Interadsorbate vibrational energy flow on stepped vicinal H/Si(111) surfaces

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We report direct measurements of the vibrational energy flow among Si-H stretching modes on hydrogen-terminated stepped vicinal Si(111) surfaces using a two-color, infrared pump/sum frequency generation probe scheme in which one vibrational mode is pumped and another one is probed. The results, which follow the vibrational energy equilibration, reveal that interadsorbate energy transfer from the terrace to the step can be the dominant relaxation channel of the terrace oscillators. Two types of surfaces have been examined. Both have monohydride terminated terraces, but one has monohydride and the other dihydride terminated steps. On the dihydride stepped surface, the terrace Si-H vibrational energy is drained by the short lifetime step modes. The energy flow on the dihydride terminated surface occurs between terrace-localized and step-localized modes and can be resolved into a kinetic model of the vibrational energy equilibration process. Stronger interadsorbate dipole couplings, on the monohydride stepped surface, delocalize the terrace and step modes and make it difficult to separate the energy flow from direct Si-H oscillators excitation. We suggest that on this surface there is a rapid equilibration of all the Si-H stretching modes followed by their collective decay. Estimates of dipole-dipole energy transfer rates are consistent with the kinetic model results and confirm the role of dipolar interactions in vibrational energy flow on the hydrogen-terminated silicon surfaces.