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Strong angular-momentum mixing in ultracold atom-ion excitation-exchange

Ruti Ben-shlomi, Romain Véxiau, Meir, Ziv, Sikorsky, Thomas, Nitzan Akerman, Meirav Pinkas, Olivier Dulieu, Ozeri, Roee

Published Dec 14, 2019• 0 citations
OpenAlex

Abstract

Atom-ion interactions occur through the electric dipole which is induced by the ion on the neutral atom. In a Langevin collision, in which the atom and ion overcome the centrifugal barrier and reach a short internuclear distance, their internal electronic states deform due to their interaction and can eventually alter. Here we explore the outcome products and the energy released from a single Langevin collision between a single cold $^{88}$Sr$^{+}$ ion initialized in the metastable $4d^2D_{5/2,3/2}$ states, and a cold $^{87}$Rb atom in the $5s^2S_{1/2}$ ground state. We found that the long-lived $D_{5/2}$ and $D_{3/2}$ states quench after roughly three Langevin collisions, transforming the excitation energy into kinetic energy. We identify two types of collisional quenching. One is an Electronic Excitation-Exchange process, during which the ion relaxes to the $S$ state and the atom is excited to the $P$ state, followed by energy release of $\sim$ 3000 K$\cdot$k$_B$. The second is Spin-Orbit Change where the ion relaxes from the higher fine-structure $D_{5/2}$ level to the lower $D_{3/2}$ level releasing $\sim$ 400 K$\cdot$k$_B$ into kinetic motion. These processes are theoretically understood to occur through Landau-Zener avoided crossings between the different molecular potential curves. We also found that these relaxation rates are insensitive to the mutual spin orientation of the ion and atoms. This is explained by the strong inertial Coriolis coupling present in ultracold atom-ion collisions due to the high partial wave involved, which strongly mixes different angular momentum states. This inertial coupling explains the loss of the total electronic angular-momentum which is transferred to the external rotation of nuclei. Our results provide deeper understanding of ultracold atom-ion inelastic collisions and offer additional quantum control tools for the cold chemistry field.

