1. a CRPP CNRS, av. A. Schweitzer , 33600 , Pessac , FRANCE
2. b CEA-IRF-DPhG-SCM, CEN Saclay , 91191 , Gif-Sur-Yvette Cedex , FRANCE
3. c Laboratoire de Spectroscopie Moléculaire et Cristalline , Université Bordeaux I , 33405 , Talence , FRANCE
4. d On leave from Theor. Phys. Chem. Institute NHSF , 116/35 , Athens , GREECE
5. e On leave from Chelyabinsk State University , 454136 , Chelyabinsk , RUSSIA