1. Contribution from the Laser Centre and Department of Physical Chemistry, Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands, Sackler Laboratory for Astrophysics, Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands, Combustion Research Facility, Mailstop 9055, Sandia National Laboratories, Livermore, California 94550, and Unidad de Láseres y Haces Moleculares, Instituto Pluridisciplinar, Universidad Complutense de Madrid, C/ Juan XXIII-1°, 28040-Madrid, Spain