Ultrafast two-step thermalization processes of photoexcited electrons at a gold surface: Application of a wavelength-selective transient reflecting grating method

We have observed two-step thermalization processes of photoexcited electrons in gold, by using a wavelength-selective transient reflecting grating method that provides information on carrier dynamics at each energy state selectively. We detected ultrafast processes around L and X points in the Brillouin zone. Within 1 ps, two relaxation components (40 +/- 10 and 280 +/- 40 fs) were observed only around L, and only the slower component was observed for every wavelength. We considered that the two relaxation processes occurred due to two kinds of electron-electron (e-e) scattering predicted from the Fermi-liquid theory, because their decay times are in good agreement with the times obtained from the theory. We concluded that faster and slower e-e scatterings physically correspond to the relaxations of the initially photoexcited electrons at the L point, and of nonthermalized electrons at various Brillouin-zone points, respectively.