1. a Institute for Physical Research , National Academy of Sciences , Ashtarak-2 , 378410 , Armenia
2. b Laboratoire de Physico-Chimie des Materiaux Luminescents, UMR 5620 CNRS , Universite Lyon I , Bat A.Kastler Campus de La Doua, 10, rue Ampere, Villeurbanne Cedex , 69622 , France