Role of Electronic Excitations in Ground-State-Forbidden Inelastic Collisions Between Ultracold Atoms and Ions

Abstract

The role of electronic excitation in charge exchange chemical reactions between ultracold Ca atoms and Ba + ions, confined in a hybrid trap, is studied. This prototypical system is energetically precluded from reacting in its ground state, allowing a particularly simple interpretation of the influence of electronic excitation. It is found that while electronic excitation of the ion can critically influence the chemical reaction rate, electronic excitation of the neutral atom is less important. It is also experimentally demonstrated that with the correct choice of the atom-ion pair, it is possible to mitigate the unwanted effects of these chemical reactions in ultracold atom-ion environments, marking an important step towards the next generation of hybrid devices. PACS numbers:Since the inception of laser-cooling, the primary focus of atomic physics has been the development of techniques for the production and study of ultracold matter -an endeavor that, at its core, is centered on gaining full control over matter at the quantum level. This work has been extremely successful, enabling many long-soughtafter goals, such as quantum degenerate gases [1, 2], quantum simulation [3], quantum information [4], and precision measurement of fundamental physics [5,6]. To gain this control, however, most ultracold matter production techniques rely on the scattering of a large number of photons from the system under study, potentially leading to a large degree of electronic excitation. Thus, the atom or molecule being cooled can be in a somewhat peculiar state: its external motion may be characterized by a temperature close to absolute zero, but its internal electronic degree of freedom may be described by a temperature approaching infinity. While this non-thermal distribution of electronic states has some notable important consequences for ultracold atoms [7], most of its effects, such as photochemical reactions [8][9][10], are largely ignored as they occur at rates that have relatively little effect on the system. However, as the field now moves towards producing more complex systems at ultracold temperatures, e.g. molecules [11] and hybrid systems [12,13], the effect of these light-assisted processes must be reevaluated as their rates may be much larger due to, among other things, an increased density of accessible product states and longer range interactions. Thus, there is presently a need to better understand the role of electronic excitation in chemical reactions at ultracold temperatures to enable the next generation of atomic physics experiments.Interestingly, the rapidly emerging field of hybrid atom-ion systems offers a unique opportunity to study ultracold chemical reactions [14][15][16][17]. Like the all-neutral systems of traditional atomic physics, the two trapped species can collide at short-enough range for chemical reactions to proceed; but, unlike all-neutral systems it is possible to maintain a product of the chemical reaction in the trap, since ion trap depths are large relative to the kinetic energy gaine...

Decoherence of a Single-Ion Qubit Immersed in a Spin-Polarized Atomic Bath

“…In our experiment we have γ L /n a = 2.1 × 10 −15 m 3 /s and we use typical densities of n a = 10 18 m −3 . Even though the Langevin rate is significantly smaller than the total collision rate γ c (see Supplementary Material), in previous experiments the Langevin process has dominated cold inelastic ion-atom collisions [2, 3,13,[16][17][18]. For spindependent processes additionally the anisotropic mag-…”

Laser-Cooling-Assisted Mass Spectrometry

“…It allows for large optical access required for these experiments, while providing isotopic identification of every trapped atomic/molecular ion. The current implementation of LA-MS will be used in ongoing studies of cold reactions of ionic atoms/molecules with neutral atoms [58][59][60], where it enables the distinction between different reaction products, and efforts towards cooling molecular ions in a hybrid atom-ion trap [61].…”

Sympathetic cooling of molecular ions with ultracold atoms

“…While some of the systems do lie in the unstable region, those experiments are able to operate by using some combination of a low buffer gas density, laser cooling of trapped ions, and a multipole trap [35]. [72]; Rb-Yb + , Bonn [73] and Ulm [74]; Rb-Ba + , Ulm [74] and Basel [75]; Ca-Yb + , UCLA [76] and Ca-BaCl + , UCLA [41]; Rb-Ca + , Basel [44]; Ca-Ba + , UCLA [45]; Na-Na + UCONN [77]; Rb-OH − , Innsbruck [78] ; Rb-N + 2 , Basel [18]; Rb-Rb + Bangalore [79] and Rb-Sr + , Weizmann [34] A further peculiarity of sympathetic cooling in an ion trap is manifested in the steadystate energy distribution of the ion, which features a power-law tail, of the form P(E) ∝ E −(ν+1) , due to the random amplification of the ion energy by collisions; it is for this reason that temperature is an ill-defined quantity for the ion and we instead refer to the steady-state (average) energy in the above. In [28] a simplified model is considered to gain a qualitative understanding of how this distribution arises.…”

“…Nonetheless, by controlling the ionic excitation to be as small as possible one can reduce the observed reaction rates by ∼ 10 × for species where ground state reactions are precluded, e.g. Ca + Ba + [45].…”