Effect of substrate photoexcitation on the dynamics of the Si-H stretch for Si(111)/H

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The effect of carriers on the dynamics of the Si-H stretch is investigated. The carriers are generated with 8 ps pulses at 532 nm and the sum-frequency spectrum of the Si-H stretch is then monitored. Modelization of the evolution of the carrier density and subsurface temperature provides a quantitative agreement with the variation of intensities and frequencies of the spectrum. For carrier densities above 0.9×10 20 cm −3, a slight reduction of the lifetime of the Si-H stretch is observed. This is analyzed with a classical image-dipole coupling resulting in an effective distance of 9 Å to the image plane. It also appears that the thermalization occurs faster than 10 ps. This is analyzed in terms of a short lifetime (<10 ps) for the 200 cm −1 surface optical phonon.