Laser-optical cycling of cryogenically cooled BN− molecular anions
Katrin Dulitz, Stanka V. Jerosimić, Pablo del Mazo‐Sevillano, Jorge Alonso de la Fuente, Cristina Sanz‐Sanz, Lola González‐Sánchez, Francesco Antonio Gianturco
Abstract We investigate an optical cycling scheme for Doppler cooling cold trapped 11B14N− ions using transitions between the X 2Σ+ ground state and the B 2Σ+ excited state, and analyze here the relevant transitions for photon cycling and repumping. Our results show that slow population decay via the first excited electronic state A 2Π cannot be neglected. To improve the optical cycling efficiency, we consider additional transitions beyond what would be expected from the highly diagonal FranckCondon factor involving the B 2Σ+(v = 0) ← X 2Σ+(v = 0) transition. We estimate that the number of cycled photons alone is not likely to be sufficient to bring buffer-gas-cooled 11B14N− to temperatures near the Doppler cooling limit. Hence, pre-cooling, e.g., using a combination of cryogenic buffer gas and photodetachment cooling, will be essential to maximize the optical cycling efficiency and to reach a regime where Coulomb crystallization occurs. To explore pre-cooling with He or Ar buffer gases, we therefore also performed extensive quantum calculations of potential energy curves, transition moments and radiative rate coefficients for the BN−–buffer gas systems, to be implemented in a later study. Our results provide key insights for generating cold negative ions. These anions have, in fact, promising applications in various fields, ranging from quantum science and technology to fundamental physics and chemistry.
Abstract We investigate an optical cycling scheme for Doppler cooling cold trapped 11B14N− ions using transitions between the X 2Σ+ ground state and the B 2Σ+ excited state, and analyze here the relevant transitions for photon cycling and repumping. Our results show that slow population decay via the first excited electronic state A 2Π cannot be neglected. To improve the optical cycling efficiency, we consider additional transitions beyond what would be expected from the highly diagonal FranckCondon factor involving the B 2Σ+(v = 0) ← X 2Σ+(v = 0) transition. We estimate that the number of cycled photons alone is not likely to be sufficient to bring buffer-gas-cooled 11B14N− to temperatures near the Doppler cooling limit. Hence, pre-cooling, e.g., using a combination of cryogenic buffer gas and photodetachment cooling, will be essential to maximize the optical cycling efficiency and to reach a regime where Coulomb crystallization occurs. To explore pre-cooling with He or Ar buffer gases, we therefore also performed extensive quantum calculations of potential energy curves, transition moments and radiative rate coefficients for the BN−–buffer gas systems, to be implemented in a later study. Our results provide key insights for generating cold negative ions. These anions have, in fact, promising applications in various fields, ranging from quantum science and technology to fundamental physics and chemistry.
Following the previous study with an extensive range of quantum calculations involving different electronic states of the BN- anion [Dulitz et al., Phys. Scripta 100, 055411 (2025)], we now extend that work by modeling the quantum dynamics of the collision cooling of its rotational states in order to investigate possible paths for bringing this molecular anion down to temperatures of a few Kelvins. This specific ionic system is of direct interest when modeling experiments in cold ion traps where He or Ar atoms can function as the chief buffer gases that drive the anions down to the low trap temperatures. We employ accurate, ab initio calculations of the potential energy surfaces for the title system in its ground electronic state, interacting with either He or Ar atoms. We then obtain a wide range of inelastic cross sections and the ensuing rate coefficients in order to model the quantum kinetics of the time evolution of the cooling steps under different temperature and trap conditions. The results are analyzed and employed to estimate the cooling efficiency paths provided by various trap arrangements for the title anion. The results show that-using either of the two investigated species-the buffer gas cooling process very efficiently brings the anions to their lowest rotational states. These findings are very promising for future applications in the field of anion laser cooling.
We report on a cycling scheme for Doppler cooling of trapped OH + ions using transitions between the electronic ground state X 3 Σ – and the first excited triplet state A 3 Π. We have identified relevant transitions for photon cycling and repumping, have found that coupling into other electronic states is strongly suppressed, and have calculated the number of photon scatterings required to cool OH + to a temperature where Raman sideband cooling can take over. In contrast to the standard approach, where molecular ions are sympathetically cooled, our scheme does not require co-trapping of another species and opens the door to the creation of pure samples of cold molecular ions with potential applications in quantum information, quantum chemistry, and astrochemistry. The laser cooling scheme identified for OH + is efficient despite the absence of near-diagonal Franck–Condon factors, suggesting that broader classes of molecules and molecular ions are amenable to laser cooling than commonly assumed.
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.
This thesis presents the development and characterisation of an experimental setup designed for photodetachment spectroscopy of boron nitride anions (BN^-), as a step towards the long-term objective of laser cooling them. Laser cooling has become a mature and widely applied technique for neutral atoms and selected cationic molecular species, while its extension to anionic systems poses considerable experimental challenges. BN^- stands out as a promising candidate due to theoretical predictions of an electronic structure that features nearly diagonal Franck–Condon factors, suggesting the potential for closed optical cycling transitions—an essential prerequisite for laser cooling of molecular ions. Realising such a system would open new frontiers in precision spectroscopy, antimatter research, and ultracold chemistry involving negative ions. To this end, the setup—comprising a plasma discharge ion source with a piezoelectric valve, a Wiley–McLaren-type time-of-flight mass spectrometer, a channeltron ion detector, and an OPO laser—was calibrated using various negative and positive ion species, enabling accurate conversion of time-of-flight data into mass-to-charge spectra with sufficient resolution to identify light ions. Initial attempts at producing BN^- via thermal decomposition of ammonia borane (H_3B-NH_3) were conducted. While unambiguous detection of BN^- remains pending, several boron-containing positive and negative ions were observed. To validate the photodetachment capabilities of the setup, proof-of-principle depletion spectroscopy was performed on the well-characterised species O^-, OH^-, and O_2^- . Significant signal reductions were observed upon laser interaction at 450 nm for all species, and at 700 nm solely for O^- and O_2^- , consistent with their known photodetachment cross sections. To enable detection of neutral species resulting from photodetachment and to mimprove charged-neutral separation, a quadrupole bender was designed, simulated using SIMION, and successfully implemented in the setup. The overall system is now operational and provides a robust platform for forthcoming spectroscopic investigations of BN^- and related molecular anions, laying the groundwork for advancing laser-cooling techniques in negatively charged systems.
