THE INFLUENCE OF THIN HELIUM FILMS ON THE PHONON-SPECTRUM OF OPTICALLY-EXCITED SILICON

We have studied the phonon spectra resulting from laser pulse excitati on of a silicon surface. We used a phonon spectrometer based on pressu re tuned boron levels in silicon (Si:B-spectrometer). A new design all owed us to use the spectrometer in vacuum for the first time. We obser ved phonon frequencies up to 1 THz. The pulse shapes indicated both, q uasidiffusion acid a long exciton lifetime. On adding helium films to the surface we have found dramatic changes of the spectrum. Full therm alisation was reached already at a thickness of 1.5 monolayers. This i ndicates a strong anharmonic interaction in the very first monolayers at these high frequencies and, in addition, a strong confinement of th e phonons to the surface by quasidiffusion.