Laser cooling of molecular anions

Vibrational Quenching of Optically Pumped Carbon Dimer Anions

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 C_{2}^{-} anions can be manipulated using optical pumping and inelastic quenching collisions with H_{2} gas. We obtained optical pumping efficiencies of about 96% into the first vibrational level of C_{2}^{-} and determined the absolute inelastic rate coefficient from v=1 to 0 to be k_{q}=(3.2±0.2_{stat}±1.3_{sys})×10^{-13}  cm^{3}/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 C_{2}^{-}, providing a solid foundation for future work on laser cooling of molecular ions.

Theoretical calculation of spectroscopy properties of selenium bromide cation

In this paper, the potential energy curves of 22 Λ-S states as well as 51 Ω states were calculated using the internally contracted multiconfiguration interaction and Davidson correction method. Through the obtained transition data, the spectroscopy data of the low excitation bound state are fitted and compared with the same main group ions. The phenomenon of avoided crossing that occurs in the Ω state is analyzed, and finally it is concluded that this phenomenon mainly occurs in the energy region between 20 000 cm^-1 and 40 000 cm^-1. The potential laser cooling transition cycle in the Ω state is analyzed. The Franck-Condon factor, radiative lifetime and Einstein coefficient between are calculated. In this paper, we argue that direct laser cooling of SeBr⁺ is not feasible. The content of our study provides a theoretical basis for subsequent calculations to explore the properties of SeBr⁺ spectrum.

Sympathetic cooling of a trapped proton mediated by an LC circuit

Efficient cooling of trapped charged particles is essential to many fundamental physics experiments1,2, to high-precision metrology3,4 and to quantum technology5,6. Until now, sympathetic cooling has required close-range Coulomb interactions7,8, but there has been a sustained desire to bring laser-cooling techniques to particles in macroscopically separated traps5,9,10, extending quantum control techniques to previously inaccessible particles such as highly charged ions, molecular ions and antimatter. Here we demonstrate sympathetic cooling of a single proton using laser-cooled Be+ ions in spatially separated Penning traps. The traps are connected by a superconducting LC circuit that enables energy exchange over a distance of 9 cm. We also demonstrate the cooling of a resonant mode of a macroscopic LC circuit with laser-cooled ions and sympathetic cooling of an individually trapped proton, reaching temperatures far below the environmental temperature. Notably, as this technique uses only image-current interactions, it can be easily applied to an experiment with antiprotons1, facilitating improved precision in matter-antimatter comparisons11 and dark matter searches12,13.

Theoretical investigation of laser cooling for BN- anion by ab inito calculation

A theoretical investigation for the feasibility of laser cooling BN^-anion is presented. An ab initio calculation on the three low-lying states Χ²Σ⁺, Α²Π and Β²Σ⁺ are performed at the CASSCF/MRCI + Q level. The calculated spectroscopic constants are in good agreement with the available theoretical and experimental data. Radiative properties including Franck-Condon factor, Einstein coefficients and radiative lifetimes are determined. The calculation shows that the transition B²Σ⁺(v^')↔X²Σ⁺(v^'') has highly diagonal FCFs, especially f₀₀ = 0.9898, and enough short radiative lifetimes. A cooling scheme by three laser beams is proposed, which requires one main pumping laser(λ₀₀ = 474.67 nm) and two repumping lasers (λ₀₁ = 514.64 nm, λ₁₂= 514.90 nm). The population dynamics of cooling is investigated with the rate equation approach. The simulation demonstrates that the population does not remain trapped within the intermediate Α²Π state. The resultant scattered photons are about2.5×10⁴, which is expected to stop BN^-anion molecule in a cryogenic beam theoretically.

