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Physical Review Letters

Vibrational Quenching of Optically Pumped Carbon Dimer Anions

Markus Nötzold, Robert Wild, Christine Lochmann, Tanja Rahim, Sruthi Purushu Melath, Katrin Dulitz, Barry P. Mant, Jan Franz, Francesco Antonio Gianturco, Roland Wester

Published Oct 30, 2023• 9 citations
Publisher

Abstract

Careful control of quantum states is a gateway to research in many areas of science such as quantum information, quantum-controlled chemistry, and astrophysical processes. Precise optical control of molecular ions remains a challenge due to the scarcity of suitable level schemes, and direct laser cooling has not yet been achieved for either positive or negative molecular ions. Using a cryogenic wire trap, we show how the internal quantum states of ${\mathrm{C}}_{2}^{\ensuremath{-}}$ anions can be manipulated using optical pumping and inelastic quenching collisions with ${\mathrm{H}}_{2}$ gas. We obtained optical pumping efficiencies of about 96% into the first vibrational level of ${\mathrm{C}}_{2}^{\ensuremath{-}}$ and determined the absolute inelastic rate coefficient from $v=1$ to 0 to be ${k}_{\mathrm{q}}=(3.2\ifmmode\pm\else\textpm\fi{}{0.2}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}{1.3}_{\mathrm{sys}})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\text{ }\text{ }{\mathrm{cm}}^{3}/\mathrm{s}$ at 20(3) K, over 3 orders of magnitude smaller than the capture limit. Reduced-dimensional quantum scattering calculations yield a small rate coefficient as well, but significantly larger than the experimental value. Using optical pumping and inelastic collisions, we also realized fluorescence imaging of negative molecular ions. Our work demonstrates high control of a cold ensemble of ${\mathrm{C}}_{2}^{\ensuremath{-}}$, providing a solid foundation for future work on laser cooling of molecular ions.

