Tight-binding molecular-dynamics study of the hydrogen vibration spectrum on a Si(111) surface

The vibration spectrum of hydrogen on a Si(111) surface is studied by molecular-dynamics simulation using an empirical tight-binding potential model. The frequencies of the stretching and wagging modes of hydrogen obtained from the simulations are in excellent agreement with experiment. An extra mode on the hydrogen overlayer at 62 meV, which has been observed in electron-energy-loss experiments, is also reproduced. We show that the extra mode originates from the coupling between the H wagging mode and a Si substrate phonon mode. The frequency shifts of the stretching and the wagging mode of H with temperature are also obtained from our simulations.