Vibrational quenching of CN- in collisions with He and Ar

The vibrational quenching cross sections and corresponding low-temperature rate constants for the ν = 1 and ν = 2 states of CN^-(¹Σ⁺) colliding with He and Ar atoms have been computed ab initio using new three-dimensional potential energy surfaces. Little work has been carried out so far on low-energy vibrationally inelastic collisions for anions with neutral atoms. The cross sections and rates calculated at energies and temperatures relevant for both ion traps and astrochemical modeling are found by the present calculations to be even smaller than those of the similar C₂ ^-/He and C₂ ^-/Ar systems, which are in turn of the order of those existing for the collisions involving neutral diatom-atom systems. The implications of our finding in the present case mainly focus on the possible role of small computed rate constants in the dynamics of molecular cooling and the evolution of astrochemical modeling networks.

State-Selective Observation of Radiative Cooling of Vibrationally Excited C2. J Phys Chem Lett 2020;11:10526-10531. [PMID: 33289570 DOI: 10.1021/acs.jpclett.0c03196]

We have observed radiative cooling of vibrationally excited C₂^- in the X²Σ_g⁺ electronic ground state via electronic transitions to near-degenerate low-lying vibrational levels of the A²Π_u electronic excited state. Combining an ion storage technique with high-resolution detachment spectroscopy, we were able to assign rovibronic transitions to the resulting complex spectra. The time evolution of the population at specific vibrational states was measured up to 60 ms, providing the first quantitative experimental support for the long-standing theoretical predictions.

The study of laser cooling of TeH- anion in theoretical approach

The probabilities of laser cooling of TeH^- anion via a spin-forbidden transition and a three-electronic-level transition are proposed. The potential energy curves of the X¹Σ⁺, a³∏, A¹∏, and b³Σ⁺ electronic states of tellurium monohydride anion (TeH^-) are calculated using multi-reference configuration interaction method. Davidson corrections, core-valence correlations and spin-orbit coupling effects are also considered. The AWCV5Z-PP pseudopotential basis set of Te atom is used. Spectroscopic parameters of the Λ-S and Ω states are obtained by solving radial Schrodinger equation. These results are reported at the first time. Permanent dipole moments of the Ω states and transition dipole moments of the a₂1↔X0⁺ and A1↔X0⁺ transitions are also calculated. Highly diagonally distributed Franck-Condon factors of the a₂1↔X0⁺ and A1↔X0⁺ transitions are obtained, the value of f₀₀ is 0.9970 and 0.9980, respectively. Spontaneous radiative lifetimes of the a₂1 and A1 excited states are predicted. i.e. τ(a₂1) = 200.3 ns and τ(A1) = 84.3 ns. Only the main pump laser is required to driving a₂1↔X0⁺ and A1↔X0⁺ transitions. The laser wavelengths both are in the visible region. Doppler temperatures and recoil temperatures of laser cooling TeH^- anion are also predicted.

In search of molecular ions for optical cycling: a difficult road

Optical cycling, a continuous photon scattering off atoms or molecules, is the key tool in quantum information science.

Candidate for Laser Cooling of a Negative Ion: High-Resolution Photoelectron Imaging of Th^{-}

Laser cooling is a well-established technique for the creation of ensembles of ultracold neutral atoms or positive ions. This ability has opened many exciting new research fields over the past 40 years. However, no negatively charged ions have been directly laser cooled because a cycling transition is very rare in atomic anions. Efforts of more than a decade currently have La^{-} as the most promising candidate. We report on experimental and theoretical studies supporting Th^{-} as a new promising candidate for laser cooling. The measured and calculated electron affinities of Th are, respectively, 4901.35(48)  cm^{-1} and 4832  cm^{-1}, or 0.607 690(60) and 0.599 eV, almost a factor of 2 larger than the previous theoretical value of 0.368 eV. The ground state of Th^{-} is determined to be 6d^{3}7s^{2} ^{4}F_{3/2}^{e} rather than 6d^{2}7s^{2}7p ^{4}G_{5/2}^{o}. The consequence of this is that there are several strong electric dipole transitions between the bound levels arising from configurations 6d^{3}7s^{2} and 6d^{2}7s^{2}7p in Th^{-}. The potential laser-cooling transition is ^{2}S_{1/2}^{o}↔^{4}F_{3/2}^{e} with a wavelength of 2.6  μm. The zero nuclear spin and hence lack of hyperfine structure in Th^{-} reduces the potential complications in laser cooling as encountered in La^{-}, making Th^{-} a new and exciting candidate for laser cooling.

