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Collisions between cold molecules in a superconducting magnetic trap

Yair Segev, Martin Pitzer, Michael Karpov, Nitzan Akerman, Julia Narevicius, Edvardas Narevicius

Published Jan 1, 2019• 35 citations
OpenAlex

Abstract

Abstract Collisions between cold molecules are essential for studying fundamental aspects of quantum chemistry, and may enable the formation of quantum degenerate molecular matter by evaporative cooling. However, collisions between trapped, naturally occurring molecules have not been directly observed so far owing to the low collision rates of dilute samples. Here we report the direct observation of collisions between cold trapped molecules, without the need for laser cooling. We magnetically capture molecular oxygen in an 800-millikelvin-deep superconducting trap and set bounds on the ratio between the elastic- and inelastic-scattering rates—the key parameter determining the feasibility of evaporative cooling. We further co-trap atoms and molecules and identify collisions between them, paving the way for studies of cold interspecies collisions in a magnetic trap.

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Collisions between cold molecules in a superconducting magnetic trap

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Synopsis We decelerate and trap molecular oxygen using time-dependent magnetic fields and superconducting magnets. The density-dependent, non-exponential decay in particle number provides a clear proof of molecule-molecule collisions within the trapped ensemble. The spatial distribution of molecules in the trap is found to change over time, allowing to set limits on the ratio between the elastic and inelastic collision cross sections. Our experimental scheme opens up new possibilities for studying quantum effects in chemistry as well as for evaporative cooling of molecules.

Journal of Physics Conference Series•Jan 1, 2020•5 citations

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Abstract Trapping of atoms and molecules in electrostatic, magnetic and optical traps has enabled studying atomic and molecular interactions on a timescale of many seconds, allowing observations of ultra-cold collisions and reactions. Here we report the first magnetic deceleration and trapping of neutral carbon atoms in a static magnetic trap. When co-trapping the carbon atoms with oxygen molecules in a superconducting trap, the carbon signal decays in a non-exponential manner, consistent with the decay model describing losses resulting from atom-molecule collisions. Our findings pave the way to studying both elastic and inelastic collisions of species that cannot be laser cooled, and specifically may facilitate the observation of reactions at low temperatures, such as C + O2 → CO + O, which is important in interstellar chemistry.

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Molecular spin relaxation in cold atom-molecule scattering

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Elastic and spin-changing inelastic collision cross sections between oxygen molecules and helium atoms are computed in the energy range 0.1--10 K. This study explores the recently demonstrated buffer-gas cooling process that produces magnetically trapped, translationally cold molecules. Conclusions are drawn about the favorable prospects of cooling oxygen molecules, and about the general feasibility of this cooling technique.

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А. V. Avdeenkov, John L. Bohn

Collision cross sections and rate constants between two ground-state oxygen molecules are investigated theoretically at translational energies below $\ensuremath{\sim}1$ K and in zero magnetic field. We present calculations for elastic and spin-changing inelastic collision rates for different isotopic combinations of oxygen atoms as a prelude to understanding their collisional stability in ultracold magnetic traps. A numerical analysis has been made in the framework of a rigid-rotor model that accounts fully for the singlet, triplet, and quintet potential-energy surfaces in this system. The results offer insights into the effectiveness of evaporative cooling and the properties of molecular Bose-Einstein condensates, as well as estimates of collisional lifetimes in magnetic traps. Specifically, ${}^{17}{\mathrm{O}}_{2}$ looks like a good candidate for ultracold studies, while ${}^{16}{\mathrm{O}}_{2}$ is unlikely to survive evaporative cooling. Since ${}^{17}{\mathrm{O}}_{2}$ is representative of a wide class of molecules that are paramagnetic in their ground state we conclude that many molecules can be successfully magnetically trapped at ultralow temperatures.

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Collisions between laser-cooled atoms and molecules

Sarunas Jurgilas

This thesis describes an experiment to produce the first laser cooled mixtures of atoms and polar molecules in magneto-optical and magnetic quadrupole traps. Inelastic collision processes between CaF molecules and Rb atoms are studied in both trapping arrangements. When molecules occupy the first rotationally excited state it is found that loss from either trap occurs at near the universal rate due to collisions with co-trapped atoms. When the molecular sample is transferred to the ground rotational state the loss rate is suppressed by at least an order of magnitude. The experimental results are compared to a quantum scattering model based on quantum defect theory and the results seem to be encouraging for observing a high ratio of elastic to inelastic collision rates in this system. In addition, improvements to the experimental apparatus are presented which led to the first measurements of the elastic cross-section for collisions between CaF and Rb. Although at the currently attainable number densities no thermalization is observed, an upper bound on the value of the cross section is placed.

