Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

Journal of Physics B Atomic Molecular and Optical Physics

Positron cooling by vibrational and rotational excitation of molecular gases

Michael R. Natisin, James R. Danielson, Clifford M. Surko

Published Nov 4, 2014• 39 citations
Publisher

Abstract

Measurements of positron temperature as a function of time are presented when a positron gas, confined in an electromagnetic trap at an elevated temperature ( K), is cooled by interactions with the molecular gases , or CO at 300 K. A simple model describing positron thermalization by coupling to vibrational and rotational modes is presented, with cooling-rate predictions calculated in the Born approximation. Comparisons to the measured positron cooling-rate curves permit estimates of the magnitudes of the relevant cross sections. The results are compared with experiment for the case of vibrational excitation, where direct measurements exist; and they provide estimates of the rotational excitation cross sections where direct measurements are not currently possible. Positron cooling rates are compared for these gases at 300 K, and estimates of their effectiveness in cooling positrons to cryogenic temperatures are discussed.

DOI
10.1088/0953-4075/47/22/225209

Related papers

Rotational cooling of molecular gases by positron impact at vanishing collision energies

J Sabin Del Valle, Enrico Bodo, Franco Antonio Gianturco

The behaviour of the rotationally inelastic cross sections for ultracold positrons injected in molecular gases is examined computationally by considering the ambient molecules to be rotationally 'hot' ( j = 10) and by letting the relative translational energies tend to zero. The size of the corresponding state-to-state transitions are obtained from an exact quantum calculation carried out in the space-frame representation, using non-empirical model interactions between e + and the target test molecules (H 2 , N 2 , O 2 , Li 2 ). The structural features of the molecules are related, via the dynamical coupling effects, to the size and efficiency of the cooling process along the above series of target gases. It is shown that positron projectiles can provide a possible route to the cooling of molecular rotations for simple homonuclear diatomics within the framework of ultracold dynamical conditions.

Journal of Physics B Atomic Molecular and Optical Physics•Jun 14, 2005•5 citations

Positron Cooling and Annihilation in Noble Gases

Dermot G. Green

Positron cooling and annihilation in room temperature noble gases is simulated using accurate scattering and annihilation cross sections calculated with many-body theory, enabling the first simultaneous probing of the energy dependence of the scattering and annihilation cross sections. A strikingly small fraction of positrons is shown to survive to thermalization: ∼0.1 in He, ∼0 in Ne, ∼0.15 in Ar, ∼0.05 in Kr, and ∼0.01 in Xe. For Xe, the time-varying annihilation rate Z[over ¯]_{eff}(τ) is shown to be highly sensitive to the depletion of the momentum distribution due to annihilation, conclusively explaining the long-standing discrepancy between gas-cell and trap-based measurements. Overall, the use of the accurate atomic data gives Z[over ¯]_{eff}(τ) in close agreement with experiment for all noble gases except Ne, the experiment for which is proffered to have suffered from incomplete knowledge of the fraction of positrons surviving to thermalization and/or the presence of impurities.

arXiv:1706.01434•Physical Review Letters•Nov 16, 2017•12 citations

Investigation of buffer gas trapping of positrons

C. J. Baker, Christopher Aled Isaac, D Edwards +4 more

Abstract A study of positron capture in a two-(pressure) stage buffer gas accumulation apparatus is presented for a variety of species, including some molecules which are known to be either efficient for positron trapping, or are frequently used to cool the particles when held in these devices. Absolute accumulation efficiencies are reported for all species. A detailed optimisation procedure, which has identified the main processes responsible for positron capture and loss in the trap, has been deployed to explore accumulation efficiency as the gas pressure and the electrostatic well depth in the trap are systematically varied. Accumulation exploiting energy loss via molecular vibrational transitions has been observed for the first time for a number of gases, though at much lower efficiency than achieved using electronic excitation processes.

