Vibrational dynamics of isolated hydrogen in germanium

Absorption line shapes associated with the stretch mode of bond-center hydrogen (H_BC ⁽⁺⁾) at 1794 cm^-1 and the twofold degenerate mode of hydrogen near the tetrahedral site (H^(-)) at 745 cm^-1 in germanium are measured with infrared-absorption spectroscopy. The H_BC ⁽⁺⁾ line shape measured at 10 K for the case of low hydrogen concentrations gives a lifetime between 15 and 23 ps. Similar in magnitude to the 7.8±0.2 ps observed for the same defect in silicon [M. Budde et al, Phys. Rev. Lett. 85, 1452 (2000)], this observation indicates that the lifetime of the bond-centered mode is largely insensitive to the phonon frequency distributions of the host material. Pure dephasing dynamics of hydrogen in germanium are studied by measuring the shape of the 1794- and 745-cm^-1 lines as a function of temperature. These measurements are analyzed using a recently formulated exchange model for vibrational dephasing. The temperature dependence of the 1794-cm^-1 line shape is attributed to thermal fluctuations in the occupation number of a pseudolocalized mode at 75±2 cm^-1.