Vibrational line shape of chemisorbed CO

The width and shape of the infrared absorption peak of the internal stretch vibrational mode of CO adsorbed on a Cu(100) and a Ni(111) surface have been studied. On Cu(100) there is no temperature dependence of the width in the 20- 130 K range, indicating that thermally excited low-frequency modes are not involved in the peak-broadening process. A peak-shape analysis showed that the broadening is predominantly homogeneous for the ordered $c(2\mathrm{}\times 2)$ structure and hence that the width is a true measure of the vibrational lifetime. Comparison is made to recent line-shape theories.