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boron nitride anions

Breitmeier, Beke Leonie

Published Jan 1, 2025• 0 citations
OpenAlex

Abstract

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.

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We propose to extend laser-cooling techniques, so far only achieved for neutral molecules, to molecular anions. A detailed computational study is performed for molecules stored in Penning traps using GPU based Monte Carlo simulations. Two cooling schemes—Doppler laser cooling and photodetachment cooling—are investigated. The sympathetic cooling of antiprotons is studied for the Doppler cooling scheme, where it is shown that cooling of antiprotons to subKelvin temperatures could becomes feasible, with impacts on the field of antimatter physics. The presented cooling schemes also have applications for the generation of cold, negatively charged particle sources and for the sympathetic cooling of other molecular anions.

New Journal of Physics•Jan 19, 2018•23 citations
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.
Atomic anions are generally not amenable to optical spectroscopy because they are loosely bound systems and rarely have bound excited states. Until recently, there was only one known negative ion with a strong bound–bound electronic transition, the osmium anion Os − . The electric-dipole transition between the 4 F e 9/2 ground and 6 D o J excited state of this ion provides unique insight into the structure of atomic anions. In addition, it may enable the preparation of ultracold ensembles of negative ions. Laser excitation of the electric-dipole transition in Os − ions could be used to laser-cool them to microkelvin temperatures. If demonstrated to be successful, the technique would allow the cooling of any species of negatively charged ions - from subatomic particles to molecular anions - to ultracold temperatures by sympathetic cooling. We have been investigating the bound-bound electric-dipole transition in Os − by high-resolution laser spectroscopy with a view to using it for the first laser cooling of negative ions. The principle of the method, its potential applications, as well as experimental results are presented.
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.
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.
Despite the tremendous advances in laser cooling of neutral atoms and positive ions, no negatively charged ion has been directly laser cooled. The negative ion of lanthanum, La(-), has been proposed as the best candidate for laser cooling of any atomic anion [ and , Phys. Rev. A 81, 032503 (2010)]. Tunable infrared laser photodetachment spectroscopy is used to measure the bound-state structure of La(-), revealing a spectrum of unprecedented richness with multiple bound-bound electric dipole transitions. The potential laser-cooling transition ((3)F(2)(e)→(3)D(1)(o)) is identified and its excitation energy is measured. The results confirm that La^{-} is a very promising negative ion for laser-cooling applications.
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 propose to extend laser-cooling techniques, so far only achieved for neutral molecules, to molecular anions. A detailed computational study is performed for molecules stored in Penning traps using GPU based Monte Carlo simulations. Two cooling schemes—Doppler laser cooling and photodetachment cooling—are investigated. The sympathetic cooling of antiprotons is studied for the Doppler cooling scheme, where it is shown that cooling of antiprotons to subKelvin temperatures could becomes feasible, with impacts on the field of antimatter physics. The presented cooling schemes also have applications for the generation of cold, negatively charged particle sources and for the sympathetic cooling of other molecular anions.