We explain the origin of recent observations of so-called dipole-forbidden adsorbate vibrations in infrared-reflection-absorption spectroscopy (IRAS) of atoms and molecules adsorbed on metal surfaces. The excitation of these modes is indirectly mediated by the metal electrons and closely related to the concept of surface resistivity. The theory is in very good agreement with the IRAS measurements of Hirschmugl et al. [Phys. Rev, Lett. 65 (1990) 480] for CO on Cu(100) and in qualitative agreement with the measurements of Lin et al. [Phys. Rev. B, in press] for CO on Ni(100).