DOI
10.1103/physrevlett.131.183002

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Careful control of quantum states is a gateway to research in many areas of science such as quantum information, quantum-controlled chemistry, and astrophysical processes. Precise optical control of molecular ions remains a challenge due to the scarcity of suitable level schemes, and direct laser cooling has not yet been achieved for either positive or negative molecular ions. Using a cryogenic wire trap, we show how the internal quantum states of C2\ensuremath−{\mathrm{C}}_{2}^{\ensuremath{-}}C2\ensuremath−​ anions can be manipulated using optical pumping and inelastic quenching collisions with H2{\mathrm{H}}_{2}H2​ gas. We obtained optical pumping efficiencies of about 96% into the first vibrational level of C2\ensuremath−{\mathrm{C}}_{2}^{\ensuremath{-}}C2\ensuremath−​ and determined the absolute inelastic rate coefficient from v=1v=1v=1 to 0 to be kq=(3.2\ifmmode±\else\textpm\fi0.2stat\ifmmode±\else\textpm\fi1.3sys)\ifmmode×\else\texttimes\fi10\ensuremath−13  cm3/s{k}_{\mathrm{q}}=(3.2\ifmmode\pm\else\textpm\fi{}{0.2}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}{1.3}_{\mathrm{sys}})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\text{ }\text{ }{\mathrm{cm}}^{3}/\mathrm{s}kq​=(3.2\ifmmode±\else\textpm\fi0.2stat​\ifmmode±\else\textpm\fi1.3sys​)\ifmmode×\else\texttimes\fi10\ensuremath−13  cm3/s at 20(3) K, over 3 orders of magnitude smaller than the capture limit. Reduced-dimensional quantum scattering calculations yield a small rate coefficient as well, but significantly larger than the experimental value. Using optical pumping and inelastic collisions, we also realized fluorescence imaging of negative molecular ions. Our work demonstrates high control of a cold ensemble of C2\ensuremath−{\mathrm{C}}_{2}^{\ensuremath{-}}C2\ensuremath−​, providing a solid foundation for future work on laser cooling of molecular ions.
The molecular anion ${{\mathrm{C}}_{2}}^{\ensuremath{-}}$ is currently of interest as a candidate for laser cooling due to its electronic structure and favorable branching ratios to the ground electronic and vibrational states. Helium has been proposed as a buffer gas to cool the molecule's internal motion. We calculate the cross sections and corresponding rates for rovibrational inelastic collisions of ${\mathrm{C}}_{2}{}^{\ensuremath{-}}$ with He, and also with Ne and Ar, on three-dimensional ab initio potential energy surfaces using quantum scattering theory. The rates for vibrational quenching with He and Ne are very small and are similar to those for small neutral molecules in collision with helium. The quenching rates for Ar, however, are far larger than those with the other noble gases, suggesting that this may be a more suitable gas for driving vibrational quenching in traps. The implications of these results for laser cooling of ${{\mathrm{C}}_{2}}^{\ensuremath{-}}$ are discussed.
The study of ion-molecule reactions plays a vital role in cold chemistry, thus implying the need of well-controlled ion ensembles in a cold environment. The internal and external degrees of freedom of molecular ions, trapped in multipole radio frequency ion traps, can be cooled via collisions with neutral atoms [1]. Usually the coolant undergoes collisions with a thermal shield mounted on a cryostat attaining temperatures of about 4 K. To overcome this lower temperature limit two approaches have been proposed. Firstly, using a laser-cooled buffer-gas localized at the center of the ion cloud [2]. Secondly, sympathetic cooling via a second laser cooled molecular anion like C2−_{2}^{-}2−​ [3]. This, in addition to its potential role in forming carbon chain anions in the interstellar medium, makes C2−_{2}^{-}2−​ an important molecule in anion research. In this contribution we present a newly developed 16-pole cryogenic radio frequency wire trap for spectroscopic and kinetic studies of molecular anions. First results of quenching rates of the first excited vibrational level of C2−_{2}^{-}2−​ due to collisions with H2_{2}2​ are presented. Furthermore, possible reaction pathways with H2_{2}2​ are discussed and recent spectroscopic measurements of the second electronic excited state of C2−_{2}^{-}2−​ are presented. References: [1] R. Wester , J. Phys. B 42, 154001 (2009) [2] B. Höltkemeier et al., Phys. Rev. A 94, 062703, (2016) [3] P. Yzombard et al., Phys. Rev. Lett. 114, 213001 (2015)
This dissertation focuses on observing and controlling cold atom-ion collisions. Such collisions have numerous applications ranging from providing insight into the formation of the interstellar medium to presenting a potential platform for developing quantum information architectures. The experimental apparatus at the center of this work is a hybrid atom-ion trapping system in which laser-cooled Ca atoms held within a magneto-optical trap are spatially overlapped with ions localized within an ion trap. The ions studied in this work are both laser-cooled atomic ions as well as sympathetically cooled molecular ions. As detailed in the remainder of this thesis, by employing tools such as optical pumping, collision energy control, and mass spectrometry, reactions between these species can not only be studied with high precision but can also be controlled, leading to expanded tools for reaction engineering as well as the creation of exotic chemical species. Finally, this work also includes efforts to use inelastic collisions between room temperature BaCl^+} molecular ions and cold Ca atoms to create ro-vibrational ground state molecules, a precursor for developing a high-fidelity qubit based on rotational levels in polar molecules.
The study of cold and controlled molecular ions is pivotal for fundamental research in modern physics and chemistry. Investigations into cooling molecular anions, in particular, have proven to be of key consequence for the production of cold antihydrogen, the creation, and study of anionic Coulomb crystals as well as in atmospheric research and astrochemistry. The commonly used anion cooling technique via collisions with a buffer gas is limited by the temperature of the used cryogenic cooling medium. Here, we demonstrate forced evaporative cooling of anions via a laser beam with photon energies far above the photodetachment threshold of the anion. We reach runaway evaporative cooling of an anionic OH−^{-}− ensemble from an initial temperature of 370(12) K down to 2.2(8) K. This corresponds to three orders of magnitude increase in the ions' phase space density approaching the near-strong Coulomb coupling regime. A quantitative analysis of the experimental results, via a full thermodynamic model including the role of intrinsic collisional heating, represents the anion cooling dynamics without any fitting parameters. This technique can be used to cool, in principle, any anionic species below liquid helium temperature, providing a novel tool to push the frontiers of anion cooling to much lower than the state-of-the-art temperature regimes.
Precision vibrational spectroscopy of the molecular hydrogen ions is of significant interest for determining fundamental constants, for searching for new forces, and for testing quantum electrodynamics calculations. Future experiments can profit from the ability of preparing molecular hydrogen ions at ultralow kinetic energy and in preselected internal states, with respect to vibration, rotation, and spin degrees of freedom. For the homonuclear ions (H2+,D2+{\mathrm{H}}_{2}{}^{+}, {\mathrm{D}}_{2}{}^{+}H2​+,D2​+), direct laser cooling of the rotational degree of freedom is not feasible. We show by quantum calculations that rotational cooling by cold He buffer gas is an effective approach. For this purpose we have computed the energy-dependent cross sections for rotationally elastic and inelastic collisions, ${\mathrm{h}}_{2}{}^{+}(v=0,\phantom{\rule{0.16em}{0ex}}N)+\mathrm{He}\phantom{\rule{0.16em}{0ex}}\ensuremath{\rightarrow} {\mathrm{h}}_{2}{}^{+}(v=0,\phantom{\rule{0.16em}{0ex}}{N}^{\ensuremath{'}})+\mathrm{He}$ (where h=H,D)\mathrm{h}=\mathrm{H},\phantom{\rule{0.16em}{0ex}}\mathrm{D})h=H,D), using ab initio coupled-channel calculations. We find that rotational cooling to the lowest rotational state is possible within tens of seconds under experimentally realistic conditions. We furthermore describe possible protocols for the preparation of a single quantum state, where also the spin state is well defined.
The molecular anion C 2 − has been of interest in the past few years as a candidate for laser cooling due to its electronic structure and favorable branching ratios to the ground electronic and vibrational states. Molecular hydrogen has been used by the Wester group at Innsbruck as a buffer gas to cool the molecule's internal rovibrational motion. In the present work we generate a five-dimensional (5D) interaction potential for the system by considering the H 2 as a rigid rotor and the C 2 − as a rotating-vibrating diatomic molecule. We then calculate the cross sections and rate coefficients for rovibrational inelastic collisions of C 2 − with both para- and ortho- H 2 on this 5D potential energy surface using quantum scattering theory for the dynamics. The rates for vibrational quenching are obtained over a range of temperatures that covers the single point measured by the experiments. A comparison is also made with earlier results which had used a simpler 3D interaction potential. Para- H 2 is found to be more efficient than ortho- H 2 (with or without also undergoing rotational excitation) in cooling C 2 − . The rate coefficients for cooling the anions have been computed by appropriately weighting the ortho- and para- H 2 and compared with the available experimental result at 20 K. The computed results are nearly two orders of magnitude larger than the single experimental point. When the vibrational deexcitation rate coefficients are employed without any concurrent rotational excitations in the final C 2 − anions (a situation considered to be nonphysical under the present experimental setup), the properly averaged results are found to get smaller and to become very close to the existing experimental measurements. The robustness of the present results is tested and confirmed. The implications of these results for laser cooling of C 2 − are analyzed and discussed.