Two-phonon bound state for the hydrogen vibration on the H/Si(111) surface

The ${\mathit{v}}_{1\to 2}$ hot band of the Si-H stretching vibration of the H/Si(111) 1×1 surface is investigated using picosecond sum-frequency generation. The hot band is probed after directly exciting the fundamental ${\mathit{v}}_{0\to 1}$ and found to be 90 ${\mathrm{cm}}^{\mathrm{-}1}$ lower than the fundamental. The shift is consistent with the Si-H anharmonicity in the gap phase and a small dispersion (11 ${\mathrm{cm}}^{\mathrm{-}1}$) for the surface layer, implying a strong two-phonon bound state.