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arXiv:0909.2913 · Physical Review A

Quenching of para- H2 with an ultracold antihydrogen atom H¯1s

Renat A. Sultanov, Sadhan K. Adhikari, Dennis C. Guster

Published Feb 8, 2010• 3 citations
arXivHTMLPDF

Abstract

In this work we report the results of calculation for quantum-mechanical rotational transitions in molecular hydrogen, ${\mathrm{H}}_{2}$, induced by an ultracold ground-state antihydrogen atom ${\overline{\mathrm{H}}}_{1s}$. The calculations are accomplished using a nonreactive close-coupling quantum-mechanical approach. The ${\mathrm{H}}_{2}$ molecule is treated as a rigid rotor. The total elastic-scattering cross section ${\ensuremath{\sigma}}_{\mathrm{el}}(\ensuremath{\epsilon})$ at energy $\ensuremath{\epsilon}$, state-resolved rotational transition cross sections ${\ensuremath{\sigma}}_{{\mathit{jj}}^{'}}(\ensuremath{\epsilon})$ between states $j$ and ${j}^{'}$, and corresponding thermal rate coefficients ${k}_{{\mathit{jj}}^{'}}(T)$ are computed in the temperature range $0.004 \mathrm{K}\ensuremath{\lesssim}T\ensuremath{\lesssim}$ 4 K. Satisfactory agreement with other calculations (variational) has been obtained for ${\ensuremath{\sigma}}_{\mathrm{el}}(\ensuremath{\epsilon})$.

