INFRARED REFLECTION-ABSORPTION SPECTROSCOPY OF DIPOLE-FORBIDDEN ADSORBATE VIBRATIONS

Formally dipole-forbidden adsorbate vibrational modes have recently be en detected using infrared reflection-absorption spectroscopy (IRAS). In an earlier publication we have proposed that the excitation of thes e modes is indirect, mediated by the metal electrons and closely relat ed to the concept of surface resistivity. Here we extend that theory t o lower frequencies; for parallel adsorbate vibrations the theory pred icts an anti-absorption resonance with an unique asymmetry which only depends on the ratio omega0/omega1 between the adsorbate vibrational f requency omega0 and omega1 = upsilon(F)/delta where upsilon(F) is the Fermi velocity and delta = c/omega(p) the skin depth. The theory is in very good agreement with the IRAS measurements of Hirschmugl et al. [ Phys. Rev. Lett. 65 (1990) 4801 for CO on Cu(100) and in qualitative a greement with the measurements of Lin et al. [Phys. Rev. B 48 (1993) 2 791] for CO on Ni(100).