Ultrafast dynamics of hot electrons and holes in copper:  Excitation, energy relaxation, and transport effects

Time-resolved two-photon photoemission (2PPE) at various photon energies is used to investigate the relaxation dynamics of hot electrons in Cu (111), applying auto- and cross-correlation techniques. The relaxation times vary from 250 fs at 0. 1 eV above the Fermi level to 20 fs at 2 eV and show a strong wavelength dependence in the vicinity of the d-band feature in the 2PPE spectra. Electrons not directly excited from the d band exhibit a much longer relaxation time than d-band electrons excited to the same intermediate-state energy. We attribute these apparently longer lifetimes to a delayed electron generation via Auger decay of d-band holes. Based on a band structure calculation, a simulation of the ballistic transport effect and its implication on the observed electron relaxation dynamics is presented for the three low-index copper surfaces. These observations suggest that d-band holes have a substantially longer lifetime than excited sp-band electrons of the corresponding excitation energy.