An excited electron in condensed matter propagates with its momentum k at an energy E(k) and experiences elastic and inelastic scattering processes, which lead to relaxation. Experiments employing femtosecond time-resolved photoelectron spectroscopy exploited so far very successfully the surface sensitivity of photo¬electron spectroscopy and probed such scattering processes locally at the surface or the surface near region in the time domain [1]. Here, we report on first experimental results which analyze the non-local dynamics of excited electrons in two-photon photoemission (2PPE). In these experiments one photon excites in a Au/Fe/MgO(001) heterostructure electrons in Fe. Electron propagation through the layer stack to the Au surface is detected in 2PPE in back-pump front-probe experiments, similar to pioneering femtosecond pump-probe experiments in condensed matter, which revealed time-dependent changes in the linear optical reflectivity of free standing Au films [2]. Electrons which propagate through the layer stack in the intermediate state are detected, similar to a time-of-flight analysis. We observe pronounced differences between front- and back-pumping the hetero¬structure which are attributed to electron transport contributions through the layer stack. Given the investigated Au film thickness 5 nm ≤ dAu ≤ 30 nm, the Fermi velocity of Au of vF=1.4 nm/fs, and the 2PPE cross correlation width on the Au surface of 70 fs we are setup to distinguish ballistic and superdiffusive transport contributions. We identify elastically and inelastically scattered electrons which propagate in a superdiffusive regime [3]. Further¬more, absence of ballistic contributions at electron energies of 1-2 eV above the Fermi level for the thicker Au films are attributed to enhanced scattering at the Fe-Au interface. We investigate the relaxation of delocalized Bloch electron in the charge density wave / Mott system 1T-TaS2 with femtosecond photoelectron spectroscopy. In addition to previous work on the ultrafast doublon dynamics [4] we observe scattering of Bloch electrons with doublon excitations, which limit the relaxation timescales at quasiparticle energies up to 1.5 eV to 50 fs. This is in clear contrast to metals in which electron-electron scattering timescales of few femtosecond are found. Calculations by a quantum Boltzmann equation [5] support the experimental observation and allow to estimate the interaction energy.
This work was funded by the Deutsche Forschungsgemeinschaft through the Collaborative Research Center CRC 1242 (project number 278162697).