Reflection absorption IR studies of vibrational energy transfer processes and adsorption energetics

Abstract

The adsorption of CO on Cu(100) and Pd(100) surfaces has been studied using reflection absorption IR spectroscopy (RAIRS) at cryogenic temperatures, in the range from ca. 23 K to the CO desorption temperature. CO is found to be adsorbed on terminal sites on Cu(100) and on bridged sites on Pd(100) in the temperature range 23–77 K. CO can occupy some terminal sites on Pd when defects which offer terminal rather than bridging sites are available, in agreement with the findings of other workers. We report on the changes in absorption profiles of ν_CO at 79 and 23 K on both substrates, detailing the change in frequency, intensity and full-width-at-half-maximum (FWHM) as a function of temperature. Below 35 K, CO is physisorbed above the chemisorbed layer on both substrates and this is found to affect the chemisorbed phase, resulting in changes in the absorption profiles of the chemisorbed CO-stretching mode. We interpret these observations in terms of the delicate energetic balance between the highest-coverage chemisorbed phases and lower-coverage ordered phases combined with a physisorbed overlayer. We have studied the absorption profiles of the CO-stretching mode for adsorbed CO (ν_CO), from 23 K and above and this has enabled us to determine the pathways of vibrational energy transfer (VET) for the CO/metal systems. We report on the frequency shifts for ν_CO for the highest-coverage CO phase on Pd(100) in the 23–80 K temperature range and compare the results with the frequency shifts for the compressed Cu(100)/CO system in the 23–120 K range, in order to assess the effect of the occupation of different sites on VET processes. On Cu(100) a major decay channel is via coupling to the frustrated translation, whereas on Pd(100) the dominant process appears to be coupling to the metal phonons.

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