Competing thermalization pathways of photoexcited hot electrons

Photoexcited hot carriers in solids can drive processes, such as photocatalytic reactions on the surface, beyond those available in thermal equilibrium. Hot-electron-mediated reaction pathways are limited by the thermalization of the nonequilibrium electron distribution through microscopic scattering events. Commonly, thermalization is exclusively attributed to electron-electron scattering, whereas electron-phonon scattering is considered relevant mainly for the energy equilibration with the lattice. With a kinetic model based on full Boltzmann collision integrals, we demonstrate that each scattering mechanism alone can thermalize the electron distribution, albeit along different trajectories in phase space. We find an opposite dependence on the excitation strength of the respective thermalization times and show that both processes can become comparable for weak excitations, corresponding to a sample temperature increase of a few Kelvin. Our results unravel the contributions of electron-electron and electron-phonon scattering to the thermalization across the full range of experimental excitation strengths up to the melting regime, thus facilitating the prediction of thermalization times for hot-carrier-based applications.