1. National Research Council, Steacie Institute for Molecular Sciences, Building 459, Stn. 18, Chalk River, Ontario, K0J 1J0, Canada, Raman Research Institute, Bangalore 560 080, India, and Materials Science and Engineering, Michigan Technological University, 504 M and ME Building, 1400 Townsend Drive, Houghton, Michigan 49931-1295