Time-resolved two-photon-photoelectron (2PPE) spectroscopy is used to study the
dynamics of non-equilibrium electron and hole distributions at bare and D2O
covered Ru(001) following optical excitation (55 fs, 800 nm pulses) with
variable fluence (0.04 - 0.6 mJcm-2). Within the first 0.5 ps we observe an
ultrafast transient of the excited carrier population and energy density at the
surface which is accompanied by pronounced deviations of the electron energy
distribution from a (thermalized) Fermi-Dirac distribution. Comparison of the
transient energy density of the photoexcited electrons at the surface with
predictions of the two-temperature model provides fair agreement up to 400 fs,
but exhibits a systematically lower energy density at later times, where
electrons and phonons are equilibrated. We propose that this reduced energy
density at the surface originates from ultrafast energy transport of non-thermal
electrons into the bulk in competition to electron-phonon coupling at the
surface. This is corroborated by extending the two-temperature model to account
for non-thermal, photoexcited electrons, whereby quantitative agreement with
experiment can only be achieved if ballistic transport and reduced
electron-phonon coupling is incorporated for non-thermal electrons. Implications
for surface femtochemistry are discussed.
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