  A transient femtosecond population inversion in graphene was recently
reported by Li et al., Phys. Rev. Lett. 108, 167401 (2012). Based on a
microscopic theory we clarify the underlying microscopic mechanism: Transient
gain and population inversion in graphene occurs due to a complex interplay of
strong optical pumping and carrier cooling that fills states close to the Dirac
point giving rise to a relaxation bottleneck. The subsequent femtosecond decay
of the optical gain is mainly driven by Coulomb-induced Auger recombination.