Bulletin of the American Physical Society•Jul 1, 2021•0 citations

Atom-Molecule Collisions in an Optically Trapped Gas

Nassim Zahzam, Thibault Vogt, Marcel Carto Constantin Mudrich +2 more

Cold inelastic collisions between confined cesium (Cs) atoms and ${\mathrm{Cs}}_{2}$ molecules are investigated inside a ${\mathrm{CO}}_{2}$ laser dipole trap. Inelastic atom-molecule collisions can be observed and measured with a rate coefficient of $\ensuremath{\sim}2.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\text{ }\text{ }{\mathrm{cm}}^{3}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$, mainly independent of the molecular rovibrational state populated. Lifetimes of purely atomic and molecular samples are essentially limited by rest gas collisions. The pure molecular trap lifetime ranges 0.3--1 s, 4 times smaller than the atomic one, as is also observed in a pure magnetic trap. We give an estimation of the inelastic molecule-molecule collision rate to be $\ensuremath{\sim}{10}^{\ensuremath{-}11}\text{ }\text{ }{\mathrm{cm}}^{3}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$.

arXiv:physics/0509197•Physical Review Letters•Jan 19, 2006•123 citations
Abstract Collisions between cold molecules are essential for studying fundamental aspects of quantum chemistry, and may enable the formation of quantum degenerate molecular matter by evaporative cooling. However, collisions between trapped, naturally occurring molecules have not been directly observed so far owing to the low collision rates of dilute samples. Here we report the direct observation of collisions between cold trapped molecules, without the need for laser cooling. We magnetically capture molecular oxygen in an 800-millikelvin-deep superconducting trap and set bounds on the ratio between the elastic- and inelastic-scattering rates—the key parameter determining the feasibility of evaporative cooling. We further co-trap atoms and molecules and identify collisions between them, paving the way for studies of cold interspecies collisions in a magnetic trap.
Synopsis We decelerate and trap molecular oxygen using time-dependent magnetic fields and superconducting magnets. The density-dependent, non-exponential decay in particle number provides a clear proof of molecule-molecule collisions within the trapped ensemble. The spatial distribution of molecules in the trap is found to change over time, allowing to set limits on the ratio between the elastic and inelastic collision cross sections. Our experimental scheme opens up new possibilities for studying quantum effects in chemistry as well as for evaporative cooling of molecules.
Abstract Trapping of atoms and molecules in electrostatic, magnetic and optical traps has enabled studying atomic and molecular interactions on a timescale of many seconds, allowing observations of ultra-cold collisions and reactions. Here we report the first magnetic deceleration and trapping of neutral carbon atoms in a static magnetic trap. When co-trapping the carbon atoms with oxygen molecules in a superconducting trap, the carbon signal decays in a non-exponential manner, consistent with the decay model describing losses resulting from atom-molecule collisions. Our findings pave the way to studying both elastic and inelastic collisions of species that cannot be laser cooled, and specifically may facilitate the observation of reactions at low temperatures, such as C + O2 → CO + O, which is important in interstellar chemistry.
Collision cross sections and rate constants between two ground-state oxygen molecules are investigated theoretically at translational energies below $\ensuremath{\sim}1$ K and in zero magnetic field. We present calculations for elastic and spin-changing inelastic collision rates for different isotopic combinations of oxygen atoms as a prelude to understanding their collisional stability in ultracold magnetic traps. A numerical analysis has been made in the framework of a rigid-rotor model that accounts fully for the singlet, triplet, and quintet potential-energy surfaces in this system. The results offer insights into the effectiveness of evaporative cooling and the properties of molecular Bose-Einstein condensates, as well as estimates of collisional lifetimes in magnetic traps. Specifically, 17O2{}^{17}{\mathrm{O}}_{2}17O2​ looks like a good candidate for ultracold studies, while 16O2{}^{16}{\mathrm{O}}_{2}16O2​ is unlikely to survive evaporative cooling. Since 17O2{}^{17}{\mathrm{O}}_{2}17O2​ is representative of a wide class of molecules that are paramagnetic in their ground state we conclude that many molecules can be successfully magnetically trapped at ultralow temperatures.
This thesis describes an experiment to produce the first laser cooled mixtures of atoms and polar molecules in magneto-optical and magnetic quadrupole traps. Inelastic collision processes between CaF molecules and Rb atoms are studied in both trapping arrangements. When molecules occupy the first rotationally excited state it is found that loss from either trap occurs at near the universal rate due to collisions with co-trapped atoms. When the molecular sample is transferred to the ground rotational state the loss rate is suppressed by at least an order of magnitude. The experimental results are compared to a quantum scattering model based on quantum defect theory and the results seem to be encouraging for observing a high ratio of elastic to inelastic collision rates in this system. In addition, improvements to the experimental apparatus are presented which led to the first measurements of the elastic cross-section for collisions between CaF and Rb. Although at the currently attainable number densities no thermalization is observed, an upper bound on the value of the cross section is placed.
Cold inelastic collisions between confined cesium (Cs) atoms and Cs2{\mathrm{Cs}}_{2}Cs2​ molecules are investigated inside a CO2{\mathrm{CO}}_{2}CO2​ laser dipole trap. Inelastic atom-molecule collisions can be observed and measured with a rate coefficient of $\ensuremath{\sim}2.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\text{ }\text{ }{\mathrm{cm}}^{3}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$, mainly independent of the molecular rovibrational state populated. Lifetimes of purely atomic and molecular samples are essentially limited by rest gas collisions. The pure molecular trap lifetime ranges 0.3--1 s, 4 times smaller than the atomic one, as is also observed in a pure magnetic trap. We give an estimation of the inelastic molecule-molecule collision rate to be $\ensuremath{\sim}{10}^{\ensuremath{-}11}\text{ }\text{ }{\mathrm{cm}}^{3}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$.