1. Saha’s Spectroscopy Laboratory, Department of Physics, University of Allahabad, Allahabad, India;
2. Centre for Environmental Science, IIDS, University of Allahabad, Allahabad, India;
3. Centre for Material Science, IIDS, University of Allahabad, Allahabad, India;
4. School of Basic and Applied Sciences, G D Goenka University, Gurugram, Haryana, India