Vibrational Linewidths of Adsorbed Molecules by Infrared Emission Spectroscopy

Two experiments are described, each of which uses an unusual spectroscopic technique to investigate the vibrational properties of adsorbed molecules on metal surfaces. Infrared emission spectroscopy is the only experimental technique capable of high resolution vibrational studies of adsorbates on well characterized single-crystal metal surfaces in the frequency range below 1000 cm('-1). The present work reports the first measurement of the linewidth of the C-Pt stretching vibration of CO on Pt(111), and the first detailed investigation of any molecule-substrate vibrational mode. The linewidth has been studied at sample temperatures between 275 and 400 K, and the variations are found to be significantly less than predicted by theories of lifetime broadening. Based on this fact, and the observed asymmetry of the line shape, it is concluded that the linewidth is predominantly inhomogeneous. The O-Pt stretching vibration of atomic oxygen on Pt(111) has also been observed. The technique of infrared direct absorption spectroscopy is particularly well suited to observations of adsorbates on thin films at low temperature. It has been used to make an extensive study of the C=O stretching vibration of CO on evaporated Ag, Au, and Cu films at 2 K. The adsorption behavior is found to depend strongly on the deposition temperature of the film. An investigation of the shift of the C=O frequency with CO exposure reveals both chemical and dynamic interactions between the molecules. No evidence is found for an enhancement of the infrared polarizability analogous to the Raman enhancement at 'active sites' proposed in some theories of Surface Enhanced Raman Scattering.