Felix H. J. Hall1, Pascal Eberle1, Gregor Hegi1, Maurice Raoult2, Mireille Aymar2, Nadia Bouloufa-Maafa2, Olivier Dulieu2 and Stefan Willitsch1
1Department of Chemistry, University of Basel, Klingelbergstrasse 80, 4056 Basel, Switzerland; 2Laboratoire Aimé Cotton du CNRS, 91405 Orsay Cedex, France
We present the results of combined experimental and theoretical investigations into cold reactive collisions between laser- or sympathetically cooled ions and Rb atoms in an ion-atom hybrid trap. We compare the collision systems of Ca+ + Rb [1,2] and Ba+ + Rb , in which both species are laser cooled, and find that for both systems the rate of reaction is considerably enhanced in electronically excited channels. These observations are rationalised with reference to computed potential energy curves, calculated using high level electronic structure methods. We find that radiative processes play an important role in both systems, verified by the observation of the radative association products CaRb+ and BaRb+, and rationalised using quantum scattering calculations.
We also compare these results on atomic ion systems with collisions of sympathetically cooled N2+ molecular ions in their vibrational ground state with ultracold Rb atoms, with average collision energies (<Ecoll>/kB) down to 20 mK . To our knowledge, this is the first time that reactive collisions with molecular ions have been studied at mK energies. We find that the reaction rate is a factor of ?4 faster than the Langevin rate from the Rb (5p) state, and is two orders of magnitude faster than in the atomic Ca+ + Rb system. The fast rate is explained by a near resonance between entrance and product channels at short range, and the strong charge-quadrupole interaction in the Rb (5p) + N2+ entrance channel at long range. A classical capture model including these effects reproduces the observed rate.
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