Dynamics of Electron Excitations in Solids

For optical radiation in the visible wavelength regions, the photons are absorbed by the creation of carriers, i.e. through the production of electron-hole pairs. Initially these carriers are far from being in equilibrium with the temperature of the substrate or even with the electron gas itself. After an ultrashort laser pulse, for example, a finite time is required for the nascent electrons to equilibrate among themselves to establish a temporally increased Fermi-Dirac distribution. As a result, the temperature of the electronic system cannot be defined in this first phase of the relaxation process. Here we are interested in the detailed dynamics of the first steps in the relaxation of the nascent distribution to Fermi-Dirac. The lifetime of the electronic excitation will be controlled by the available decay channels, which generally include collisions with other electrons, holes, phonons, plasmons, defects, and impurities. In the case of metals without a restriction of unoccupied orbitals due to a band gap, the dominant relaxation process is electron electron scattering. Therefore, the lifetime of individual excited electronic states is always short, typically in the order of only a few femtoseconds. Our main goal is to gain a basic understanding of the dynamics of single excited electrons in solids, ultrathin films and nanoparticles, by using time-resolved two-photon photoemission.
Most recent work focuses on the dynamics of excited (hot) electrons in transition metals. Due to the high electron DOS in the vicinity of the Fermi level these metal show much shorter lifetimes than the sp-like noble metals. Still, significant divergences in the relaxation dynamics between different transition metals can be resolved. Figure A compares for instance time-resolved 2PPE data for the 4d-transition metals rhodium and molybdenum (filled and open symbols). The experimental results show a rather good agreement with theoretical results from the Echenique group in San Sebastian (solid and dashed lines) providing a deeper insight in the relevant mechanism determining the decay of bulk electron excitations (i.e. the influence of the bulk electronic structure or the scattering processes involved).

Figure A: lifetime of bulk electron excitations in Rh and Mo.

• Lifetimes of excited electrons in Ta: experimental time-resolved photoemission data and first-principle GW+T theory
  V. P. Zhukov, O. Andreyev, D. Hoffmann, M. Bauer, M. Aeschlimann, E. V. Chulkov, P. M. Echenique; Physical Review B 70, 233106 (2004). [Paper as PDF]
• Dynamics of excited electrons in metals, thin films and nanostructures
  M. Bauer, M. Aeschlimann; J. Electr. Spectr. 124, (2002) 225. [Paper as PDF]
• Transport and dynamics of optically excited electrons in metals
  M. Aeschlimann, M. Bauer, S. Pawlik, R. Knorren, G. Bouzerar, K. H. Bennemann; Appl. Phys. A 71 (2000) 1. [Paper as PDF]
• Dynamics of excited electrons in copper and ferromagnetic transition metals:Theory and experiment
  R. Knorren, K. H. Bennemann, R. Burgermeister, M. Aeschlimann; Phys. Rev. B 61 (2000) 9427.
• Elektronische Relaxation in Metallen
  M. Aeschlimann
• Dynamik in kondensierterMaterie, 31. IFF Ferienkurs 2000, Jülich, E3.1-E3.13 Femtosekundendynamik in Metallen - Das kurze Leben heißer Elektronen
  M. Wolf und M. Aeschlimann; Phys. Bl. 54 (1998) 145.
• Lifetime difference of photoexcited electrons between intraband and interband transitions
  S. Pawlik, M. Bauer, M. Aeschlimann; Surf. Sci. 377-379 (1997) 206. [Paper as PDF]
• Competing nonradiative channels for hot electron induced surface photochemistry
  M. Aeschlimann, M. Bauer, S. Pawlik; Chem. Phys., 205 (1996) 127. [Paper as PDF]
• Femtosecond Time-Resolved Measurement of Electron Relaxation at Metal Surfaces
  M. Aeschlimann, M. Bauer, S. Pawlik; Ber. Bunsenges. Phys. Chem. 99 (1995) 1504.
• Femtosecond Studies of Carrier Relaxation Processes at Single Crystal Metal Surfaces
  C. A. Schmuttenmaer, M. Aeschlimann, J. W. Herman, R. J. D. Miller, D. A. Mantell, J. Cao, , Y. Gao; “Laser Techniques for Surface Sciences”, H.L. Dai and S.J. Silbener, Eds., Proc. of the SPIE 2125, 98 (1994).
• Femtosecond Photoemission Studies of Electron Relaxation at Cu Surfaces
  C. A. Schmuttenmaer, R. J. D. Miller, M. Aeschlimann, J. Cao, Y. Gao, , D. Mantell; “Ultrafast Phenomena IX”, A.H. Zewail, G. Mourou, W. Knox, and P.F. Barbara, Eds., May 1994 .
• Time resolved Two Photon Photoemission from Cu(100): Energy Dependence of Electron Relaxation
  C. A. Schmuttenmaer, M. Aeschlimann, H. E. Elsayed-Ali, R. J. D. Miller, D. Mantell, J. Cao, , Y. Gao; Phys. Rev. B. 50, 8957 (1994). [Paper as PDF]

financial support:

• DFG priority program 1133 "Ultrafast magnetization processes"