We present an ab-initio study of photocarrier dynamics in graphene due to electron-phonon (EP) interactions. Using the Boltzmann relaxation-time approximation with parameters determined from density functional theory (DFT) and a complementary, explicitly solvable model we show that the photocarrier thermalization time changes by orders of magnitude, when the excitation energy is reduced from 1 eV to the 100 meV range. In detail, the ultrafast thermalization at low temperatures takes place on a femtosecond timescale via optical phonon emission, but slows down to picoseconds once excitation energies become comparable with these optical phonon energy quanta. In the latter regime, thermalization times exhibit a pronounced dependence on temperature. Our DFT model includes all the inter- and intraband transitions due to EP scattering. Thanks to the high melting point of graphene we extend our studies up to 2000~K and show that such high temperatures reduce the photocarrier thermalization time through phonon absorption.
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