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Atom-ion interactions occur through the electric dipole which is induced by the ion on the neutral atom. In a Langevin collision, in which the atom and ion overcome the centrifugal barrier and reach a short internuclear distance, their internal electronic states deform due to their interaction and can eventually alter. Here we explore the outcome products and the energy released from a single Langevin collision between a single cold 88^{88}88Sr+^{+}+ ion initialized in the metastable 4d2D5/2,3/24d^2D_{5/2,3/2}4d2D5/2,3/2​ states, and a cold 87^{87}87Rb atom in the 5s2S1/25s^2S_{1/2}5s2S1/2​ ground state. We found that the long-lived D5/2D_{5/2}D5/2​ and D3/2D_{3/2}D3/2​ states quench after roughly three Langevin collisions, transforming the excitation energy into kinetic energy. We identify two types of collisional quenching. One is an Electronic Excitation-Exchange process, during which the ion relaxes to the SSS state and the atom is excited to the PPP state, followed by energy release of ∼\sim∼ 3000 K⋅\cdot⋅kB_BB​. The second is Spin-Orbit Change where the ion relaxes from the higher fine-structure D5/2D_{5/2}D5/2​ level to the lower D3/2D_{3/2}D3/2​ level releasing ∼\sim∼ 400 K⋅\cdot⋅kB_BB​ into kinetic motion. These processes are theoretically understood to occur through Landau-Zener avoided crossings between the different molecular potential curves. We also found that these relaxation rates are insensitive to the mutual spin orientation of the ion and atoms. This is explained by the strong inertial Coriolis coupling present in ultracold atom-ion collisions due to the high partial wave involved, which strongly mixes different angular momentum states. This inertial coupling explains the loss of the total electronic angular-momentum which is transferred to the external rotation of nuclei. Our results provide deeper understanding of ultracold atom-ion inelastic collisions and offer additional quantum control tools for the cold chemistry field.
Ultracold atom-ion collisions are an emerging field of research that can ultimately lead to their precise quantum control. In collisions in which the ion is prepared in an excited state, previous studies showed that the dominant reaction pathway was charge exchange. Here, we explored the outcome products and the energy released from a single ultracold collision between a single 88Sr+^{88}\mathrm{Sr}^{+}88Sr+ ion and a single 87Rb^{87}\mathrm{Rb}87Rb atom prepared in excited metastable and ground electronic states, respectively, with control over their relative spins. We found that the ion's long-lived D5/2{D}_{5/2}D5/2​ and D3/2{D}_{3/2}D3/2​ states quench after roughly three collisions, acquiring immense kinetic energy in the process. By performing single-shot thermometry on the ion after the collision, we identified two dominant reaction pathways: electronic excitation exchange and spin-orbit change. In contrast to previous experiments, we observed no charge-exchange events. These processes are theoretically understood to occur through Landau-Zener avoided crossings leading to the observed reaction pathways. We also found that spin orientation has almost no effect on the reaction pathways, due to strong Coriolis-spin mixing. Our results provide a deeper understanding of ultracold atom-ion inelastic collisions and offer additional quantum control tools for the cold chemistry field.
The quantum dynamics of ultracold collisions between rubidium atoms and excited metastable strontium ions is treated in the laboratory frame, enlightening the importance of the coupling between internal angular momenta of the particles and their mutual rotation. The study reveals a subtle competition between electronic excitation exchange and fine structure quenching, with no charge exchange, which is found to be very sensitive to the details of ion-atom interactions. The rate constant for electronic excitation exchange is found in agreement with the experimental results of Ben-Shlomi \textit{et al.} (Phys. Rev. A \textbf{102}, 031301(R) (2020)), while the rate for fine structure quenching is predicted to strongly depend on the initial polarization of the reactants.
Ultracold hybrid ion–atom traps offer the possibility of microscopic manipulation of quantum coherences in the gas using the ion as a probe. However, inelastic processes, particularly charge transfer can be a significant process of ion loss and has been measured experimentally for the ion immersed in a Rb vapour. We use first-principles quantum chemistry codes to obtain the potential energy curves and dipole moments for the lowest-lying energy states of this complex. Calculations for the radiative decay processes cross sections and rate coefficients are presented for the total decay processes; and . Comparing the semi-classical Langevin approximation with the quantum approach, we find it provides a very good estimate of the background at higher energies. The results demonstrate that radiative decay mechanisms are important over the energy and temperature region considered. In fact, the Langevin process of ion–atom collisions dominates cold ion–atom collisions. For spin-dependent processes [ 1 ] the anisotropic magnetic dipole–dipole interaction and the second-order spin–orbit coupling can play important roles, inducing coupling between the spin and the orbital motion. They measured the spin-relaxing collision rate to be approximately five orders of magnitude higher than the charge-exchange collision rate [ 1 ]. Regarding the measured radiative charge transfer collision rate, we find that our calculation is in very good agreement with experiment and with previous calculations. Nonetheless, we find no broad resonances features that might underly a strong isotope effect. In conclusion, we find, in agreement with previous theory that the isotope anomaly observed in experiment remains an open question.
Abstract Hybrid atom-ion systems are a rich and powerful platform for studying chemical reactions, as they feature both excellent control over the electronic state preparation and readout as well as a versatile tunability over the scattering energy, ranging from the few-partial wave regime to the quantum regime. In this work, we make use of these excellent control knobs, and present a joint experimental and theoretical study of the collisions of a single 138Ba+ ion prepared in the 5 d 2 D 3 / 2 , 5 / 2 metastable states with a ground state 6Li gas near quantum degeneracy. We show that in contrast to previously reported atom-ion mixtures, several non-radiative processes, including charge exchange, excitation exchange and quenching, compete with each other due to the inherent complexity of the ion-atom molecular structure. We present a full quantum model based on high-level electronic structure calculations involving spin-orbit couplings. Results are in excellent agreement with observations, highlighting the strong coupling between the internal angular momenta and the mechanical rotation of the colliding pair, which is relevant in any other hybrid system composed of an alkali-metal atom and an alkaline-earth ion.
We present a joint experimental and theoretical study of spin dynamics of a single ^{88}Sr^{+} ion colliding with an ultracold cloud of Rb atoms in various hyperfine states. While spin exchange between the two species occurs after 9.1(6) Langevin collisions on average, spin relaxation of the Sr^{+} ion Zeeman qubit occurs after 48(7) Langevin collisions, which is significantly slower than in previously studied systems due to a small second-order spin-orbit coupling. Furthermore, a reduction of the endothermic spin-exchange rate is observed as the magnetic field is increased. Interestingly, we find that while the phases acquired when colliding on the spin singlet and triplet potentials vary largely between different partial waves, the singlet-triplet phase difference, which determines the spin-exchange cross section, remains locked to a single value over a wide range of partial waves, which leads to quantum interference effects.
Charge-transfer cold Yb+{}^{+}++++ Rb collision dynamics is investigated theoretically using high-level ab initio potential energy curves, dipole moment functions, and nonadiabatic coupling matrix elements. Within the scalar-relativistic approximation, the radiative transitions from the entrance $A{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Sigma}}^{+}$ to the ground $X{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Sigma}}^{+}$ state are found to be the only efficient charge-transfer pathway. The spin-orbit coupling does not open other efficient pathways, but alters the potential energy curves and the transition dipole moment for the $A\ensuremath{-}X$ pair of states. The radiative, as well as the nonradiative, charge-transfer cross sections calculated within the ${10}^{\ensuremath{-}3}--10$ cm${}^{\ensuremath{-}1}$ collision energy range exhibit all features of the Langevin ion-atom collision regime, including a rich structure associated with centrifugal barrier tunneling (orbiting) resonances. Theoretical rate coefficients for two Yb isotopes agree well with those measured by immersing Yb+{}^{+}+ ions in an ultracold Rb ensemble in a hybrid trap. Possible origins of discrepancy in the product distributions and relations to previously studied similar processes are discussed.