1. School of Physical Sciences, University of Kent, Canterbury Kent CT2 7NH, U. K.
2. Materials Measurement Science Division, NIST, Gaithersburg, Maryland 20899, United States
3. Department of Chemistry, University of Houston, 112 Fleming Building, Houston, Texas 77204, United States
4. Department of Materials Sciences and Engineering, University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield S1 3JD, U. K.
5. Physique Théorique des Matériaux, Q-MAT, CESAM, Université de Liège, Allée du 6 août 19, B-4000 Sart Tilman, Belgium