Phonon-Mediated Excited State Decay in High Energy Unoccupied Bands of the Topological Insulator Bi_1.5Sb_0.5Te_1.7 Se_1.3

Excited state electron populations in topological insulator materials provide a fertile proving ground for understanding non-equilibrium dynamics in solids. These populations can be prepared far from thermal equilibrium in the solid and decay by interactions with thermalized populations of electrons, phonons, etc. Using time and angle resolved photoemission spectroscopy (trARPES), we monitor the excited state decay in the topological insulator Bi_1.5Sb_0.5Te_1.7Se_1.3 at high energies much greater than those of the dominant phonon modes. We consider the decay rates throughout both the bulk conduction band and the topological surface state and find that they increase linearly with energy. We explain the observed rate as a function of energy as reflecting the shape of the electronic density of states by considering an electron-phonon scattering model that was previously applied only to low energy rate behavior. This shows that emission of low energy phonons can be a dominant mechanism of decay of highly excited electron populations.