1. Departments of Physics and Chemistry, and School of Pharmacy, University of Reading, Whiteknights Park, Reading RG6 6AF, U.K., Medical Physics, Royal Berkshire Hospital, Reading RG1 5AN, U.K., and Department of Chemical and Processing Engineering, University of Strathclyde, James Weir Building, 75 Montrose Street, Glasgow G1 1XJ, U.K.