The influence of thin helium films on the phonon spectrum of optically excited silicon

We have studied the phonon spectra resulting from laser pulse excitation of a silicon surface. We used a phonon spectrometer based on pressure tuned boron levels in silicon (Si: B-spectrometer). A new design allowed us to use the spectrometer in vacuum for the first time. We observed phonon frequencies up to 1 THz. The pulse shapes indicated both, quasidiffusion and a long exciton lifetime. On adding helium films to the surface we have found dramatic changes of the spectrum. Full thermalisation was reached already at a thickness of 1.5 monolayers. This indicates a strong anharmonic interaction in the very first monolayers at these high frequencies and, in addition, a strong confinement of the phonons to the surface by quasidiffusion.