1. Department of Chemistry and Regional Sophisticated Instrumentation Centre, Indian Institute of Technology Madras, Chennai 600 036, India, Optics Group, Raman Research Institute, Bangalore 560 080, India, Physical and Materials Chemistry Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India, and Laboratory for Electro-Optics Systems, A6 Peenya Industrial Estate, Bangalore 560 058, India