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Physical Review A

Molecular spin relaxation in cold atom-molecule scattering

John L. Bohn

Published Mar 6, 2000• 26 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.61.040702

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arXiv:1209.3071•The Journal of Chemical Physics•Sep 10, 2012•10 citations
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.
Collision cross sections between oxygen molecules and helium atoms are computed at translational energies between 0.1 K and 10.0 K, motivated by the recently demonstrated cooling of molecules by a helium buffer gas. Detailed calculations based on a rigid-rotor model demonstrate the differences among various isotopic combinations of oxygen atoms and also serve to illustrate resonant features unique to cold molecules. Comparisons of elastic versus spin-changing inelastic collision rates suggest that buffer-gas cooling is likely to be a widely applicable tool for producing cold molecular samples.
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.
We report on the buffer-gas cooling and trapping of CrH and MnH molecules in a magnetic quadrupole trap with densities on the order of ${10}^{6}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ at a temperature of 650mK650\phantom{\rule{0.3em}{0ex}}\mathrm{mK}650mK. Storage times of up to 180ms180\phantom{\rule{0.3em}{0ex}}\mathrm{ms}180ms have been observed, corresponding to a 20-fold lifetime enhancement with respect to the field-free diffusion through the 3He^{3}\mathrm{He}3He buffer-gas. Using Monte Carlo trajectory simulations, inelastic molecule-3He^{3}\mathrm{He}3He collision cross sections of $1.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}$ and $3.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}$ are extracted for CrH and MnH, respectively. Furthermore, elastic molecule-3He^{3}\mathrm{He}3He collision cross sections of $1.4(\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}$ are determined for both species. We conclude that the confinement time of these molecules in a magnetic trapping field is limited by inelastic collisions with the helium atoms leading to Zeeman relaxation.
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.
Cold collisions of polyatomic molecular radicals with S-state atoms in a magnetic field: An ab initio study of He + CH2(X~){\rm CH}_2(\tilde{X})CH2​(X~) CH 2(X) collisions
We develop a rigorous quantum mechanical theory for collisions of polyatomic molecular radicals with S-state atoms in the presence of an external magnetic field. The theory is based on a fully uncoupled space-fixed basis set representation of the multichannel scattering wave function. Explicit expressions are presented for the matrix elements of the scattering Hamiltonian for spin-1/2 and spin-1 polyatomic molecular radicals interacting with structureless targets. The theory is applied to calculate the cross sections and thermal rate constants for spin relaxation in low-temperature collisions of the prototypical organic molecule methylene [CH(2)(X(3)B(1))] with He atoms. To this end, two accurate three-dimensional potential energy surfaces (PESs) of the He-CH(2)(X(3)B(1)) complex are developed using the state-of-the-art coupled-cluster method including single and double excitations along with a perturbative correction for triple excitations and large basis sets. Both PESs exhibit shallow minima and are weakly anisotropic. Our calculations show that spin relaxation in collisions of CH(2), CHD, and CD(2) molecules with He atoms occurs at a much slower rate than elastic scattering over a large range of temperatures (1 μK-1 K) and magnetic fields (0.01-1 T), suggesting excellent prospects for cryogenic helium buffer-gas cooling of ground-state ortho-CH(2)(X(3)B(1)) molecules in a magnetic trap. Furthermore, we find that ortho-CH(2) undergoes collision-induced spin relaxation much more slowly than para-CH(2), which indicates that magnetic trapping can be used to separate nuclear spin isomers of open-shell polyatomic molecules.