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arXiv:1703.07192 · The Journal of Physical Chemistry A

Ab Initio Study of Chemical Reactions of Cold SrF and CaF Molecules with Alkali-Metal and Alkaline-Earth-Metal Atoms: The Implications for Sympathetic Cooling

Maciej B. Kosicki, Dariusz Kędziera, Piotr Szymon Żuchowski

Published May 1, 2017• 17 citations
arXivHTMLPDF

Abstract

We investigate the energetics of the atom exchange reaction in the SrF + alkali-metal atom and CaF + alkali-metal atom systems. Such reactions are possible only for collisions of SrF and CaF with the lithium atoms, while they are energetically forbidden for other alkali-metal atoms. Specifically, we focus on SrF interacting with Li, Rb, and Sr atoms and use ab initio methods to demonstrate that the SrF + Li and SrF + Sr reactions are barrierless. We present potential energy surfaces for the interaction of the SrF molecule with the Li, Rb, and Sr atoms in their energetically lowest-lying electronic spin states. The obtained potential energy surfaces are deep and exhibit profound interaction anisotropies. We predict that the collisions of SrF molecules in the rotational or Zeeman excited states most likely have a strong inelastic character. We discuss the prospects for the sympathetic cooling of SrF and CaF molecules using ultracold alkali-metal atoms.