Laser-Assisted Evaporative Cooling of Anions

We report the first cooling of atomic anions by laser radiation. O^{-} ions confined in a linear Paul trap were cooled by selectively photodetaching the hottest particles. For this purpose, anions with the highest total energy were illuminated with a 532 nm laser at their maximal radial excursion. Using laser-particle interaction, we realized a both colder and denser ion cloud, achieving a more than threefold temperature reduction from 1.15 to 0.33 eV. Compared with the interaction with a dilute buffer gas, the energy-selective addressing and removal of anions resulted in lower final temperatures, yet acted 10 times faster and preserved twice as large a fraction of ions in the final state. An ensemble of cold negative ions affords the ability to sympathetically cool any other negative ion species, enabling or facilitating a broad range of fundamental studies from interstellar chemistry to antimatter gravity. The technique can be extended to any negative ion species that can be neutralized via photodetachment.

Ultracold Anions for High-Precision Antihydrogen Experiments

Experiments with antihydrogen (H[over ¯]) for a study of matter-antimatter symmetry and antimatter gravity require ultracold H[over ¯] to reach ultimate precision. A promising path towards antiatoms much colder than a few kelvin involves the precooling of antiprotons by laser-cooled anions. Because of the weak binding of the valence electron in anions-dominated by polarization and correlation effects-only few candidate systems with suitable transitions exist. We report on a combination of experimental and theoretical studies to fully determine the relevant binding energies, transition rates, and branching ratios of the most promising candidate La^{-}. Using combined transverse and collinear laser spectroscopy, we determined the resonant frequency of the laser cooling transition to be ν=96.592 713(91)  THz and its transition rate to be A=4.90(50)×10^{4}  s^{-1}. Using a novel high-precision theoretical treatment of La^{-} we calculated yet unmeasured energy levels, transition rates, branching ratios, and lifetimes to complement experimental information on the laser cooling cycle of La^{-}. The new data establish the suitability of La^{-} for laser cooling and show that the cooling transition is significantly stronger than suggested by a previous theoretical study.

AEgIS at ELENA: outlook for physics with a pulsed cold antihydrogen beam

The efficient production of cold antihydrogen atoms in particle traps at CERN's Antiproton Decelerator has opened up the possibility of performing direct measurements of the Earth's gravitational acceleration on purely antimatter bodies. The goal of the AEgIS collaboration is to measure the value of g for antimatter using a pulsed source of cold antihydrogen and a Moiré deflectometer/Talbot-Lau interferometer. The same antihydrogen beam is also very well suited to measuring precisely the ground-state hyperfine splitting of the anti-atom. The antihydrogen formation mechanism chosen by AEgIS is resonant charge exchange between cold antiprotons and Rydberg positronium. A series of technical developments regarding positrons and positronium (Ps formation in a dedicated room-temperature target, spectroscopy of the n=1-3 and n=3-15 transitions in Ps, Ps formation in a target at 10 K inside the 1 T magnetic field of the experiment) as well as antiprotons (high-efficiency trapping of [Formula: see text], radial compression to sub-millimetre radii of mixed [Formula: see text] plasmas in 1 T field, high-efficiency transfer of [Formula: see text] to the antihydrogen production trap using an in-flight launch and recapture procedure) were successfully implemented. Two further critical steps that are germane mainly to charge exchange formation of antihydrogen-cooling of antiprotons and formation of a beam of antihydrogen-are being addressed in parallel. The coming of ELENA will allow, in the very near future, the number of trappable antiprotons to be increased by more than a factor of 50. For the antihydrogen production scheme chosen by AEgIS, this will be reflected in a corresponding increase of produced antihydrogen atoms, leading to a significant reduction of measurement times and providing a path towards high-precision measurements.This article is part of the Theo Murphy meeting issue 'Antiproton physics in the ELENA era'.

