A study of adsorbate vibrations at cryogenic temperatures

Reflection-absorption infrared spectroscopy (RAIRS) has been used to probe adsorbate vibrations in the temperature range below 77 K (the base temperature obtained in most RAIRS experiments), by cooling the metal sample with liquid helium. Spectra can be recorded routinely at any desired temperature from 23 K to the desorption temperature during any experiment using the apparatus which we have constructed. We report on the adsorption of CO on Pd(100) where CO adopts only bridging sites below 77 K. A significant interaction between the physisorbed and chemisorbed layers is observed using RAIRS. RAIRS spectra of physisorbed CO2 on Cu(100) are presented and illustrate the potential of the application of RAIRS to the study of growth and phase formation of molecular crystals.