  The dynamics of photo-generated electron-hole pairs in solids are dictated by
many-body interactions such as electron-electron and electron-phonon
scattering. Hence, understanding and controlling these scattering channels is
crucial for many optoelectronic applications, ranging from light harvesting to
optical amplification. Here we measure the formation and relaxation of the
photo-generated non-thermal carrier distribution in monolayer graphene with
time- and angle-resolved photoemission spectroscopy. Using sub 10fs pulses we
identify impact ionization as the primary scattering channel, which dominates
the dynamics for the first 25fs after photo-excitation. Auger recombination is
found to set in once the carriers have accumulated at the Dirac point with time
scales between 100 and 250fs, depending on the number of non-thermal carriers.
Our observations help in gauging graphene's potential as a solar cell and
TeraHertz lasing material.
