1. Institute of Chemistry, Universidade Federal do Rio Grande do Sul, UFRGS, Av. Bento Gonçalves 9500, P.O. Box 15003, 91501-970, Porto Alegre, RS, Brazil, and Institute of Physics, UFRGS, Av. Bento Gonçalves 9500, P.O. Box 15051, 91501-970, Porto Alegre, RS, Brazil