Dephasing of vibrationally excited molecules at surfaces: CO/Ru(001)

The ?=0?1 fundamental and the ?=1?2 hot-band transitions of the C-O stretch vibration of CO chemisorbed on Ru(0 0 1) are investigated as a function of temperature using broadband-IR sum-frequency generation spectroscopy. Although the linewidths are markedly different for the two transitions, we demonstrate that the temperature-dependent contribution to the linewidths from anharmonic coupling to the low-frequency frustrated translation mode (giving rise to pure dephasing) is identical for both transitions. This implies that the anharmonic coupling between the high-frequency and the low-frequency mode is independent of the degree of vibrational excitation.