  The unique optical properties of graphene, with broadband absorption and
ultrafast response, make it a critical component of optoelectronic and
spintronic devices. Using time-resolved momentum microscopy with high data rate
and high dynamic range, we report momentum-space measurements of electrons
promoted to the graphene conduction band with visible light, and their
subsequent relaxation. We observe a pronounced non-thermal distribution of
nascent photoexcited electrons with lattice pseudospin polarization in
remarkable agreement with results of simple tight-binding theory. By varying
the excitation fluence, we vary the relative importance of electron-electron
vs. electron-phonon scattering in the relaxation of the initial distribution.
Increasing the excitation fluence results in increased noncollinear
electron-electron scattering and reduced pseudospin polarization, although
up-scattered electrons retain a degree of polarization. These detailed
momentum-resolved electron dynamics in graphene demonstrate the capabilities of
high-performance time-resolved momentum microscopy in the study of 2D materials
and can inform the design of graphene devices.
