Vibrational phase relaxation at surfaces: The role of lateral interaction

We have studied the temperature and coverage dependence of the vibrational line shape of the C–O stretch vibration for CO on Ru(001). We find that the narrow IR line profile which is observed for the ordered structure of CO on Ru(001) is due to the strong dipole–dipole coupling and that for low CO coverage (or in the isotopic dilution limit), where the lateral interaction is effectively zero, the linewidth of the C–O stretch mode increases in accordance with theory. The implication of this result is briefly discussed.