1. Contribution from the Department of Chemistry, Stony Brook University, Stony Brook, New York 11794-3400, Central Laser Facility, Science and Technology Facilities Council, Harwell Science and Innovation Campus, Didcot, Oxon, OX11 0QX, U.K., and School of Chemical Sciences and Pharmacy, University of East Anglia, Norwich NR4 7TJ, U.K.