THE METAL-MOLECULE STRETCH VIBRATIONAL-MODE STUDIED BY INFRARED-SPECTROSCOPY - CO ON PT(111)

The metal-molecule stretch vibrational mode of CO chemisorbed on a Pt(111) surface has been studied by infrared reflection spectroscopy. The shape of the infrared absorption peak exhibits very strong variation both with temperature and adsorbate coverage. It is shown that these variational are caused by order-disorder transitions in the overlayer, as well as vibrational phase relaxation. For the best ordered structure the width of the infrared absorption peak is dominated by the instrumental resolution. This gives an upper limit to the homogeneous broadening of 1 cm-1. It is argued that the vibrational damping is caused by excitations of electron-hole pairs in the metal. The dynamical dipole moment, which is proportional to the oscillating charge in the Pt-CO bond, decreases with increasing coverage, which correlates with a corresponding decrease in binding energy.