When: Thursday May 3, 2007: 4 pm

Where: SSRL/SLAC Building 137, LOS 3rd Floor Conference Room, #322

Electron dynamics at ice-metal interfaces

Uwe Bovensiepen
Freie Universität Berlin

Elementary processes in condensed matter like e.g. electron transfer, electron-electron and electron-phonon scattering proceed typically in the femtosecond timescale. The analysis of transport studies or of spectral lineshapes presents a well established way to investigate these processes in thermal equilibrium. Based on rapid advances in laser technology, time-resolved experiments with femtosecond time-resolution are by now routine and first experiments using attosecond XUV laser pulses have been demonstrated. In such pump-probe experiments an initial pump laser pulse generates a non-equilibrium state whose relaxation is studied by a time-delayed laser pulse which probes the excited state directly in the time domain. Detailed insight into the nature of elementary processes and of complex phenomena, which consist out of subsequent individual steps, has been gained in biology, chemistry, and physics.

In this talk recent experiments using time- and angle-resolved two-photon photoelectron spectroscopy at ice-metal interfaces are presented. In amorphous ice layers these electrons localize directly after injection into the ice conduction band at favorable sites and are solvated by rearrangement of surrounding water dipoles. This leads to an increase in binding energy of these solvated electrons with time. [1,2] This stabilization process competes with electron back transfer to the substrate, where the electron decays by electron hole pair excitations to the Fermi level. Using different substrates Cu(111) and Ru(001) we have identified a substrate-dominated and a barrier-determined regime of electron transfer. [3] In combination with low temperature scanning tunneling microscopy the correlation of microscopic structure and dynamics has been correlated on amorphous ice-cluster on Cu(111). [4] Finally, the long-living nature of deep-trapped electrons in preexisting defects in crystalline ice will be discussed. [5] We observed that such electrons are stable over minutes even 2 eV above the Fermi level at few nanometer distance from the metal substrate. First principle calculations show that these excess electrons are residing at the ice surface in agreement with experimental observations and are extremely well screened from the metal. Stabilization proceeds also over minutes as suggested by a thermally activated relaxation through conformational substates. In addition, microscopic insight has been obtained from first principles for the initial part of the molecular rearrangement.

1. C. Gahl, U. Bovensiepen, C. Frischkorn, M. Wolf, Phys. Rev. Lett. 89, 107402 (2002).
2. U. Bovensiepen, Progress in Surface Science 78, 87 (2005).
3. J. Stähler, C.Gahl, U. Bovensiepen, M. Wolf, J. Phys. Chem. B 110, 9637 (2006).
4. J. Stähler, M. Mehlhorn, U. Bovensiepen, M. Meyer, D. O. Kusmierek, K. Morgenstern, M. Wolf, Phys. Rev. Lett., in press (2007).
5. U. Bovensiepen, C. Gahl, J. Stähler, M. Bockstedte, F. Baletto, S. Scandolo, X.-Y. Zhu, A. Rubio, M. Wolf, submitted (2007).