Femtosecond dynamics of electrons in noble metals: Orientational relaxation in momentum space and the Drude relaxation rate

We investigate the momentum-resolved dynamics of conduction electrons in noble metals following ultrashort optical excitation with linearly polarized light. Using a momentum-resolved Boltzmann equation approach for electron-phonon interaction, we solve for the combined effects of orientational relaxation, thermalization, and cooling. We introduce momentum orientational relaxation as the initial step in the equilibration of an optically excited nonequilibrium electron gas by highlighting its importance for the optical response of noble metals as the dephasing of the Drude model. The numerical results for gold reveal that orientational relaxation exists already for linear optical excitations and dominates on time scales on the first tens of femtoseconds after excitation. Also incorporating thermalization and cooling on times up to a few picoseconds, our approach provides a simultaneous description of optical and thermal properties of noble metals under optically induced nonequilibrium conditions.