Thermalization of photoexcited electrons in bismuth investigated by time-resolved THz spectroscopy and {\it ab initio} calculations*

The charge carrier dynamics of photoexicted bismuth generates a Drude response that evolves in time. In contrast to graphite, the plasma frequency of bismuth displays an initial increase and a subsequent decay. We have performed {\it ab initio} calculations of bulk bismuth within the density functional theory, and show that the non-monotonic behaviour of the plasma frequency is due to the presence of local minima in the conduction band: most of the photoexicted electrons first accumulate in these local minima, and reach the L point only 0.6 ps after photoexcitation.