DOI
10.1021/acs.jpca.7b01523

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We investigate the energetics of the atom exchange reaction in the SrF + alkali-metal atom and CaF + alkali-metal atom systems. Such reactions are possible only for collisions of SrF and CaF with the lithium atoms, while they are energetically forbidden for other alkali-metal atoms. Specifically, we focus on SrF interacting with Li, Rb, and Sr atoms and use ab initio methods to demonstrate that the SrF + Li and SrF + Sr reactions are barrierless. We present potential energy surfaces for the interaction of the SrF molecule with the Li, Rb, and Sr atoms in their energetically lowest-lying electronic spin states. The obtained potential energy surfaces are deep and exhibit profound interaction anisotropies. We predict that the collisions of SrF molecules in the rotational or Zeeman excited states most likely have a strong inelastic character. We discuss the prospects for the sympathetic cooling of SrF and CaF molecules using ultracold alkali-metal atoms.
) with alkali-metal (Li, Na, K, Rb, Cs) and alkaline-earth-metal (Mg, Ca, Sr, Ba) atoms. State-of-the-art ab initio techniques are applied to compute the potential energy surfaces (PESs) for these systems. The coupled cluster method restricted to single, double, and noniterative triple excitations, CCSD(T), is employed and the scalar relativistic effects in heavier metal atoms are modeled within the small-core energy-consistent pseudopotentials. The leading long-range isotropic and anisotropic induction and dispersion interaction coefficients are obtained within the perturbation theory. The PESs are characterized in detail and their universal similarities typical for systems dominated by the induction interaction are discussed. The two-dimensional PESs are provided for selected systems and can be employed in scattering calculations. The possible channels of chemical reactions and their control are analyzed based on the energetics of the reactants. The present study of the electronic structure is the first step towards the evaluation of prospects for sympathetic cooling and controlled chemistry of linear polyatomic ions with ultracold atoms.
We use accurate ab initio and quantum scattering calculations to explore the prospects for sympathetic cooling of the heavy molecular radical $\mathrm{SrOH}{(}^{2}{\mathrm{\ensuremath{\Sigma}}}^{+})$ by ultracold Li atoms in a magnetic trap. Our ab initio calculations show that the chemical reaction between spin-polarized Li and SrOH, which occurs on the triplet Li-SrOH potential energy surface (PES), is strongly endothermic and hence energetically forbidden at ultralow temperatures. The chemical reaction $\mathrm{Li}(^{2}S)+\mathrm{SrOH}{(}^{2}{\mathrm{\ensuremath{\Sigma}}}^{+})\ensuremath{\rightarrow}\phantom{\rule{4pt}{0ex}}\mathrm{Sr}(^{1}S)+\mathrm{LiOH}{(}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+})$ occurs barrierlessly on the singlet PES and is exothermic by 2505 ${\mathrm{cm}}^{\ensuremath{-}1}$. A two-dimensional PES for the triplet electronic state of Li-SrOH is calculated ab initio using the partially spin-restricted coupled cluster method with single, double, and perturbative triple excitations and a large correlation-consistent basis set. The highly anisotropic PES has a deep global minimum in the skewed Li-HOSr geometry with ${D}_{e}=4932\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ and saddle points in collinear configurations. Our quantum scattering calculations predict low spin-relaxation rates in fully spin-polarized Li + SrOH collisions with the ratios of elastic to inelastic collision rates well in excess of 100 over a wide range of magnetic fields (1--1000 G) and collision energies (${10}^{\ensuremath{-}5}$ to 0.1 K), suggesting favorable prospects for sympathetic cooling of SrOH molecules with spin-polarized Li atoms in a magnetic trap. We find that spin relaxation in Li + SrOH collisions occurs via a direct mechanism mediated by the magnetic dipole-dipole interaction between the electron spins of Li and SrOH, and that the indirect (spin-rotation) mechanism is strongly suppressed. The upper limit to the Li + SrOH reaction rate coefficient calculated for the singlet PES using adiabatic capture theory is found to decrease from $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{3}$/s to a limiting value of $3.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{3}$/s with decreasing temperature from 0.1 K to 1 $\ensuremath{\mu}\mathrm{K}$.
We explore the potential energy surfaces for NH molecules interacting with alkali-metal and alkaline-earth atoms using highly correlated ab initio electronic structure calculations. The surfaces for interaction with alkali-metal atoms have deep wells dominated by covalent forces. The resulting strong anisotropies will produce strongly inelastic collisions. The surfaces for interaction with alkaline-earth atoms have shallower wells that are dominated by induction and dispersion forces. For Be and Mg the anisotropy is small compared to the rotational constant of NH, so that collisions will be relatively weakly inelastic. Be and Mg are thus promising coolants for sympathetic cooling of NH to the ultracold regime.
We have performed a systematic ab initio study on alkali and alkaline earth hydroxide neutral (MOH) and anionic (MOH−) species where M = Li, Na, K, Rb, Cs or Be, Mg, Ca, Sr, Ba. The CCSD(T) method with extended basis sets and Dirac-Fock relativistic effective core potentials for the heavier atoms has been used to study their equilibrium geometries, interaction energies, electron affinities, electric dipole moment, and potential energy surfaces. All neutral and anionic species exhibit a linear shape with the exception of BeOH, BeOH−, and MgOH−, for which the equilibrium structure is found to be bent. Our analysis shows that the alkaline earth hydroxide anions are valence-bound whereas the alkali hydroxide anions are dipole bound. In the context of sympathetic cooling of OH− by collision with ultracold alkali and alkaline earth atoms, we investigate the 2D MOH− potential energy surfaces and the associative detachment reaction M + OH→− MOH + e−, which is the only energetically allowed reactive channel in the cold regime. We discuss the implication for the sympathetic cooling of OH− and conclude that Li and K are the best candidates for an ultracold buffer gas.
We theoretically characterize interactions, energetics, and chemical reaction paths in ionic two-body and three-body systems of alkali-metal and alkaline-earth-metal atoms in the context of modern experiments with cold hybrid ion-atom mixtures. Using ab initio techniques of quantum chemistry such as the coupled-cluster method, we calculate ground-state electronic properties of all diatomic AB+A{B}^{+}AB+ and most of the triatomic A2B+{A}_{2}{B}^{+}A2​B+ molecular ions consisting of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, and Yb atoms. Different geometries and wave-function symmetries of the ground state are found for different classes of molecular ions. We analyze intermolecular interactions in the investigated systems including additive two-body and nonadditive three-body ones. As an example we provide two-dimensional interaction potential-energy surfaces for KRb++K{\mathrm{KRb}}^{+}+\mathrm{K}KRb++K and Rb++Sr2{\mathrm{Rb}}^{+}+{\mathrm{Sr}}_{2}Rb++Sr2​ mixtures. We identify possible channels of chemical reactions based on the energetics of the reactants. The present results may be useful for investigating controlled chemical reactions and other applications of molecular ions formed in cold hybrid ion-atom systems.
The prospects of sympathetic cooling of polar molecules with magnetically cotrapped alkali-metal atoms are generally considered poor due to strongly anisotropic atom-molecule interactions leading to large spin relaxation rates. Using rigorous quantum scattering calculations based on ab initio interaction potentials, we show that inelastic spin relaxation in low-temperature collisions of CaH(${}^{2}\ensuremath{\Sigma}$) molecules with Li and Mg atoms occurs at a slow rate despite the strongly anisotropic interactions. This unexpected result, which we rationalize using multichannel quantum-defect theory, opens up the possibility of sympathetic cooling of polar ${}^{2}\ensuremath{\Sigma}$ molecules with alkali-metal atoms in a magnetic trap and with alkaline-earth-metal atoms in an optical dipole trap.