Rotationally induced Penning ionization of ultracold photoassociated helium dimers

¹ Ecole Normale Supérieure and Collège de France, Laboratoire Kastler Brossel 24 rue Lhomond, 75231 Paris Cedex 05, France
² Debye Institute, Atom Optics and Ultrafast Dynamics, Utrecht University P.O. Box 80000, 3508 TA Utrecht, The Netherlands

Received: 19 January 2005
Accepted: 17 February 2005

Abstract

We have studied photoassociation of metastable  helium atoms near the - asymptote by both ion detection in a magneto-optical trap and trap-loss measurements in a magnetic trap. A detailed comparison between the results of the two experiments gives insight into the mechanism of the Penning ionization process. We have identified four series of resonances corresponding to vibrational molecular levels belonging to different rotational states in two potentials. The corresponding spin states become quasi-purely quintet at small interatomic distance, and Penning ionization is inhibited by spin conservation rules. Only a weak rotational coupling is responsible for the contamination by singlet spin states leading to a detectable ion signal. However, for one of these series, Bose statistics does not enable the rotational coupling and the series detected through trap-loss does not give rise to sufficient ionization for detection.