REFLECTION-ABSORPTION IR STUDIES OF VIBRATIONAL-ENERGY TRANSFER PROCESSES AND ADSORPTION ENERGETICS

The adsorption of CO on Cu(100) and Pd(100) surfaces has been studied using reflection absorption IR spectroscopy (RAIRS) at cryogenic tempe ratures, 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 br idging sites are available, in agreement with the findings of other wo rkers. We report on the changes in absorption profiles of v(CO) at 79 and 23 K on both substrates, detailing the change in frequency,intensi ty and full-width-at-half-maximum (FWHM) as a function of temperature. Below 35 K, CO is physisorbed above the chemisorbed layer on both sub strates and this is found to affect the chemisorbed phase, resulting i n changes in the absorption profiles of the chemisorbed GO-stretching mode. We interpret these observations in terms of the delicate energet ic balance between the highest-coverage chemisorbed phases and lower-c overage ordered phases combined with a physisorbed overlayer. We have studied the absorption profiles of the GO-stretching mode for adsorbed CO (v(CO)), from 23 K and above and this has enabled us to determine the pathways of vibrational energy transfer (VET) for the CO/metal sys tems. We report on the frequency shifts for v(CO) for the highest-cove rage 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 sy stem in the 23-120 K range, in order to assess the effect of the occup ation of different sites on VET processes. On Cu(100) a major decay ch annel is via coupling to the frustrated translation, whereas on Pd(100 ) the dominant process appears to be coupling to the metal phonons.