Abstract
We have used infrared emission spectroscopy to study the molecule-substrate vibrational mode of on-top CO on Pt(111), as a function of coverage and of temperature, in the range from 210 to 400 K. Little change in the line is observed as a function of coverage, except for an abrupt shift, of ~ 6 cm −1, to lower frequency, at coverages > 0.5. The line shape is asymmetric, and the linewidth varies much more slowly with temperature than is predicted by theories of lifetime broadening by phonon emission. Based on these observations, our conclusion is that the line is inhomogeneously broadened. Our measurements place an upper limit on the homogeneous linewidth for this mode. The conclusion that the line is inhomogeneously broadened is in contrast to suggestions that the corresponding mode of CO on Ni(100) is lifetime broadened by a two-phonon decay process. We also report the observation of the atom-substrate mode of O on Pt(111).
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