Journal of Physics B Atomic Molecular and Optical Physics•Jun 29, 2020•9 citations

Collisional cooling of internal rotation in MgH+ ions trapped with He atoms: Quantum modeling meets experiments in Coulomb crystals

Lola González‐Sánchez, Roland Wester, Franco Antonio Gianturco

Using the ab initio computed potential energy surface for the electronic interaction of the ${\mathrm{MgH}}^{+}(^{1}\mathrm{\ensuremath{\Sigma}}$) ion with the He($^{1}S$) atom, we calculate the relevant state-changing rotationally inelastic collision cross sections from a quantum treatment of the multichannel scattering problem. We focus on the quantum dynamics at the translationally low energies for the present partners discussed in earlier, cold ion trap experiments (see below), which we wish to model in detail. The corresponding state-changing rates computed between the lower rotational states of the molecular ion are employed to describe the time-evolution kinetics followed by recent experiments on Coulomb-crystallized ${\mathrm{MgH}}^{+}(^{1}\mathrm{\ensuremath{\Sigma}}$), where the ions are rotationally cooled by micromotion tuning after being uploaded into the trap of He as a buffer gas. The present computational modeling of the final ion rotational temperatures in the experiments turns out to agree very well with the observations and points at a fast equilibration between rotational and thermal temperatures of the ions.

arXiv:2010.14458•Physical Review A•Nov 15, 2018•5 citations

Role of Binding Energy in Feshbach-Resonant Positron-Molecule Annihilation

Jason A. Young, Clifford M. Surko

Measurements of positron-on-molecule annihilation have established that positrons bind to a variety of molecules via vibrational Feshbach resonances. Data for deeply bound states in benzene and 1-chlorohexane and for positronically excited (i.e., second) bound states in alkanes are used to establish the dependence of annihilation rates on the binding energy and incident positron energy. With this dependence removed, annihilation rates for a broad class of molecules lie on a universal curve as a function of the number of molecular vibrational degrees of freedom. The implications of these results for theoretical models are discussed.

Physical Review Letters•Sep 28, 2007•24 citations

Positron scattering from atoms and molecules using a magnetized beam

James Patrick Sullivan, S. J. Gilbert, Joan P. Marler +3 more

The development of buffer-gas trapping and magnetized beam formation has provided positron beams with significantly improved energy resolution $(\ensuremath{\Delta}E<~0.025\mathrm{eV})$ compared to those available previously. Analysis techniques have been developed to take advantage of the fact that the beam is in a magnetic field of $\ensuremath{\sim}0.1\mathrm{T}.$ This has enabled scattering experiments at lower energies and with significantly improved state selectivity for excitation experiments than had been possible previously. A detailed description of these techniques is presented. Data are presented for a variety of cross sections for scattering of positrons from atomic and molecular targets.

