1. Department of Physics, Pondicherry University, Puducherry, India 605 014
2. Atomic & Molecular Physics Division, Bhabha Atomic Research Centre, Mumbai, India 400 085
3. College of Science, Institute of Materials Physics and Chemistry, Nanjing Forestry University, Nanjing 210037, P. R. China
4. Theoretical Physics and Applied Mathematics Department, Ural Federal University, Mira Street 19, 620002 Ekaterinburg, Russia