Rotationally induced Penning ionization of ultracold pho- toassociated helium dimers

– We have studied photoassociation of metastable 2S1 helium atoms near the 2 S1-2 P2asymptote 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. Recently there have been many experimental efforts to achieve Bose-Einstein condensation (BEC) for metastable rare-gas atoms [1–5]. The achievement of BEC with ultracold metastable He(2S1) atoms (He*) [1, 2] followed the prediction [6, 7] that spin conservation prohibits Penning ionization in the fully stretched molecular spin state and thus prevents the cold cloud of spin-polarized metastable atoms to ionize before it Bose condenses. By contrast, Penning ionization is less effectively suppressed in heavier rare gases, which must be trapped in a metastable P state and are therefore subject to stronger spin-orbit collisional couplings [8]. In this context, there is general interest in both experimental and theoretical studies of the dynamics that lead to Penning ionization and its suppression in trapped metastable rare gases. For a magnetically trapped sample in the He* state, the common spin orientation imposes a strict conservation rule that permits collisional Penning ionization (He*(2S1) + He*(2 S1) → () Permanent address: Institut de Physique et Chimie des Matériaux de Strasbourg, France. () Permanent address: Dept. of Science & Technology and MESA+ research institute, University of Twente, The Netherlands. () Permanent address: Calvin College, Grand Rapids, MI, USA.