REFLECTION-ABSORPTION IR SPECTROSCOPY AT CRYOGENIC TEMPERATURES

Reflection-absorption IR spectroscopy (RAIRS) has been used to study the adsorption and behaviour of a number of adsorbates on a Cu(100) surface at cryogenic temperatures (in the range ca. 20-77 K). Sample cooling to temperatures below 77 K presents a wide range of opportunities for definitive experiments on the nature of the surface-adsorbate and adsorbate-adsorbate interactions. Adsorption of methane and carbon dioxide on Cu(100) at 24 K results in physisorbed mono- and multi-layers; for CO2 the effect of orientation of the layer can be clearly seen. Carbon monoxide adsorption on Cu(100) produces both chemisorbed and physisorbed molecules. The physisorbed multilayers are desorbed at 26 K, leaving chemisorbed and physisorbed monolayers. Above 35 K, a transition occurs where the physisorbed species disappears and additional CO is chemisorbed. The final example is the use of an argon matrix to dilute the adsorbate and disrupt adsorbate-adsorbate interactions, for adsorbed chromium hexacarbonyl on copper.