Berlin 2008 – wissenschaftliches Programm
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O: Fachverband Oberflächenphysik
O 52: Time-Resolved Spectroscopy III
O 52.7: Vortrag
Mittwoch, 27. Februar 2008, 16:45–17:00, MA 042
Strong temperature dependence of vibrational relaxation of H/Ge(100) — Xu Han, Kristian Lass, and •Eckart Hasselbrink — Department of Chemistry, University of Duisburg-Essen, D-45117 Essen, Germany
The population relaxation of vibrational excitations of adsorbates is the result of various dynamical couplings. However, the underlying microscopic mechanisms at semiconductor surfaces are not yet fully understood. Germanium provides special research interests because of its lower surface Debye temperature than silicon. On Ge(100) surface the Debye temperature is more than two times smaller than in bulk, which implies a strong anharmonicity on the surface.
IR pump-SFG probe measurements of the vibrational population relaxation on H (2 × 1)/Ge(100) surfaces revealed a strong temperature dependence of the vibrational lifetime. T1 was measured to be 0.5 ns for the Ge-H symmetric stretch at RT. The relaxation of vibrational energy follows a single exponential decay. The order of measured decay constants decreases from one nanosecond to 100 picoseconds, with increasing the substrate temperature from 273 K to 400 K. The model fitting of the temperature dependence suggests that relaxation of the Ge-H stretch mode initiates simultaneous excitation of three Ge-H bending quanta and four bulk phonons above RT. Our DFT calculations suggest that the vibration-phonon coupling, via anharmonicity, dominates the vibrational decay processes. The strong temperature dependence is facilitated by the quick increase of the density of states associated with low-frequency phonons.