Vibrational dynamics of hydrogen on Ge surfaces.

The vibrational dynamics of the H stretch excitation on the Ge(100)-(2x1) and Ge(111)-(1x1) surfaces has been studied using picosecond IR pump-SFG probe spectroscopy. Moreover, the temperature dependence and an isotope mixture effect are reported. The symmetric stretching mode at 1994 cm(-1) on the Ge(100)-(2x1):H surface shows a single-exponential relaxation with a decay constant of 4.8+/-0.6 ns at 100 K with a strong temperature dependence, while the Ge-H stretch at 1975 cm(-1) on the Ge(111)-(1x1):H surface relaxes four times faster with a 1.3+/-0.2 ns lifetime also exhibiting a weaker temperature dependence. The lifetime decreases with increasing temperature to 1.6 and 0.74 ns at 400 K on Ge(100) and Ge(111), respectively. We find that the decay rate increases by a factor of 3-6 depending on sample temperature when the Ge(100) surface dimers are saturated with an isotope mixture of H and D. Such an effect upon isotope mixing is not observed for the Ge(111) surface. The results suggest for the Ge(100)-(2x1):H system that a decay into three bending mode quanta requires the creation of two-optical phonons to satisfy energy conservation, whereas the decay into four bending quanta requires the annihilation of only one phonon. The three bending quanta process is hence the slower one. However, the decay into four bending quanta shows a strong temperature dependence. For an isotope mixture covered surface a larger number of combinations of low-frequency adsorbate modes exist facilitating a faster decay of the stretching excitation.