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Inelastic collisions of atomic thorium and molecular thorium monoxide with cold helium-3

Yat Shan Au

Published Jun 6, 2014• 0 citations
OpenAlex

Abstract

We measure inelastic cross sections for atomic thorium (Th) and molecular thorium monoxide (ThO) in collisions with $^3$He at temperatures near 1 K. We determine the Zeeman relaxation cross section for Th ($^3$F$_2$) to be $\\sim 2 \\times 10^{-17}$~cm$^{-2}$ at 800~mK. We study electronic inelastic processes in Th ($^3$P$_0$) and find no quenching even after $10^6$ collisions at 800~mK. We measure the vibrational quenching cross section for ThO~(X,~$\\nu=1$) to be $(7.9 \\pm 2.7) \\times 10^{-19}$~cm$^{-2}$ at 800~mK. Finally, we observe indirect evidence for ThO (X, $\\nu=0$)--$^3$He van der Waals complex formation, and measure the 3-body recombination rate constant to be $\\Gamma_3 = (8 \\pm 2) \\times 10^{-33}$~cm$^6$s$^{-1}$ at 2.4~K. The stability of the ground Th ($^3$F$_2$) state, metastable Th ($^3$P$_0$) state, and vibrational excited ThO (X, $\\nu=1$) state provides data on anisotropic interactions in new systems and opens up the possibility for further studies and experiments, including trapping.

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Physical Review A•Aug 2, 2006•15 citations
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}sim2times10−17cm−2^{-2}−2 at 800mK. We study electronic inelastic processes in Th (3^33P0_00​) and find no quenching even after 10610^6106 collisions at 800mK. 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)times10−19cm−2^{-2}−2 at 800mK. 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)times10−33cm6^66s−1^{-1}−1 at 2.4K. 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.
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].
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.
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=1v=1v=1, J=0J=0J=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, ${\ensuremath{\Gamma}}_{3}=8\ifmmode\pm\else\textpm\fi{}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}33}$ cm6{\mathrm{cm}}^{6}cm6 ${\mathrm{s}}^{\ensuremath{-}1}$.
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.
NH radicals are magnetically trapped and their Zeeman relaxation and energy transport collision cross sections with helium are measured. Continuous buffer-gas loading of the trap is direct from a room-temperature molecular beam. The Zeeman relaxation (inelastic) cross section of magnetically trapped electronic, vibrational, and rotational ground state NH molecules in collisions with 3He^{3}\mathrm{He}3He is measured to be $3.8\ifmmode\pm\else\textpm\fi{}1.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}\text{ }\text{ }{\mathrm{cm}}^{2}$ at 710 mK. The NH-He energy transport cross section is also measured, indicating a ratio of diffusive to inelastic cross sections of $\ensuremath{\gamma}=7\ifmmode\times\else\texttimes\fi{}{10}^{4}$, in agreement with recent theory [R. V. Krems, H. R. Sadeghpour, A. Dalgarno, D. Zgid, J. K\l{}os, and G. Cha\l{}asi\ifmmode \acute{n}\else \'{n}\fi{}ski, Phys. Rev. A 68, 051401 (2003)].
The dynamics of magnetic relaxation of Tm atoms in cold helium gas is investigated using ab initio interaction potentials between Tm(2F)\mathrm{Tm}(^{2}F)Tm(2F) and He atoms. The interaction of Tm(2F)\mathrm{Tm}(^{2}F)Tm(2F) with He gives rise to four adiabatic potentials of ${\ensuremath{\Sigma}}^{+}$, $\ensuremath{\Pi}$, $\ensuremath{\Delta}$, and $\ensuremath{\Phi}$ symmetries, which are found to be degenerate to within $0.1\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ at all interatomic distances larger than R=5.4A˚R=5.4\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}R=5.4A˚. The small splitting between the interaction potentials leads to suppression of Zeeman transitions in Tm-He collisions. Our quantum scattering calculations yield a small rate constant of $1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{3}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ for Zeeman relaxation of the Tm(2F7∕2)\mathrm{Tm}(^{2}F_{7∕2})Tm(2F7∕2​) atoms in the maximally stretched M=7∕2M=7∕2M=7∕2 sublevel at the temperature 0.8K0.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}0.8K, in reasonable agreement with the measured value $(5.0\ifmmode\pm\else\textpm\fi{}2.1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{3}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. The sensitivity of Zeeman-relaxation cross sections to details of the interaction potentials is examined. To our knowledge, this is one of only two comparisons of ab initio quantum scattering calculations with experimental measurements of inelastic collisions at subkelvin temperatures near one Kelvin. Our work thus demonstrates the feasibility of describing cold collisions from first principles.