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arXiv:1612.08437 · Journal of Physics B Atomic Molecular and Optical Physics

Resonances at very low temperature for the reaction D 2 + H

I Simbotin, R Côté

Published Mar 21, 2017• 2 citations
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Abstract

Abstract We present numerical results for rate coefficients of reaction and vibrational quenching in the collision of H with D 2 ( v , j ) at cold and ultracold temperatures. We explore both ortho-D 2 ( j = 0 ) and para-D 2 ( j = 1 ) for several initial vibrational states ( v ≤ 5 ) , and find resonant structures in the energy range 0.01–10 K, which are sensitive to the initial rovibrational state ( v , j ). We compare the reaction rates for D 2 + H with our previously obtained results for the isotopologue reaction H 2 + D, and discuss the implications of our detailed study of this benchmark system for ultracold chemistry.

DOI
10.1088/1361-6455/aa6821

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Abstract We present numerical results for rate coefficients of reaction and vibrational quenching in the collision of H with D 2 ( v , j ) at cold and ultracold temperatures. We explore both ortho-D 2 ( j = 0 ) and para-D 2 ( j = 1 ) for several initial vibrational states ( v ≤ 5 ) , and find resonant structures in the energy range 0.01–10 K, which are sensitive to the initial rovibrational state ( v , j ). We compare the reaction rates for D 2 + H with our previously obtained results for the isotopologue reaction H 2 + D, and discuss the implications of our detailed study of this benchmark system for ultracold chemistry.
We report results for reaction and vibrational quenching of the collision D with para-H 2 ( ) and ortho-H 2 ( ) at cold and ultracold temperatures. We investigate the effect of nuclear spin symmetry for barrier dominated processes ( ) and for one barrierless case ( v = 5). We find resonant structures for energies in the range corresponding to 0.01–10 K, which depend on the nuclear spin of H 2 , arising from contributions of specific partial waves. We discuss the implications on the results in this benchmark system for ultracold chemistry.
Theoretical results for ultracold atom-molecule collisions involving exoergic reactions are presented. Systems with a reaction barrier are considered; specifically, we report the results of extensive computations for D + H 2 and Cl + H2. We analyzed the extreme situation of translationally cold collisions between an atom and an internally hot molecule; namely, for D + H2 we explored the role played by the internal vibrational excitation of the diatomic target for initial states H2(υ) with υ = 0,1,2,…, 8. The υ-dependence of the zero-temperature limit of the reaction rate coefficient shows two distinct regimes: a barrier dominated regime for υ < 4, and a barrierless regime for υ > 4. Also, for highly excited initial states, the distribution over the final states of the products shows an approximate conservation of the internal vibrational energy. For Cl + H2 we studied in detail the isotopic effect by varying continuously the mass of H, which allowed us to find resonance effects in the threshold behavior of cross sections. These resonances are caused by long-lived van der Waals complexes (Cl···H2) which have vanishingly small binding energy, or by virtual states of the Cl···H 2 complex. ^
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.
We report quantum mechanical scattering calculations of zero temperature quenching rate coefficients for vibrationally and rotationally excited O 2 in collisions with 3 He. It is found that vibrational energy is channeled into more specific rotational levels in the ultracold regime than at thermal energies where a broad range of rotational levels are populated. The influence of the van der Waals interaction on the low-temperature quenching rate coefficients and the possibility of trapping vibrationally excited O 2 by the buffer gas cooling technique are also discussed.
We report quantum dynamics calculations of F((2)P)+HCl(v,j)-->HF(v('),j('))+Cl((2)P) and F+DCl(v,j)-->DF(v('),j('))+Cl reactions at cold and ultracold temperatures. The effect of rotational and vibrational excitations of the HCl molecule on the reactivity is investigated. It is found that, in the ultracold regime, vibrational excitation of the HCl molecule from v=0 to v=2 enhances the reactivity by four orders of magnitude. The rotational excitation from j=0 to j=1 decreases the reactivity while the rotational excitation from j=0 to j=2 increases the reactivity. The overall effect of rotational excitation was found to be much smaller than vibrational excitation. The reactivity of the F+DCl system is significantly lower than that of the F+HCl case indicating the importance of quantum tunneling at low energies. For both reactions, Feshbach resonances corresponding to Fcdots, three dots, centered HCl or Fcdots, three dots, centeredDCl triatomic states occur at low energies. We also explored the validity of the coupled-states approximation for cold collisions taking the F+HCl(v=0,j=0) reaction as an illustrative example. It is found that the coupled-states approximation is generally valid for the background scattering even at low energies but it is inadequate to accurately describe the rich resonances in the energy dependence of the cross section resulting from the decay of van der Waals complexes. It is further shown that the coupled-states approximation cannot be used for scattering in the Wigner threshold regime when the molecule is initially in a rotationally excited level.
The H3 system has served as a prototype for geometric phase (GP) effects in bimolecular chemical reactions for over three decades. Despite a large number of theoretical and experimental efforts, no conclusive evidence of GP effects in the integral cross section or reaction rate has been presented until recently [B. Kendrick et al., Phys. Rev. Lett. 115, 153201 (2015)]. Here we report a more detailed account of GP effects in the H + H2(v = 4, j = 0) → H + H2(v′, j′) (para-para) reaction rate coefficients for temperatures between 1 μK (8.6 × 10−11 eV) and 100 K (8.6 × 10−3 eV). The GP effect is found to persist in both vibrationally resolved and total rate coefficients for collision energies up to about 10 K. The GP effect also appears in rotationally resolved differential cross sections leading to a very different oscillatory structure in both energy and scattering angle. It is shown to suppress a prominent shape resonance near 1 K and enhance a shape resonance near 8 K, providing new experimentally verifiable signatures of the GP effect in the fundamental hydrogen exchange reaction. The GP effect in the D + D2 and T + T2 reactions is also examined in the ultracold limit and its sensitivity to the potential energy surface is explored.