  Femtosecond laser excitation of solid-state systems creates non-equilibrium
hot electrons that cool down by transferring their energy to other degrees of
freedom and ultimately to lattice vibrations of the solid. By combining ab
initio calculations with ultrafast diffuse electron scattering we gain a
detailed understanding of the complex non-equilibrium energy transfer between
electrons and phonons in laser-excited Ni metal. Our experimental results show
that the wavevector resolved population dynamics of phonon modes is distinctly
different throughout the Brillouin zone and are in remarkable agreement with
our theoretical results. We find that zone-boundary phonon modes become
occupied first. As soon as the energy in these modes becomes larger than the
average electron energy a backflow of energy from lattice to electronic degrees
of freedom occurs. Subsequent excitation of lower-energy phonon modes drives
the thermalization of the whole system on the picosecond timescale. We
determine the evolving non-equilibrium phonon occupations which we find to
deviate markedly from thermal occupations.
