INFRARED REFLECTION-ABSORPTION SPECTROSCOPY OF DIPOLE-FORBIDDEN ADSORBATE VIBRATIONS

Formally dipole-forbidden adsorbate vibrational modes have recently been detected using infrared reflection-absorption spectroscopy (IRAS). In an earlier publication we have proposed that the excitation of these modes is indirect, mediated by the metal electrons and closely related to the concept of surface resistivity. Here we extend that theory to lower frequencies; for parallel adsorbate vibrations the theory predicts an anti-absorption resonance with an unique asymmetry which only depends on the ratio omega0/omega1 between the adsorbate vibrational frequency 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 agreement with the measurements of Lin et al. [Phys. Rev. B 48 (1993) 2791] for CO on Ni(100).