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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