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arXiv:cond-mat/0307466 · Physical Review Letters

Interaction of NH(X Σ−3) Molecules with Rubidium Atoms: Implications for Sympathetic Cooling and the Formation of Extremely Polar Molecules

Pavel Soldán, Jeremy M. Hutson

Published Apr 23, 2004• 61 citations
arXivHTMLPDF

Abstract

The Rb-NH interaction is investigated as a prototype for interactions between alkali-metal atoms and stable molecules. For spin-aligned Rb and NH that interact on a quartet surface (4A"), the interaction is relatively weak, with a well depth of 0.078 eV. However, there are also doublet surfaces of ion-pair character that are very much deeper (well depth 1.372 eV). They may be important for atom-molecule collision rates and offer the possibility of forming strongly dipolar molecules by photoassociation. Similar deeply bound ion-pair states are likely to exist for other alkali atom-molecule pairs.

DOI
10.1103/physrevlett.92.163202

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The Rb-NH interaction is investigated as a prototype for interactions between alkali-metal atoms and stable molecules. For spin-aligned Rb and NH that interact on a quartet surface (4A"), the interaction is relatively weak, with a well depth of 0.078 eV. However, there are also doublet surfaces of ion-pair character that are very much deeper (well depth 1.372 eV). They may be important for atom-molecule collision rates and offer the possibility of forming strongly dipolar molecules by photoassociation. Similar deeply bound ion-pair states are likely to exist for other alkali atom-molecule pairs.
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.
Using ab initio calculations for the lowest, spin-stretched, quartet potential energy surfaces for Rb and Cs neutral atoms interacting with the $\text{NH}({^{3}\ensuremath{\Sigma}}^{\ensuremath{-}})$ polar target, the cold collision quantum dynamics is analyzed at ultralow energies to examine possible propensity rules existing for the relaxation of either spin or rotational target states (or both) via sympathetic cooling induced by the partner atoms. The specific coupling of Hund's case (b) is adopted for the dynamical treatment of spin and rotational angular momenta and the coupled channel approach is employed to solve the scattering equations. Both rotational and/or spin relaxation cross sections are obtained down to ${10}^{\ensuremath{-}6}\text{ }{\text{cm}}^{\ensuremath{-}1}$ of energy and corresponding cooling rates are analyzed and compared between the two systems.
We investigate the possibility of producing ultracold NH3\mathrm{N}{\mathrm{H}}_{3}NH3​ molecules by sympathetic cooling in a bath of ultracold atoms. We consider the interactions of NH3\mathrm{N}{\mathrm{H}}_{3}NH3​ with alkali-metal and alkaline-earth-metal atoms, and with Xe, using ab initio coupled-cluster calculations. For $\mathrm{Rb}\text{\ensuremath{-}}\mathrm{N}{\mathrm{H}}_{3}$ and $\mathrm{Xe}\text{\ensuremath{-}}\mathrm{N}{\mathrm{H}}_{3}$ we develop full potential energy surfaces, while for the other systems we characterize the stationary points (global and local minima and saddle points). We also calculate isotropic and anisotropic van der Waals C6{C}_{6}C6​ coefficients for all the systems. The potential energy surfaces for interaction of NH3\mathrm{N}{\mathrm{H}}_{3}NH3​ with alkali-metal and alkaline-earth-metal atoms all show deep potential wells and strong anisotropies. The well depths vary from $887\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ for $\mathrm{Mg}\text{\ensuremath{-}}\mathrm{N}{\mathrm{H}}_{3}$ to $5104\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ for $\mathrm{Li}\text{\ensuremath{-}}\mathrm{N}{\mathrm{H}}_{3}$. This suggests that all these systems will exhibit strong inelasticity whenever inelastic collisions are energetically allowed and that sympathetic cooling will work only when both the atoms and the molecules are already in their lowest internal states. $\mathrm{Xe}\text{\ensuremath{-}}\mathrm{N}{\mathrm{H}}_{3}$ is more weakly bound and less anisotropic.
The relative size of the elastic cross sections between NH\mathrm{NH}NH molecules colliding with Cs and Rb atoms on the lowest potential energy surfaces of spin-stretched quartet symmetry are computed using the full quantum treatment and taking the target molecule to be in its ground rotational state. Results are compared between the two systems under similar dynamics and the Rb atom is found to yield larger cross sections at ultralow energies. The process is seen to be dominated by dispersion interaction and an analysis of the effects on the cross sections induced by changes on the C6{C}_{6}C6​ coefficients is carried out in detail for both systems.
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The potential energy curves, permanent and transition dipole moments, and the static dipo-lar polarizability, of molecular ions composed of one alkali-metal atom and a Strontium ion are determined with a quantum chemistry approach. The molecular ions are treated as effective two-electron systems and are treated using effective core potentials including core polarization, large gaussian basis sets, and full configuration interaction. In the perspective of upcoming experiments aiming at merging cold atom and cold ion traps, possible paths for radiative charge exchange, photoassociation of a cold Lithium or Rubidium atom and a Strontium ion are discussed, as well as the formation of stable molecular ions. PACS numbers: 31.15.AR,31.15.Ct,31.50.Be,31.50.Df ∗O. Dulieu: