1. Laser Centre and Department of Physics and Astronomy, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands, Department of Physics, Washington State University, Webster Building, Pullman, Washington 99164-2814, and 3. Physikalisches Institut, Fakultät für Physik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany