Dephasing of localized and delocalized vibrational modes: CO adsorbed on Ru(001)

The influence of the dynamic dipole coupling on the dephasing of vibrational modes has been examined for the ordered (√3×√3)R30^∘ CO/Ru(001) adsorbate layer as a model system. The temperature dependences of the line position and the linewidth of the C-O stretch mode are found to be mainly caused by the anharmonic coupling to the hindered translational (parallel to the surface) CO mode at 47cm^-1. In agreement with theoretical predictions, the delocalized band of the majority ¹²C¹⁶O species exhibits only very little broadening with increasing temperatures, which contrasts with the much larger broadening of the localized C-O stretch mode of naturally abundant ¹³C¹⁶O. The opposite behavior is found for the line shifts, which is larger for the majority ¹²C¹⁶O derived band as compared to the ¹³C¹⁶O mode, again in excellent agreement with theory. Similar temperature dependences of the linewidth and the line shifts of a dilute CO layer, and of the isotopically diluted ¹³C¹⁶O species within the homogeneous (√3×√3)R30^∘ ¹²C¹⁶O/Ru(001) layer, indicate that the anharmonic coupling of the C-O stretch mode to low-frequency modes is largely independent of structural details, such as the long-range order of the adsorbate layer, but displays a pronounced dependence on the extent of delocalization of the respective mode under study.