The ground and low-lying excited states and feasibility of laser cooling for GaH+ and InH+ cations

The potential energy curves and transition dipole moments of 1²Σ⁺ and 1²Π states of GaH⁺ and InH⁺ cations are performed by employing ab initio calculations. Based on the potential energy curves, the rotational and vibrational energy levels of the two states are obtained by solving the Schrödinger equation of nuclear movement. The spectroscopic parameters are deduced with the obtained rovibrational energy levels. The spin-orbit coupling effect of the ²Π states for both the GaH⁺ and InH⁺ cations are also calculated. The feasibility of laser cooling of GaH⁺ and InH⁺ cations are examined by using the results of the electronic and spectroscopic properties. The highly diagonal Franck-Condon factors and appropriate radiative lifetimes are determined by using the potential energy curves and transition dipole moments for the ²Π_{1/2, 3/2}↔1²Σ⁺ transitions. The results indicate that the ²Π_{1/2, 3/2}↔1²Σ⁺ transitions of both GaH⁺ and InH⁺ cations are appropriate for the close cycle transition of laser cooling. The optical scheme of the laser cooling is constructed for the GaH⁺ and InH⁺ cations.

Bound and continuum-embedded states of cyanopolyyne anions

Equation-of-motion coupled-cluster calculations reveal systematic trends across bound and continuum-embedded excited states in cyanopolyyne anions.

The low-lying electronic states and optical schemes for the laser cooling of the BH+ and BH- ions

The potential energy curves and transition dipole moments for the 1²Σ⁺, 2²Σ⁺, 1²Π and 2²Π electronic states of the two molecules are calculated using multi-reference configuration interaction and the large basis sets aug-cc-pwCV5Z. Based on the obtained potential energy curves, the rotational and vibrational energy levels of the states are obtained by solving the Schrödinger equation of nuclear motion, and the spectroscopic parameters are then obtained by fitting the energy levels to Dunham series expansions. The spin-orbit coupling effect of the ²Π states for both the BH⁺ cation and BH^- anion are calculated. Highly diagonally distributed Franck-Condon factors are determined for the 1²Σ⁺ (v″=0)↔1²Π (v'=0) transition, ƒ₀₀ (BH⁺)=0.943, while the Franck-Condon factors for the 1²Π (v″=0)↔1²Σ⁺ (v'=0) transition is ƒ₀₀ (BH^-)=0.942. Moreover, the radiative lifetime of 38.2ns for the excited 1²Π state of the BH⁺ and 91.8ns for the 1²Σ⁺ state of the BH^- are obtained, which are short enough for rapid laser cooling. A three-step optical scheme of the laser cooling is constructed for either the BH⁺ cation or the BH^- anion.

Ab initio study of the neutral and anionic alkali and alkaline earth hydroxides: Electronic structure and prospects for sympathetic cooling of OH. J Chem Phys 2017;146:194309. [PMID: 28527437 PMCID: PMC5438307 DOI: 10.1063/1.4983627]

We have performed a systematic ab initio study on alkali and alkaline earth hydroxide neutral (MOH) and anionic (MOH-) species where M = Li, Na, K, Rb, Cs or Be, Mg, Ca, Sr, Ba. The CCSD(T) method with extended basis sets and Dirac-Fock relativistic effective core potentials for the heavier atoms has been used to study their equilibrium geometries, interaction energies, electron affinities, electric dipole moment, and potential energy surfaces. All neutral and anionic species exhibit a linear shape with the exception of BeOH, BeOH-, and MgOH-, for which the equilibrium structure is found to be bent. Our analysis shows that the alkaline earth hydroxide anions are valence-bound whereas the alkali hydroxide anions are dipole bound. In the context of sympathetic cooling of OH- by collision with ultracold alkali and alkaline earth atoms, we investigate the 2D MOH- potential energy surfaces and the associative detachment reaction M + OH→- MOH + e-, which is the only energetically allowed reactive channel in the cold regime. We discuss the implication for the sympathetic cooling of OH- and conclude that Li and K are the best candidates for an ultracold buffer gas.

