1. Laboratoire de Chimie Physique CNRS UMR8000; Université Paris-Sud; 91405 Orsay Cedex France
2. CEA Saclay, IRAMIS, LIDyl; Physico-Chimie et Rayonnement; UMR3299, CEA-CNRS 91191 Gif-sur-Yvette Cedex France
3. Centre for Plasma Physics, School of Mathematics and Physics; Queen's University Belfast; BT7 1NN UK