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arXiv:1310.7279 · Physical Review A

Vibrational quenching of the electronic ground state in ThO in cold collisions with He3

Yat Shan Au, Colin B. Connolly, Wolfgang Ketterle, John M. Doyle

Published Sep 3, 2014• 4 citations
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Abstract

We measure the ratio $\ensuremath{\gamma}$ of the momentum transfer--to--vibrational quenching cross section for molecular thorium monoxide (ThO) [$X{(}^{1}{\ensuremath{\Sigma}}^{+})$, $v=1$, $J=0$] in collisions with atomic helium between 800 mK and 2.4 K. We find $\ensuremath{\gamma}\ensuremath{\sim}{10}^{4}$. We also observe indirect evidence for ThO-He van der Waals complex formation, which has been predicted by theory [Tscherbul, Sayfutyarova, Buchachenko, and Dalgarno, J. Chem. Phys. 134, 144301 (2011)], and in conjunction, we determine the three-body recombination rate constant at 2.4 K, ${\ensuremath{\Gamma}}_{3}=8\ifmmode\pm\else\textpm\fi{}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}33}$ ${\mathrm{cm}}^{6}$ ${\mathrm{s}}^{\ensuremath{-}1}$.

DOI
10.1103/physreva.90.032703

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We measure the ratio \ensuremathγ\ensuremath{\gamma}\ensuremathγ of the momentum transfer--to--vibrational quenching cross section for molecular thorium monoxide (ThO) [X(1\ensuremathΣ+)X{(}^{1}{\ensuremath{\Sigma}}^{+})X(1\ensuremathΣ+), v=1v=1v=1, J=0J=0J=0] in collisions with atomic helium between 800 mK and 2.4 K. We find \ensuremathγ\ensuremath∼104\ensuremath{\gamma}\ensuremath{\sim}{10}^{4}\ensuremathγ\ensuremath∼104. We also observe indirect evidence for ThO-He van der Waals complex formation, which has been predicted by theory [Tscherbul, Sayfutyarova, Buchachenko, and Dalgarno, J. Chem. Phys. 134, 144301 (2011)], and in conjunction, we determine the three-body recombination rate constant at 2.4 K, \ensuremathΓ3=8\ifmmode±\else\textpm\fi2\ifmmode×\else\texttimes\fi10\ensuremath−33{\ensuremath{\Gamma}}_{3}=8\ifmmode\pm\else\textpm\fi{}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}33}\ensuremathΓ3​=8\ifmmode±\else\textpm\fi2\ifmmode×\else\texttimes\fi10\ensuremath−33 cm6{\mathrm{cm}}^{6}cm6 s\ensuremath−1{\mathrm{s}}^{\ensuremath{-}1}s\ensuremath−1.
We measure the ratio γ of the momentum transfer–to–vibrational quenching cross section for molecular thorium monoxide (ThO) [X([superscript 1]Σ[superscript +]), v = 1, J = 0] in collisions with atomic helium between 800 mK and 2.4 K. We find γ ∼ 10[superscript 4]. We also observe indirect evidence for ThO-He van der Waals complex formation, which has been predicted by theory [Tscherbul, Sayfutyarova, Buchachenko, and Dalgarno, J. Chem. Phys. 134, 144301 (2011)], and in conjunction, we determine the three-body recombination rate constant at 2.4 K, Γ[subscript 3] = 8 ± 2 × 10[superscript −33] cm[superscript 6] s[superscript −1].
We measure inelastic cross sections for atomic thorium (Th) and molecular thorium monoxide (ThO) in collisions with 3^33He at temperatures near 1 K. We determine the Zeeman relaxation cross section for Th (3^33F2_22​) to be sim2times10−17\\sim 2 \\times 10^{-17}sim2times10−17~cm−2^{-2}−2 at 800~mK. We study electronic inelastic processes in Th (3^33P0_00​) and find no quenching even after 10610^6106 collisions at 800~mK. We measure the vibrational quenching cross section for ThO~(X,~nu=1\\nu=1nu=1) to be (7.9pm2.7)times10−19(7.9 \\pm 2.7) \\times 10^{-19}(7.9pm2.7)times10−19~cm−2^{-2}−2 at 800~mK. Finally, we observe indirect evidence for ThO (X, nu=0\\nu=0nu=0)--3^33He van der Waals complex formation, and measure the 3-body recombination rate constant to be Gamma3=(8pm2)times10−33\\Gamma_3 = (8 \\pm 2) \\times 10^{-33}Gamma3​=(8pm2)times10−33~cm6^66s−1^{-1}−1 at 2.4~K. The stability of the ground Th (3^33F2_22​) state, metastable Th (3^33P0_00​) state, and vibrational excited ThO (X, nu=1\\nu=1nu=1) state provides data on anisotropic interactions in new systems and opens up the possibility for further studies and experiments, including trapping.
