Study of adsorption states and interactions of CO on evaporated noble metal surfaces by infrared absorption spectroscopy: II. Gold and copper

Abstract

We have used infrared absorption spectroscopy to study the adsorption of CO at low temperature on evaporated films of gold and copper as a function of the coverage of CO and the deposition temperature of the metal. For both metals we observe two distinct adsorption regimes when the cold metal is exposed to CO gas. These regimes arise depending on whether the deposition temperature of the metal (or the highest temperature at which the metal has been annealed) is above or below a threshold temperature. Gold films deposited at temperatures below 290 K contain chemically active sites at which CO chemisorbs, while films deposited at higher temperatures do not support CO chemisorption. This behavior is very similar to that observed on silver films. Chemisorption of CO occurs on copper films regardless of deposition temperature, but the vibrational spectrum is radically different for films deposited above and below the threshold temperature of 250 K. For both metals, the shift of the vibrational frequency with CO coverage has been analyzed, and the static and dynamic contributions separated. The dynamic shift is found to be well modeled by dipole-dipole coupling, with no evidence for vibrational coupling through the metal. An analysis of infrared intensities and vibrational polarizabilities shows no evidence for any special infrared enhancement analogous to the large enhancement in Raman cross section peculiar to adsorption on rough metal films, particularly noble metal films, deposited at low temperatures.