Study of adsorption states and interactions of CO on evaporated noble metal surfaces by infrared absorption spectroscopy: I. Silver

Abstract

We have used infrared absorption spectroscopy to study the adsorption of CO at low temperature on evaporated silver films as a function of the coverage of CO and the deposition temperature of the silver. We observe two adsorption regimes when a cold silver film is exposed to CO gas. If the silver deposition temperature, (or the highest temperature at which the silver has been annealed), is above the threshold temperature of 150 K, then only physisorbed CO is observed. For sample temperatures below 25 K, these physisorbed molecules are oriented perpendicular to the metal surface. Films deposited at temperatures below 150 K, however, contain ≈ 0.01 monolayer of chemically active sites at which CO chemisorbs. The infrared band due to chemisorbed CO shifts to lower frequency with increasing coverage. We have analyzed this shift and separated the static and dynamic contributions. The static, chemical shift is caused in part by the change in the work function induced by surrounding adsorbates. The dynamics shift is fully explained by a dipole-dipole interaction; we find no evidence for a vibrational coupling through the metal. We have analyzed the vibrational polarizability and infrared absorption strength of the absorbed CO, and find no evidence for the infrared enhancement suggested by some theories of surface enhanced Raman scattering.