Vibrational energy flow at stepped H/Si(111): phonons, dipoles and screening

Abstract

Direct measurements of interadsorbate vibrational energy flow between Si—H stretching modes on hydrogen-terminated, stepped vicinal H/Si(111) surface have been made. A two-colour picosecond infrared method has been used in which one vibrational mode is pumped by a resonant infrared pulse and other vibrational modes are probed by vibrationally resonant sum frequency generation to observe energy transfer. The surfaces are prepared by chemical etching in HF solutions and have monohydride-terminated (111)-(1 × 1) terraces with average terrace widths of ca. 5 atoms, and dihydride-terminated steps. The results confirm that interadsorbate energy transfer competes efficiently with slow multiphonon energy relaxation to the substrate. The energy flow is analysed to give a kinetic model of the energy equilibration pathways. The model confirms that the fast relaxing dihydride-terminated steps (60–120 ps lifetime) drain a large fraction (ca. 2/3) of the terrace Si—H mode energy (the terrace mode intrinsic lifetime is fit to be ca. 1.4 ns). The model is consistent with terrace–step energy transfer by dipole–dipole coupling between Si—H oscillators. The dipole coupling also causes normal-mode delocalization, and unusual spectroscopic changes in pump–probe experiments.