Sequential vibrational relaxation of polyatomic molecules at surfaces: C2HD and C2H2 scattered from LiF(001)

Reported here is a detailed study of the vibrational relaxation of C2H2 and C2HD upon scattering from LiF(001). While direct (specular) scattering sho ws no vibrational quenching for either molecule, the residence times associ ated with trapping-desorption are long enough to give significant quenching . By studying this quenching as a function of surface temperature, thereby varying the residence times, we show that vibrational relaxation proceeds v ia multiple steps. The differences between C2H2 and C2HD quenching can be u nderstood in terms of the presence of a Fermi resonance in the former, whic h enhances the relaxation rates associated with the early steps. A simple k inetic model is used to fit the data, confirming that vibrational quenching proceeds via a cascading mechanism, which is likely to be quite general fo r polyatomic molecules. (C) 1999 American Institute of Physics. [S0021-9606 (99)70241-2].