1. Combustion Research Facility, Mail Stop 9055, Sandia National Laboratories, Livermore, California 94551
2. FOM Instituut voor Atoom en Molecuul Fysica (AMOLF), Kruislaan 407, 1098 SJ Amsterdam, The Netherlands
3. Laser Centre and Department of Physical Chemistry, Faculty of Exact Sciences, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands