1. Department of Physical Chemistry, School of Chemistry, University of Nottingham, Nottingham NG7 2RD, U.K., Department of Physics, University of Warwick, Coventry CV4 7AL, U.K., CLRC Daresbury Laboratory, Warrington WA4 4AD, U.K., and Department of Physical Chemistry, School of Chemistry, University of New South Wales, Sydney 2052, Australia