ON THE ORIGIN OF ANTI-ABSORPTION RESONANCES IN ADSORBATE VIBRATIONAL SPECTROSCOPY

We explain the origin of recent observations of anti-absorption peaks in infrared-reflection-absorption spectroscopy (IRAS) of atoms and molecules adsorbed on metal surfaces. It is shown that parallel frustrated translations of adsorbates always give rise to relatively strong anti-absorption peaks in IRAS, even though these modes are dipole-forbidden with respect to a normal electric field. The excitation mechanism is indirect, involving the metal electrons. The practical importance of this result is emphasized.