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arXiv:1305.6282 · Physical Review Letters

Ultracold Hydrogen Atoms: A Versatile Coolant to Produce Ultracold Molecules

Maykel Leonardo González-Martínez, Jeremy M. Hutson

Published Nov 12, 2013• 16 citations
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Abstract

We show theoretically that ultracold hydrogen atoms have very favorable properties for sympathetic cooling of molecules to microkelvin temperatures. We calculate the potential energy surfaces for spin-polarized interactions of H atoms with the prototype molecules $\mathrm{NH}(^{3}\ensuremath{\Sigma}^{\ensuremath{-}})$ and $\mathrm{OH}(^{2}\ensuremath{\Pi})$ and show that they are shallow (50 to $80\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$) and only weakly anisotropic. We carry out quantum collision calculations on $\mathrm{H}+\mathrm{NH}$ and $\mathrm{H}+\mathrm{OH}$ and show that the ratio of elastic to inelastic cross sections is high enough to allow sympathetic cooling from temperatures well over 1 K for NH and around 250 mK for OH.

DOI
10.1103/physrevlett.111.203004

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Acta Physica Sinica•Jan 1, 2018•0 citations
We show theoretically that ultracold hydrogen atoms have very favorable properties for sympathetic cooling of molecules to microkelvin temperatures. We calculate the potential energy surfaces for spin-polarized interactions of H atoms with the prototype molecules NH(3\ensuremathΣ\ensuremath−)\mathrm{NH}(^{3}\ensuremath{\Sigma}^{\ensuremath{-}})NH(3\ensuremathΣ\ensuremath−) and OH(2\ensuremathΠ)\mathrm{OH}(^{2}\ensuremath{\Pi})OH(2\ensuremathΠ) and show that they are shallow (50 to 80  cm\ensuremath−180\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}80  cm\ensuremath−1) and only weakly anisotropic. We carry out quantum collision calculations on H+NH\mathrm{H}+\mathrm{NH}H+NH and H+OH\mathrm{H}+\mathrm{OH}H+OH and show that the ratio of elastic to inelastic cross sections is high enough to allow sympathetic cooling from temperatures well over 1 K for NH and around 250 mK for OH.
We consider the prospect of using ultracold hydrogen atoms for sympathetic cooling of fluorine atoms to microkelvin temperatures. We carry out quantum-mechanical calculations on collisions between cold F and H atoms in magnetically trappable states and show that the ratio of elastic to inelastic cross sections remains high across a wide range of temperatures and magnetic fields. For F atoms initially in the spin-stretched state (2P3/2{}^{2}{P}_{3/2}2P3/2​, f=mf=+2f={m}_{f}=+2f=mf​=+2), sympathetic cooling appears likely to succeed from starting temperatures around 1 K or even higher. This occurs because inelastic collisions are suppressed by ppp-wave and ddd-wave barriers that are 600 mK and 3.2 K high, respectively. In combination with recent results on H+NH\mathrm{H}+\mathrm{NH}H+NH and H+OH\mathrm{H}+\mathrm{OH}H+OH collisions [M. L. Gonz\'alez-Mart\'{\i}nez and J. M. Hutson, Phys. Rev. Lett. 111, 203004 (2013)], this establishes ultracold H atoms as a very promising and versatile coolant for atoms and molecules that cannot be laser-cooled.
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 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.
Sympathetic cooling is one of the most promising techniques for producing ultracold molecules from precooled molecules. Previous researches have shown that it is inadequate to use the ultracold alkali-metal atoms as coolant for sympathetic cooling. To explore the possibility of ultracold alkali-earth-metal atoms as coolant, in this paper a theoretical investigation is performed of the cold collision dynamics for Xe-NH(X3∑-) system in magnetic fields. The interaction potential energies of Xe-NH complex are calculated respectively by using the single and double excitation coupled-cluster theory with the noniterative treatment of triple excitations[CCSD(T)] method and complete basis set limit extrapolated method. An analytic express of potential energy surface (PES) is given for the first time. A single global minimum value occurs at R=7.14a0, θ=102.76° with an energy of-153.54 cm-1, and the PES has a weak anisotropy. Combine the ab initio PES with quantum scattering theory, then the cold collisional dynamics of Xe-NH system in a magnetic field will be studied. The elastic and inelastic transition cross sections and their ratios of NH molecules in the lowest low-field following state (n=0, mj=1) under different magnetic fields and collisional energies are calculated. The results show that the elastic cross section is independent of magnetic field, and the inelastic cross section changes with magnetic field, especially at an ultracold temperature. A common rule of thumb is that to successfully implement cooling, the ratio of elastic cross section to inelastic cross section needs to reach 100 at least. The results suggest that it is likely to be a challenging work to perform sympathetic cooling of NH molecule by ultracold Xe atom.