Physical Review A•Oct 15, 2002•99 citations
Measurements of positron temperature as a function of time are presented when a positron gas, confined in an electromagnetic trap at an elevated temperature ( K), is cooled by interactions with the molecular gases , or CO at 300 K. A simple model describing positron thermalization by coupling to vibrational and rotational modes is presented, with cooling-rate predictions calculated in the Born approximation. Comparisons to the measured positron cooling-rate curves permit estimates of the magnitudes of the relevant cross sections. The results are compared with experiment for the case of vibrational excitation, where direct measurements exist; and they provide estimates of the rotational excitation cross sections where direct measurements are not currently possible. Positron cooling rates are compared for these gases at 300 K, and estimates of their effectiveness in cooling positrons to cryogenic temperatures are discussed.
The behaviour of the rotationally inelastic cross sections for ultracold positrons injected in molecular gases is examined computationally by considering the ambient molecules to be rotationally 'hot' ( j = 10) and by letting the relative translational energies tend to zero. The size of the corresponding state-to-state transitions are obtained from an exact quantum calculation carried out in the space-frame representation, using non-empirical model interactions between e + and the target test molecules (H 2 , N 2 , O 2 , Li 2 ). The structural features of the molecules are related, via the dynamical coupling effects, to the size and efficiency of the cooling process along the above series of target gases. It is shown that positron projectiles can provide a possible route to the cooling of molecular rotations for simple homonuclear diatomics within the framework of ultracold dynamical conditions.
Positron cooling and annihilation in room temperature noble gases is simulated using accurate scattering and annihilation cross sections calculated with many-body theory, enabling the first simultaneous probing of the energy dependence of the scattering and annihilation cross sections. A strikingly small fraction of positrons is shown to survive to thermalization: ∼0.1 in He, ∼0 in Ne, ∼0.15 in Ar, ∼0.05 in Kr, and ∼0.01 in Xe. For Xe, the time-varying annihilation rate Z[over ¯]_{eff}(τ) is shown to be highly sensitive to the depletion of the momentum distribution due to annihilation, conclusively explaining the long-standing discrepancy between gas-cell and trap-based measurements. Overall, the use of the accurate atomic data gives Z[over ¯]_{eff}(τ) in close agreement with experiment for all noble gases except Ne, the experiment for which is proffered to have suffered from incomplete knowledge of the fraction of positrons surviving to thermalization and/or the presence of impurities.
Abstract A study of positron capture in a two-(pressure) stage buffer gas accumulation apparatus is presented for a variety of species, including some molecules which are known to be either efficient for positron trapping, or are frequently used to cool the particles when held in these devices. Absolute accumulation efficiencies are reported for all species. A detailed optimisation procedure, which has identified the main processes responsible for positron capture and loss in the trap, has been deployed to explore accumulation efficiency as the gas pressure and the electrostatic well depth in the trap are systematically varied. Accumulation exploiting energy loss via molecular vibrational transitions has been observed for the first time for a number of gases, though at much lower efficiency than achieved using electronic excitation processes.
Using the ab initio computed potential energy surface for the electronic interaction of the ${\mathrm{MgH}}^{+}(^{1}\mathrm{\ensuremath{\Sigma}}$) ion with the He(1S^{1}S1S) atom, we calculate the relevant state-changing rotationally inelastic collision cross sections from a quantum treatment of the multichannel scattering problem. We focus on the quantum dynamics at the translationally low energies for the present partners discussed in earlier, cold ion trap experiments (see below), which we wish to model in detail. The corresponding state-changing rates computed between the lower rotational states of the molecular ion are employed to describe the time-evolution kinetics followed by recent experiments on Coulomb-crystallized ${\mathrm{MgH}}^{+}(^{1}\mathrm{\ensuremath{\Sigma}}$), where the ions are rotationally cooled by micromotion tuning after being uploaded into the trap of He as a buffer gas. The present computational modeling of the final ion rotational temperatures in the experiments turns out to agree very well with the observations and points at a fast equilibration between rotational and thermal temperatures of the ions.
Measurements of positron-on-molecule annihilation have established that positrons bind to a variety of molecules via vibrational Feshbach resonances. Data for deeply bound states in benzene and 1-chlorohexane and for positronically excited (i.e., second) bound states in alkanes are used to establish the dependence of annihilation rates on the binding energy and incident positron energy. With this dependence removed, annihilation rates for a broad class of molecules lie on a universal curve as a function of the number of molecular vibrational degrees of freedom. The implications of these results for theoretical models are discussed.
The development of buffer-gas trapping and magnetized beam formation has provided positron beams with significantly improved energy resolution $(\ensuremath{\Delta}E<~0.025\mathrm{eV})$ compared to those available previously. Analysis techniques have been developed to take advantage of the fact that the beam is in a magnetic field of $\ensuremath{\sim}0.1\mathrm{T}.$ This has enabled scattering experiments at lower energies and with significantly improved state selectivity for excitation experiments than had been possible previously. A detailed description of these techniques is presented. Data are presented for a variety of cross sections for scattering of positrons from atomic and molecular targets.