1. Institute of Scientific Instruments of the CAS, Kràlovopolskà 147, 612
64 Brno, Czech Republic
2. CNR-IPCF,
Istituto per i Processi Chimico-Fisici, Consiglio Nazionale delle Ricerche, Viale F. Stagno D’Alcontres 37, I-98158 Messina, Italy
3. Department
of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, U.K.
4. H. H.
Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, U.K.