2-PHONON BOUND-STATE FOR THE HYDROGEN VIBRATION ON THE H/SI(111) SURFACE

The upsilon-1 --> 2 hot band of the Si-H stretching vibration of the H/Si(111) 1 x 1 surface is investigated using picosecond sum-frequency generation. The hot band is probed after directly exciting the fundamental upsilon-0 --> 1 and is found to be 90 cm-1 lower than the fundamental. The shift is consistent with the Si-H anharmonicity in the gas phase and a small dispersion (11 cm-1) for the surface layer, implying a strong two-phonon bound state.