ADSORBATE-SUBSTRATE RESONANT INTERACTIONS OBSERVED FOR CO ON CU(100) AND (111) IN THE FAR-IR USING SYNCHROTRON RADIATION

The vibrations associated with CO adsorbed on Cu(100) and Cu(111) were studied in the spectral region 50 cm-1 to 2500 cm-1 using infrared reflection absorption spectroscopy. Below 500 cm-1 we used synchrotron radiation as the source. In addition to the Cu-CO stretch modes at 345 cm-1 and 346 cm-1 respectively, we observe sharp derivative-like features at 285 and 279 cm.-1 On the basis of isotopic shift measurements we assign these to the CO frustrated rotation mode which is usually dipole forbidden. We believe that the infrared activity of these modes is due to a resonant interaction with an adsorbate induced electronic surface state which accounts for the Fano line shape. The measured width of this level puts a lower limit on the lifetime of 1 ps for this electronic damping.