Formation of molecular ions by radiative association of cold trapped atoms and ions

Humberto Da Silva Jr,Mireille Aymar,Olivier Dulieu,Maurice Raoult

doi:10.1088/1367-2630/17/4/045015

Abstract

Radiative emission during cold collisions between trapped laser-cooled Rb atoms and alkaline-earth ions (Ca+, Sr+, Ba+) and Yb+, and between Li and Yb+, are studied theoretically, using accurate effective core potential based quantum chemistry calculations of potential energy curves and transition dipole moments of the related molecular ions. Radiative association of molecular ions is predicted to occur for all systems with a cross section two to ten times larger than the radiative charge transfer one. Partial and total rate constants are also calculated and compared to available experiments. Narrow shape resonances are expected, which could be detectable at low temperature with an experimental resolution at the limit of the present standards. Vibrational distributions are also calculated, showing that the final molecular ions are not created in their ground state level.

Highlights

One of the novel developments of ultracold matter research is exemplified by the experiments aiming at merging a cold atom trap and a trap of laser-cooled ions
Following the theoretical prospect of Ref. [15] with cold Na trapped in a magnetooptical trap (MOT) and lasercooled Ca+ ions in a Paul trap, other groups have carried out pioneering experiments with various combinations of atoms and atomic ions: Yb atoms with Yb+ ions [17], Rb atoms with Ca+ [18, 19] and Ba+ [20] ions, and Ca atoms with Yb+ ions [21] or Ba+ ions [22]
After recalling the basics of the derivation of expressions for the cross section and the rates (Section 2), we describe the electronic structure of the relevant molecular ions (AlkAlke)+ using accurate quantum chemistry calculations based on the representation of the colliding partners with effective core potentials (ECP) including core polarization potentials (Section 3) which are compared to other determinations when available

Summary

Introduction

One of the novel developments of ultracold matter research is exemplified by the experiments aiming at merging a cold atom trap and a trap of laser-cooled ions. [18]) that the cooling and/or trapping light most often assist the reaction through the excitation of one of the colliding partners, inducing large rates described by the Langevin regime close to the unitarity limit Among all these experiments, it is striking that so far, the direct observation of molecular ions resulting from the association of a cold atom and a cold ion has been reported only in two cases, namely RbCa+ [18, 19] and RbBa+ [20]. The collision energies will usually be expressed in units of (milli-)Kelvin, Ecoll ≡ Ecoll/kb, where kb is the Boltzmann constant

Theoretical approach for RA and RCT

Cross sections and rates for RA and RCT

Discussion and prospects