Quantum gases of diatomic molecules are many body systems that offer unprecedented control over their intrinsic long and short range interactions. This is due to the unique nature of the interactions between molecules, which can be much stronger than those in magnetic atoms and which can be tuned independently through external electromagnetic fields. As a result, diatomic molecules enable both the exploration of novel supersolid states and the creation of dipole dominated and self assembled quantum solids. The molecular properties and internal structure of calcium monofluoride (CaF) make it a promising candidate molecule for the realization of several kinds of strongly correlated quantum systems, such as strongly dipolar Bose Einstein condensates, quantum crystals and arrays of molecules in optical tweezers. This dissertation summarizes the first steps towards the realization of such systems using directly laser cooled CaF molecules. The two major experimental steps which are summarized in this thesis are cryogenic buffer gas cooling and transversal laser cooling of CaF molecules. For the first achievement, cryogenic buffer gas cooling, it is important to observe that CaF molecules are radicals and therefore do not exist in a stable gas phase. As a result, the production of a beam of molecules requires a cryogenic buffer gas cell setup, in which the molecules are produced via laser ablation of a solid precursor target. Two different kinds of ablation target are used to produce a molecular beam. The first is a pellet of a mixture of CaF2/BaF2. The second is a calcium metal target in addition to sulfurhexafluoride gas, which acts as a fluorine donor. The metal target is found to be the much more stable molecule production method, and the buffer gas cooling setup produces a bright and slow beam of molecules with a flux of around 1011 - 1013sr-1 molecules in the ground state of the laser cooling cycle. The second achievement is a significant improvement of the techniques for laser cooling of the resulting molecular beam. Notably, trapped cold gases of directly laser cooled molecules have not reached the quantum degenerate regime yet, but this milestone is within reach. In our experiment we aim to improve the loading of the magneto optical trap from the laser slowed beam, as this has been the most inefficient step in previous experiments with CaF. Our approach is to collimate the molecular beam through transversal laser cooling on the 𝑋 - 𝐴 transition. Due to the angular momentum structure of the type-II cooling transition, Doppler cooling forces in molecules are typically accompanied by sub Doppler Sisyphus heating forces and vice versa. This has made the collimation of molecular beams difficult in previous experiments, because efficient Doppler cooling over a large velocity range is perturbed by the appearance of sub Doppler heating forces at low transversal velocities. Here, we present a new way to address the hyperfine structure in the molecules using carefully chosen laser sidebands, which overcomes this limitation. The sideband spectrum of the cooling laser consists of one strong sideband with ≈ 80% of the laser power addressing a single hyperfine state and several weaker sidebands which repump population from the other hyperfine states. Depending on the detuning of the cooling laser, the strong sideband generates sub Doppler cooling forces on the 𝑋2Σ+(𝐹 = 2) - 𝐴2Π1/2 (𝐹 = 1) or the 𝑋2Σ+(𝐹 = 1+) - 𝐴2Π1/2 (𝐹 = 0) transition. The measured transversal density profiles of the molecular beam suggest that, for certain detunings of the cooling laser, both mechanisms lead to forces that counteract the molecular motion, leading to the simultaneous appearance of Doppler and sub Doppler cooling. This result is contrary to the conventional tendency of type II systems, where sub Doppler cooling forces are usually opposed by Doppler heating forces. The observed behavior is supported by numerical simulations of the laser cooling forces and molecular trajectories based on the optical Bloch equations. Overall, the results highlight the importance of a well tailored cooling laser sideband structure for molecular laser cooling to control the interplay between both cooling mechanisms. Through the collimation of the molecular motion towards the capture region of a magneto-optical trap, the transversal cooling of the molecular beam with simultaneous Doppler and sub-Doppler cooling forces should enable a more efficient trap loading. The experimental results presented here therefore form the basis for future experiments on laser slowing, magneto optical trapping and evaporative cooling towards Bose Einstein condensation or the loading of large optical tweezer arrays of cold CaF molecules.
Abstract We demonstrate optical cycling and laser cooling of a cryogenic buffer-gas beam of calcium monohydride (CaH) molecules. We measure vibrational branching ratios for laser cooling transitions for both excited electronic states A and B. Furthermore, we measure that repeated photon scattering via the A ← X transition is achievable at a rate of ∼ 1.6 × 1 0 6 photons s−1 and demonstrate interaction-time limited scattering of ∼ 200 photons by repumping the largest vibrational decay channel. We also demonstrate a sub-Doppler cooling technique, namely the magnetically assisted Sisyphus effect, and use it to cool the transverse temperature of a molecular beam of CaH. Using a standing wave of light, we lower the transverse temperature from 12.2(1.2) mK to 5.7(1.1) mK. We compare these results to a model that uses optical Bloch equations and Monte Carlo simulations of the molecular beam trajectories. This work establishes a clear pathway for creating a magneto-optical trap (MOT) of CaH molecules. Such a MOT could serve as a starting point for production of ultracold hydrogen gas via dissociation of a trapped CaH cloud.