DOI
10.1103/physreva.81.022705

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In this work we report the results of calculation for quantum-mechanical rotational transitions in molecular hydrogen, H2{\mathrm{H}}_{2}H2​, induced by an ultracold ground-state antihydrogen atom H‾1s{\overline{\mathrm{H}}}_{1s}H1s​. The calculations are accomplished using a nonreactive close-coupling quantum-mechanical approach. The H2{\mathrm{H}}_{2}H2​ molecule is treated as a rigid rotor. The total elastic-scattering cross section \ensuremathσel(\ensuremathϵ){\ensuremath{\sigma}}_{\mathrm{el}}(\ensuremath{\epsilon})\ensuremathσel​(\ensuremathϵ) at energy \ensuremathϵ\ensuremath{\epsilon}\ensuremathϵ, state-resolved rotational transition cross sections \ensuremathσjj′(\ensuremathϵ){\ensuremath{\sigma}}_{{\mathit{jj}}^{'}}(\ensuremath{\epsilon})\ensuremathσjj′​(\ensuremathϵ) between states jjj and j′{j}^{'}j′, and corresponding thermal rate coefficients kjj′(T){k}_{{\mathit{jj}}^{'}}(T)kjj′​(T) are computed in the temperature range 0.004K\ensuremath≲T\ensuremath≲0.004 \mathrm{K}\ensuremath{\lesssim}T\ensuremath{\lesssim}0.004K\ensuremath≲T\ensuremath≲ 4 K. Satisfactory agreement with other calculations (variational) has been obtained for \ensuremathσel(\ensuremathϵ){\ensuremath{\sigma}}_{\mathrm{el}}(\ensuremath{\epsilon})\ensuremathσel​(\ensuremathϵ).
A close coupling quantum-mechanical calculation is performed for rotational energy transfer in a HD+HD collision at very low energy, down to the ultracold temperatures: $T\ensuremath{\sim}{10}^{\ensuremath{-}8}$ K. A global six-dimensional H2{}_{2}2​-H2{}_{2}2​ potential-energy surface is adopted from a previous work [Boothroyd et al., J. Chem. Phys. 116, 666 (2002)]. State-resolved integral cross sections ${\ensuremath{\sigma}}_{ij\ensuremath{\rightarrow}{i}^{\ensuremath{'}}{j}^{\ensuremath{'}}}({\ensuremath{\varepsilon}}_{\text{kin}})$ of different quantum-mechanical rotational transitions $ij\ensuremath{\rightarrow}{i}^{\ensuremath{'}}{j}^{\ensuremath{'}}$ in the HD molecules and corresponding state-resolved thermal rate coefficients ${k}_{ij\ensuremath{\rightarrow}{i}^{\ensuremath{'}}{j}^{\ensuremath{'}}}(T)$ have been computed. Additionally, for comparison, H2{}_{2}2​+H2{}_{2}2​ calculations for a few selected rotational transitions have also been performed. The hydrogen and deuterated hydrogen molecules are treated as rigid rotors in this work. A pronounced isotope effect is identified in the cross sections of these collisions at low and ultracold temperatures.
The study of molecular inelastic collision processes at cold and ultracold temperatures is of considerable interest as schemes are currently being developed to efficiently cool and trap neutral polar molecules. In this work, quantum-mechanical scattering calculations are presented for the quenching of rotationally excited CO due to collisions with H, He and H 2 for collision energies between 10 −5 and 10 cm −1 . The calculations were performed using the close-coupling approach and the coupled-states approximation. Cross sections for the quenching of the j = 1–10, 20, 30 and 40 levels of CO are presented and mechanism and trends of energy transfer in these systems are discussed.
In view of the current interest in trapping antihydrogen H‾\overline{\mathrm{H}}H atoms below 1 K, the s-wave elastic-scattering parameters for the antihydrogen scattering off atomic helium target are calculated in the energy range $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}--1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}\mathrm{a}.\mathrm{u}.$ using close-coupling models. The predicted cross section will help to simulate the evolution of a realistic mixture of He and H2{\mathrm{H}}_{2}H2​ through the process involving ejection of H‾\overline{\mathrm{H}}H from the trap. The trend of the present results with different basis sets conclusively indicates the reliability of the predicted results.
We present results of quantum mechanical scattering calculations including full rovibrational coupling on the relaxation of vibrationally excited H2{\mathrm{H}}_{2}H2​ molecules in collision with ultracold 3He{}^{3}\mathrm{He}3He and 4He{}^{4}\mathrm{He}4He atoms. The exothermic vibrational deactivation cross sections exhibit minima at translational temperatures near 10 K and show an inverse velocity dependence in accordance with Wigner's threshold law in the limit of zero kinetic energy. The corresponding rate coefficients exhibit minima before attaining constant values below ${10}^{\ensuremath{-}3}$ K. The rate coefficients increase by 3 orders of magnitude when the initial vibrational level of the molecule is raised from v=1v\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1v=1 to v=10v\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}10v=10. The results suggest that quasibound resonance states of 3HeH2{}^{3}{\mathrm{HeH}}_{2}3HeH2​ and 4HeH2{}^{4}{\mathrm{HeH}}_{2}4HeH2​ exist, associated with each excited vibrational state of H2{\mathrm{H}}_{2}H2​, and that both the triatomic species 3HeH2{}^{3}{\mathrm{HeH}}_{2}3HeH2​ and 4HeH2{}^{4}{\mathrm{HeH}}_{2}4HeH2​ have one bound vibrational state.
Collisions of cold and ultracold BH in the ν = 0 level with the He atom are investigated using the quantum mechanical scattering formulation. The elastic and the inelastic cross sections are calculated using the two-dimensional ab initio potential energy surface. It is shown that the elastic cross section is larger than the inelastic one. When the collision energy is very low, the elastic cross section follows the Wigner threshold law and is one order of magnitude larger than that of He-O 2 , while it is much smaller than that of He-H 2 . The efficiency of the rotationally quenching state is given. The Δ j = −1 transition is most efficient. The resonances are also found to occur at about the same translational energy (0.1–1 cm −1 ), which gives rise to steps in the rate coefficient at temperatures around 0.1–1 K.
Cold and ultracold collisions involving rotationally hot CO\mathrm{CO}CO molecules are investigated using quantum mechanical coupled channel, coupled states, and effective potential scattering formulations. Quenching rate coefficients are given for initial rotational levels near the dissociation threshold. The stability of the CO\mathrm{CO}CO ``super rotors'' against collisional decay is compared to previous investigations involving homonuclear molecules. It is found that quasiresonant transitions provide a significantly stronger contribution to the total relaxation rate than in the comparable case of O2{\mathrm{O}}_{2}O2​. As in the case of H2{\mathrm{H}}_{2}H2​, sharp structures in the distribution of total quenching rate coefficients are found at rotational levels where quasiresonant scattering is not allowed.