Abstract

We have measured the evolution of the electron energy distribution excited in a 300-ÅAu film by a 180-fs optical pulse. The distribution is measured directly by uv photoemission with a time resolution of 270 fs and an electron energy resolution of −0.05 eV. Earlier, we reported the first time-resolved photoemission measurement of laser-heated metals with a time resolution of ~700fs. We found that the distribution function, especially around 0.3 eV above the Fermi level, deviated from Fermi-Dirac until ~800 fs after the heating pulse.¹ Here, with better time resolution we are now able to observe (a) evidence of the nascent (as photoexcited.) electron distribution, (b) the time evolution from the nascent to a Fermi Dirac distribution, and (c) that thermalization is more rapid for higher excitation levels. These results are in marked contrast to experiments in which femtosecond visible reflection spectroscopy² was used to deduce the electron-phonon constant coupling³ g with a model⁴ that assumes that the electrons are thermalized much faster than the duration of the optical pulse. These results show that electron thermalization occurs on the same time scale as electron-phonon coupling and that electron- lattice energy transfer is much mote complicated than previously thought.

© 1992 Optical Society of America