1. Division of Ion Physics, Ångström Laboratory, Uppsala University, Box 534, 75121 Uppsala, Sweden, Department of Physics, Escuela Politécnica Nacional, P.O. Box 1701 2759, Quito, Ecuador, Department of Chemical Engineering, Mälardalen University, Box 325, 63105 Eskilstuna, Sweden, and Department of Biochemistry, Uppsala University, Box 576, 75123 Uppsala, Sweden