Inelastic cross sections for collisions between thorium (Th) and helium (3He^{3}\mathrm{He}3He) are measured. For Th[3F2{}^{3}{F}_{2}3F2​]--3He^{3}\mathrm{He}3He, we determine the ratio of momentum transfer to Zeeman relaxation cross sections to be $\ensuremath{\gamma}\ensuremath{\sim}500$ at 800 mK. For Th[3P0{}^{3}{P}_{0}3P0​]--3He^{3}\mathrm{He}3He, we find no quenching of this metastable state during 106{10}^{6}106 collisions. We measure the radiative lifetime of Th[3P0{}^{3}P{}_{0}3P0​] to be $\ensuremath{\tau}>130$ ms. The observed stability of Th[3P0{}^{3}{P}_{0}3P0​] opens up the possibility of trapping this metastable species.
Inelastic cross sections for collisions between thorium (Th) and helium ([superscript 3]He) are measured. For Th[[superscript 3]F[subscript 2]]–[superscript 3]He, we determine the ratio of momentum transfer to Zeeman relaxation cross sections to be γ ∼ 500 at 800 mK. For Th[[superscript 3]P[subscript 0]]–[superscript 3]He, we find no quenching of this metastable state during 10[superscript 6] collisions. We measure the radiative lifetime of Th[[superscript 3]P[subscript 0]] to be τ > 130 ms. The observed stability of Th[[superscript 3]P[subscript 0]] opens up the possibility of trapping this metastable species.
Helium dimer in its metastable triplet electronic state, He2(a 3Σu+)_2(a~^3Σ_u^+)2​(a 3Σu+​), is a promising candidate for direct laser cooling and precision measurements. Interactions and collisions involving He2(a 3Σu+)_2(a~^3Σ_u^+)2​(a 3Σu+​) remain unexplored in ultracold and cold energy domains. Here, we present the first theoretical investigation of intermolecular interactions and collision dynamics of He2(a 3Σu+)_2(a~^3Σ_u^+)2​(a 3Σu+​)+Ne van der Waals complex. For the ground electronic state, X 3A′X~^3\mathrm{A}'X 3A′, we employ a spin-restricted coupled cluster method with single, double, and noniterative triple excitations. The resulting interaction potential exhibits an extremely shallow well with a depth of 4.612 cm−1^{-1}−1. We construct a three-dimensional vibrationally averaged potential energy surface. Using this surface, we perform full close-coupling calculations including fine-structure levels of He2(a 3Σu+)_2(a~^3Σ_u^+)2​(a 3Σu+​) to investigate collisional excitation and quenching induced by Ne. We analyze the low-energy scattering dynamics and model resonant features associated with quasi-bound states supported by interaction potential. Propensity rule for fine-structure conserving transitions is established. We additionally consider He2_22​+Ar collisions and compare their energy dependence and resonance structure with those of He2_22​+Ne. The He2_22​+Ar system exhibits a richer resonance structure, highlighting the sensitivity of low-energy scattering to interaction potential and reduced mass. Calculated PES and scattering properties provide a reliable benchmark for description of He2_22​+Ne van der Waals complex and may be useful for interpretation of future experimental spectra and collision measurements.
The molecular anion ${{\mathrm{C}}_{2}}^{\ensuremath{-}}$ is currently of interest as a candidate for laser cooling due to its electronic structure and favorable branching ratios to the ground electronic and vibrational states. Helium has been proposed as a buffer gas to cool the molecule's internal motion. We calculate the cross sections and corresponding rates for rovibrational inelastic collisions of ${\mathrm{C}}_{2}{}^{\ensuremath{-}}$ with He, and also with Ne and Ar, on three-dimensional ab initio potential energy surfaces using quantum scattering theory. The rates for vibrational quenching with He and Ne are very small and are similar to those for small neutral molecules in collision with helium. The quenching rates for Ar, however, are far larger than those with the other noble gases, suggesting that this may be a more suitable gas for driving vibrational quenching in traps. The implications of these results for laser cooling of ${{\mathrm{C}}_{2}}^{\ensuremath{-}}$ are discussed.