  We investigate the excited state electronic structure of the model phase
transition system In/Si(111) using femtosecond time- and angle-resolved
photoemission spectroscopy (trARPES) with an XUV laser source at 500 kHz .
Excited state band mapping is used to characterize the normally unoccupied
electronic structure above the Fermi level in both structural phases of indium
nanowires on Si(111): the metallic (4x1) and the gapped (8x2) phases. The
extracted band positions are compared with the band structure calculated using
density functional theory (DFT) within both the LDA and GW approximations.
While good overall agreement is found between the GW calculated band structure
and experiment, deviations in specific momentum regions may indicate the
importance of excitonic effects not accounted for at this level of
approximation. To probe the dynamics of these excited states, their
momentum-resolved transient population dynamics are extracted. The transient
intensities are then simulated by a spectral function determined by a state
population employing a transient elevated electronic temperature as determined
experimentally. This allows the momentum-resolved population dynamics to be
quantitatively reproduced, revealing important insights into the transfer of
energy from the electronic system to the lattice. In particular, a comparison
between the magnitude and relaxation time of the transient electronic
temperature observed by trARPES with those of the lattice as probed in previous
ultrafast electron diffraction studies imply a highly non-thermal phonon
distribution at the surface following photo-excitation. This suggests the
energy from the excited electronic system is initially transferred to high
energy optical phonon modes followed by cooling and thermalization of the
photo-excited system by much slower phonon-phonon coupling.
