1. Sandia National Laboratories, Albuquerque, NM 87185, USA
2. Current address: Oregon State University, Department of Chemistry, Corvallis, OR 97331, USA, Fax: +1‐515‐294‐0689, http://www.chem.orst.edu/nyman
3. US DOE Ames Laboratory and Iowa State University, The Ames Laboratory, 148 Spedding Hall, Ames, IA 50011‐3020, USA, http://www.ameslab.gov/dmse/xfang
4. Current address: Department of Geology, Kangwon National University, Chuncheon, 200‐701, Korea