Vibrational energy transfer among adsorbate modes: Picosecond dynamics on stepped H/Si(111)

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Abstract
Direct measurements of interadsorbate vibrational energy flow among Si–H stretching modes on hydrogen-terminated, stepped vicinal H/Si(111) surfaces are made. A two-color picosecond infrared method is 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, and average terrace widths of approximately five atoms. Two types of surfaces, differing in having either monohydride- or dihydride-terminated steps, are examined. The results on both surfaces confirm that interadsorbate energy transfer competes efficiently with energy relaxation to the substrate. On the dihydride-stepped surface, the energy flow is analyzed to give a relatively complete 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 (∼2/3) of the terrace Si–H mode energy (the terrace mode intrinsic lifetime is fit to be ∼1.4 ns). The model is consistent with terrace–step energy transfer by dipole–dipole coupling between Si–H oscillators. On the monohydride-stepped surface, the experimental results suggest even stronger terrace–step coupling, but the monohydride step lifetime is long (≳500 ps) and does not drain the terrace mode energy. The coupling of the monohydride steps to the terraces by dipole interactions is in fact calculated to be strong enough so that the step and terrace modes mix, and detailed kinetic analysis of the monohydride-stepped surface is therefore ambiguous because of strong spectral interactions of the modes.