Knowledge Bank

Vibrational relaxation processes have been investigated for CO physisorbed on NaCl (100) single crystal surfaces. Two approaches were employed for the study; the temperature dependence of the IR absorption line profile, and laser-induced photodesorption. At 4K, the full width at half-maximum (FWHM) is $0.11cm-1$ partly due to inhomogeneous broadening, placing the lower limit for the lifetime of 50 ps for $CO(v=1)$. The linewidth increases monitored between 4K and 55K. This behavior can be accounted for by a vibrational phase relaxation process, where the thermal coupling between the CO stretching motion and a low-frequency surface-CO bond motion causes dephasing of the vibrational motion. The frequency and the lifetime of the low-frequency mode was determined to be $50cm-1$ and 3 ps, respectively. At 22K, indirect measurement of the photodesorption rate showed a quantum yield of this process to be less than $10-6$. It is believed that the low desorption efficiency is a consequence of energy mismatch between the vibrational quantum and the surface bond.