  We examine the intrinsic energy dissipation steps in electrically biased
graphene channels. By combining in-situ measurements of the spontaneous optical
emission with a Raman spectroscopy study of the graphene sample under
conditions of current flow, we obtain independent information on the energy
distribution of the electrons and phonons. The electrons and holes contributing
to light emission are found to obey a thermal distribution, with temperatures
in excess of 1500 K in the regime of current saturation. The zone-center
optical phonons are also highly excited and are found to be in equilibrium with
the electrons. For a given optical phonon temperature, the anharmonic downshift
of the Raman G-mode is smaller than expected under equilibrium conditions,
suggesting that the electrons and high-energy optical phonons are not fully
equilibrated with all of the phonon modes.
