Direct measurement of nonequilibrium electron-energy distributions in subpicosecond laser-heated gold films

The electron-energy distribution in a Au film was measured with approx 700 fs time-resolved photoemission spectroscopy following laser heating by a 400 fs visible laser pulse. The measured distribution can be fitted by the Fermi--Dirac function at an elevated temperature except within 800 fs of the heating pulse (time-resolution limited), when a reproducible departure is observed. As a result, the relaxation of nonequilibrium electrons was found to be inadequately described by the standard electron--phonon coupling model.