  Using a theory which treats on equal footing transport of excited electrons
and electron-phonon scattering, we are able to explain the temperature
dependence of the relaxation time in Cu as recently observed by Petek, Nagano,
and Ogawa. We show that the unexpected increase of the relaxation time with
temperature results from the drastic change of the electron motion due to the
efficiency of electron-phonon scattering: the transport is ballistic at low
temperature and gets diffusive at room temperature. Finally, our theory also
reproduces the experimental measurements of the two-photon photoemission (2PPE)
intensity as a function of the pump-probe delay.
