1. Institute of Physical Chemistry, Kossuth Lajos University, H-4010 Debrecen, P.O. Box 7, Hungary
2. Physical Chemistry Laboratory, Central Research Institute for Physics, H-1525 Budapest, P.O. Box 49, Hungary
3. Facultad de Ciencias, U.A.E.M., Av. Universidad 1001, Col. Chamilpa, C.P. 62210, Cuernavaca, Morelos, Mexico