Cooling and stabilization by collisions in a mixed ion–atom system

Abstract

In mixed systems of trapped ions and cold atoms, the ions and atoms can coexist at different temperatures. This is primarily due to their different trapping and cooling mechanisms. The key questions of how ions can cool collisionally with cold atoms and whether the combined system allows stable coexistence, need to be answered. Here we experimentally demonstrate that rubidium ions cool in contact with magneto-optically trapped rubidium atoms, contrary to the general experimental expectation of ion heating. The cooling process is explained theoretically and substantiated with numerical simulations, which include resonant charge exchange collisions. The mechanism of single collision swap cooling of ions with atoms is discussed. Finally, it is experimentally and numerically demonstrated that the combined ion-atom system is intrinsically stable, which is critical for future cold chemistry experiments with such systems.

Collisional Cooling of Light Ions by Cotrapped Heavy Atoms

“…Since K and Rb are different species RCE is not possible, but nonresonant charge exchange (nRCE) is. However, careful experimental tests allow us to conclude that this channel is Rb in earlier experiments [12,24]. The essence of the present experiment is that in the ideal situation with no background gas, a spatially small MOT at the precise centre of the ion trap would always cool a trapped ion via elastic collisions, irrespective of the ion-atom mass ratio but at different rates.…”

“…This is partly because calculations show that trapped ions in a uniform buffer gas will heat up when the ratio of the atom mass ( ) to the ion mass ( ) exceeds a critical value [15][16][17][18]. In recent years, theoretical studies suggest a possible experimental resolution by using a localized ensemble of ultracold neutral atoms placed precisely at the centre of the ion trap [12,19].In ) we demonstrate cooling for, exceed the critical mass ratios (CMRs) [15][16][17][18] beyond which ion heating is predicted for uniform atomic gas densities. The experiment is consistent with our Monte Carlo (MC) simulations and other theoretical models [19] that consider collisions with centrally localized density (as opposed to uniform density) of cold atoms.…”

“…1(a) and 1(b)]. When this localized and precisely centred nature of the atomic density profile is taken into account, the trapped ions are expected to be cooled by elastic collisions with the ultracold atoms irrespective of the atom-ion mass ratio [12,17,19]. At the centre of the ion trap, the ion's micromotion is negligible while the ion's secular speed is the greatest -thus a collision with an ultracold atom, that is essentially at rest, always results in reduction in the ion's secular motion and hence in cooling of the ion [see Figs.…”

Charge-exchange collisions between ultracold fermionic lithium atoms and calcium ions

“…As an example, the elastic and inelastic collision cross-sections between ultracold atoms and ions have been successfully determined for various atom-ion combinations [2][3][4][5][6][7][8][9]. More recently, a charge exchange collision, which is one of the elementary processes occurring in chemical reactions, was observed at the single-particle level using a trapped 174 Yb + ion immersed in a BoseEinstein condensate of 87 Rb atoms [7].…”

Photodissociation of Trapped Rb2+ : Implications for Simultaneous Trapping of Atoms and Molecular Ions

“…In particular, atomic ion-atom collisions, charge exchange collisions [6-10], sympathetic cooling of ions by ultracold trapped atoms [5,[10][11][12][13], three body reactions [14][15][16] and molecular ion formation processes [3,17] have been investigated. Two complementary directions motivate key goals for future work, (a) the low partial wave ionatom collisions which explores quantum scattering and many particle physics and (b), the controlled collisions between the cold molecular ions produced in the ionatom traps with co-trapped neutral atoms [18] and with light.…”