Ultrafast dynamics of laser-excited electron distributions in silicon

Time-resolved photoelectron spectroscopy is used to directly observe the energy relaxation of optically excited electrons in silicon with 150 fsec resolution. We observe an electron-electron thermalization time of 120 fsec, an extremely fast initial electron cooling rate, followed by an electron-phonon thermalization time of {~}1 psec, and overall a strong energy dependence to the electron scattering rate. Here, we also report a new effect in two-photon photoemission unique to ultrashort laser pulses, resulting in a sensitive new monitor of electron dynamics.