Ultrafast dynamics of electron thermalization, cooling and transport effects in Ru(001)
M. Lisowski, P. A. Loukakos, U. Bovensiepen, J. Stähler, C. Gahl, M. Wolf.
App. Phys. A 78, 165176 (2004)
Timeresolved twophotonphotoelectron (2PPE) spectroscopy is used to study the dynamics of nonequilibrium electron and hole
distributions at bare and D_{2}O 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)
FermiDirac distribution. Comparison of the transient energy density of the photoexcited electrons at the surface with predictions
of the twotemperature 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 nonthermal electrons into the bulk in competition to electronphonon coupling at the surface. This is corroborated
by extending the twotemperature model to account for nonthermal, photoexcited electrons, whereby quantitative agreement with experiment
can only be achieved if ballistic transport and reduced electronphonon coupling is incorporated for nonthermal electrons. Implications
for surface femtochemistry are discussed.
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