Laser cooling of the AlCl molecule with a three-electronic-level theoretical model

Feasibility of laser-cooling AlCl molecule is investigated using ab initio quantum chemistry. Potential energy curves, permanent dipole moments, and transition dipole moments for the X(1)Σ(+), a(3)Π, and A(1)Π states are studied based on multi-reference configuration interaction plus Davidson corrections (MRCI+Q) method with ACVQZ basis set, spin-orbit coupling effects are considered at the MRCI+Q level. Highly diagonally distributed Franck-Condon factors (f00 = 0.9988 and f11 = 0.9970) and branching ratios (R00 = 0.9965, R01 = 2.85 × 10(-3), R02 = 6.35 × 10(-4), and R03 = 2.05 × 10(-6)) for the A(1)Π1(ν(')=0)→X(1)Σ0(+) (+)(ν(″)=0) transition are determined. A sufficiently radiative lifetime τ (A(1)Π1) = 4.99 ns is predicted for rapid laser cooling. The proposed cooling wavelength is deep in the ultraviolet region at λ00 = 261.75 nm. Total emission rates for the a(3)Π0(+) →X(1)Σ0(+) (+), a(3)Π1→X(1)Σ0(+) (+), A(1)Π1 → a(3)Π0(+) , and A(1)Π1 → a(3)Π1 transitions are particularly small (∼10 s(-1)-650 s(-1)). The calculated vibrational branching loss ratio to the intermediate a(3)Π0(+) and a(3)Π1 states can be negligible. The results imply the probability of laser cooling AlCl molecule with three-electronic-level.

Cold interactions and chemical reactions of linear polyatomic anions with alkali-metal and alkaline-earth-metal atoms

Cold interactions and channels of chemical reactions between linear polyatomic anions and atoms are investigated, opening the way for sympathetic cooling and controlled chemistry in these systems.

Laser cooling of the OH− molecular anion in a theoretical investigation

The schemes for laser cooling of the OH⁻ anion are proposed using an ab initio method.

Slowing and cooling of heavy or light (even with a tiny electric dipole moment) polar molecules using a novel, versatile electrostatic Stark decelerator

To meet some demands for realizing precise measurements of an electric dipole moment of electron (eEDM) and examining cold collisions or cold chemical physics, we have proposed a novel, versatile electrostatic Stark decelerator with an array of true 3D electric potential wells, which are created by a series of horizontally-oriented, U-shaped electrodes with time-sequence controlling high voltages (± HV) and two guiding electrodes with a constant voltage. We have calculated the 2D electric field distribution, the Stark shifts of the four lowest rotational sub-levels of PbF molecules in the X1(2)Π1/2(v = 0) electronic and vibrational ground states as well as the population in the different rotational levels. We have discussed the 2D longitudinal and transverse phase-space acceptances of PbF molecules in our decelerator. Subsequently, we have simulated the dynamic processes of the decelerated PbF molecules using the 3D Monte-Carlo method, and have found that a supersonic PbF beam with a velocity of 300 m s(-1) can be efficiently slowed to about 5 m s(-1), which will greatly enhance the sensitivities to research a parity violation and measure an eEDM. In addition, we have investigated the dependences of the longitudinal velocity spread, longitudinal temperature and bunching efficiency on both the number of guiding stages and high voltages, and found that after bunching, a cold packet of PbF molecules in the J = 7/2, MΩ = -7/4 state with a longitudinal velocity spread of 0.69 m s(-1) (corresponding to a longitudinal temperature of 2.35 mK) will be produced by our high-efficient decelerator, which will generate a high energy-resolution molecular beam for studying cold collision physics. Finally, our novel decelerator can also be used to efficiently slow NO molecules with a tiny electric dipole moment (EDM) of 0.16 D from 315 m s(-1) to 28 m s(-1). It is clear that our proposed new decelerator has a good slowing performance and experimental feasibility as well as wide applications in the field of precise measurements and cold molecule physics.