1. Atomic and Molecular Physics Laboratory, Department of Physics, P.O. Box 1186, University of Ioannina, GR-451 10 Ioannina, Greece, Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, Scotland, U.K., and Central Laser Facility, Rutherford Appleton Laboratory, Didcot, Oxon, OX11 0QX, England, U.K.