1. a Department of Chemistry , University of Cambridge , Lensfield Road, Cambridge , CB2 1EW , England
2. c A. F. Ioffe Physico-Technical Institute , Leningrad , 194021 , U.S.S.R
3. b A. F. Ioffe Physico-Technical Institute , Leningrad , 194021 , U.S.S.R