1. Istituto Applicazioni Interdisciplinari Fisica, CNR, 1-90123 Palermo, Italy
2. Istituto di Fisica, Universita di Palermo and IAIF-CNR. 1-90123 Palermo, Italy
3. Max-Planck-Institut für Chemie (Otto-Hahn-Institut), D-6500 Mainz, FRG
4. Permanent address: Central Research Institute for Chemistry. Hungarian Academy o f Sciences, Budapest, Hungary.