1. a Department of Physics , University College of Sciences, Acharya Nagarjuna University , Nagarjuna Nagar 522 510 , India
2. b Department of Physics , S.V. Arts College , Tirupati 517 502 , India
3. c Physical Sciences, Sri Padmavathi Mahila Viswavidyalayam , Tirupati 517 502 , India
4. d Department of Chemistry , Pondicherry University , Pondicherry 605 014 , India