INTERADSORBATE VIBRATIONAL-ENERGY FLOW ON STEPPED VICINAL H SI(111)) SURFACES/

We report direct measurements of tile vibrational energy flow among Si -H stretching modes on hydrogen-terminated stepped vicinal Si (111) su rfaces using a two-color, infrared pump/sum frequency generation probe scheme in which one vibrational mode is pumped and another one is pro bed. The results, which follow the vibrational energy equilibration, r eveal that interadsorbate energy transfer from tile terrace to the ste p can be the dominant relaxation channel of the terrace oscillators. T wo types of surfaces have been examined. Both have monohydride termina ted terraces, but one has monohydride and the other dihydride terminat ed steps. On the dihydride stepped surface, the terrace Si-H vibration al energy is drained by tile short lifetime step modes. The energy flo w 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 t errace and step modes and make it difficult to separate the energy flo w from direct Si-H oscillators excitation. We suggest that on this sur face there is a rapid equilibration of all the Si-H stretching modes f ollowed by their collective decay. Estimates of dipole-dipole energy t ransfer rates are consistent with the kinetic model results and confir m the role of dipolar interactions in vibrational energy flow on the h ydrogen-terminated silicon surfaces.