1. Department of Chemical Engineering, Mälardalen University, Box 325, S-631 05, Eskilstuna, Sweden, Division of Ion Physics, Ångström Laboratory, Uppsala University, Box 534, S-751 21, Uppsala, Sweden, Laboratory of Atomic and Surface Physics, Engineering Physics, University of Virginia, Charlottesville, Virginia 22901, and Department of Material Science and Engineering, Thornton Hall, University of Virginia, Charlottesville, Virginia 22901