1. Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, Department of Physics, Sammilani Mahavidyalaya, Baghajatin Station, E.M. Bypass, Kolkata 700 075, and Department of Physics, Raja Rammohan Ray Mahavidyalaya, P.O. Nagulpara, Hooghly 712 406, India