1. Department of Physics, Indian Institute of Technology Madras, Chennai 600 036, India
2. Defence Metallurgical Research Laboratory, Hyderabad 500 058, India
3. Department of Chemistry, Moscow State University, Moscow, GSP-2, 119992, Russia
4. Tata Institute of Fundamental Research, Mumbai 400 005, India
5. Departamento de F´isica, Universidade Federal do Rio Grande do Norte, Natal 59072-970, Brazil