Vibrational relaxation of adsorbates at semiconductor surfaces: H on Ge(100)

Infrared pump-sum frequency generation (IR pump-SFG) probe spectroscopy is used to study the vibrational energy relaxation of hydrogen adsorbed on a Ge(100) surface. We found that the symmetric Ge-H stretch mode at 1992 cm-1 (υs = 1) for the Ge(100)-(2 x 1):H surface has a population relaxation time (T1) of 0.70 ± 0.07 ns at a substrate temperature of 323 K which is of the same order as Si(100)-(2 x 1):H. The population of the stretch mode decays exponentially in the temperature range tested from 273 to 400 K. The observed decay constants decrease from a value of the order of one nanosecond to a value of the order of 100 picoseconds, when the substrate temperature is increased from 273 to 400 K. This strong temperature dependence is consistent with a decay via a multiphonon process, which may be facilitated by the rapid increase of the density of states associated with the low-frequency TA mode. From the evaluation of the temperature dependence we suggest that the Ge-H stretch mode decays by simultaneously exciting three bending mode quanta at 530 cm-1 and more than two bulk Ge phonons