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The Journal of Chemical Physics

F + D 2 reaction at ultracold temperatures

Enrico Bodo, Franco Antonio Gianturco, A. Dalgarno

Published May 9, 2002• 107 citations
Publisher

Abstract

We present a quantum mechanical study of the reaction F+D2 at ultracold temperatures based on the potential energy surface of Stark and Werner. The reaction cross section at low energies is controlled by the tunneling through the activation barrier, a mechanism that is favored by the long duration of the collision at those energies. Differences are found in the behavior of the reactive cross section compared to that for F+H2, due to the changed mass and zero point energy. A new feature is detected in the reaction probabilities and is attributed to a Feshbach resonance corresponding to a metastable state in the exit channel.

DOI
10.1063/1.1472515

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We review the recent progress achieved in the theoretical description of chemical reactions at low temperatures. In particular, we discuss the crucial role played by the van der Waals interaction potential in quantum-mechanical scattering calculations of atom–diatom collisions in the cold and ultracold regimes, where abstraction reactions proceed by tunnelling. The importance of zero-energy resonances in enhancing the reactivity in the zero-temperature limit is assessed. The impact of Feshbach resonances associated with the decay of metastable states of van der Waals complexes on reactive scattering is addressed through a series of examples. Finally, we discuss the sensitivity of the reaction dynamics to the topology of the van der Waals well. Importance of long-range interactions in chemical reactions at cold and ultracold temperaturesAll authorsPhilippe F. Weck & N. Balakrishnanhttps://doi.org/10.1080/01442350600791894Published online:22 February 2007Table Download CSVDisplay Table

International Reviews in Physical Chemistry•Jul 1, 2006•105 citations
We present a quantum mechanical study of the reaction F+D2 at ultracold temperatures based on the potential energy surface of Stark and Werner. The reaction cross section at low energies is controlled by the tunneling through the activation barrier, a mechanism that is favored by the long duration of the collision at those energies. Differences are found in the behavior of the reactive cross section compared to that for F+H2, due to the changed mass and zero point energy. A new feature is detected in the reaction probabilities and is attributed to a Feshbach resonance corresponding to a metastable state in the exit channel.
The main characteristics of cold and ultracold chemical reactions are reviewed through the illustrative study of the O(3P) + H2 reaction dynamics. Using separate analytic representations of the lowest H2O(3A'') electronic state which differ essentially by their descriptions of long-range forces, quantum-mechanical scattering calculations show the crucial role played by the van der Waals interaction potential in chemical reactions at low temperatures. Furthermore, the presence of zero-energy resonances is found to significantly enhance chemical reactivity in the ultracold regime. At translational energies comparable to the well depth of the van der Waals potential, initial-state-selected probabilities and excitation functions are characterized by Feshbach resonances arising from the decay of quasibound states supported by the van der Waals well in the entrance channel of the reaction.
The behaviour of reactive scattering at ultracold temperatures is explored by calculating the real and imaginary parts of the scattering length for the reaction of F with a molecule composed of a pair of pseudo-hydrogen atoms of arbitrary mass. The origin of a low energy feature in the cross section for the reaction of F with H 2 and its absence for the reaction with D 2 is investigated. Close-coupling calculations of the scattering matrix show that the F–H 2 feature arises from the presence of a virtual state associated with the van der Waals well in the entrance channel and that the virtual state is responsible for the enhanced zero temperature rate coefficient of the F–H 2 reaction. For a mass of about 1.12 hydrogen masses the virtual state turns into a zero energy resonance and the corresponding zero temperature rate coefficient is 1 × 10 −9 cm 3 s −1 despite an energy barrier of 300 K. Evidence in support of the virtual state is also provided by the detection of a deep Ramsauer–Townsend minimum in the elastic component of the total cross section for F–H 2 which the present calculations predict to occur at low energies.
Abstract We report a quantum dynamics study of the Li + HF → LiF + H reaction at low temperatures of interest to cooling and trapping experiments. Contributions from non-zero partial waves are analyzed and results show narrow resonances in the energy dependence of the cross section that survive partial wave summation. The computations are performed using the ABC code and a simple modification of the ABC code that enables separate energy cutoffs for the reactant and product rovibrational energy levels is found to dramatically reduce the basis set size and computational expense. Results obtained using two ab initio electronic potential energy surfaces for the LiHF system show strong sensitivity to the choice of the potential. In particular, small differences in the barrier heights of the two potential surfaces are found to dramatically influence the reaction cross sections at low energies. Comparison with recent measurements of the reaction cross section (Bobbenkamp et al 2011 J. Chem. Phys. 135 204306 ) shows similar energy dependence in the threshold regime and an overall good agreement with experimental data compared to previous theoretical results. Also, usefulness of a recently introduced method for ultracold reactions that employ the quantum close-coupling method at short-range and the multichannel quantum defect theory at long-range, is demonstrated in accurately evaluating product state-resolved cross sections for D + H2 and H + D2 reactions.
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 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.
We review the recent progress achieved in the theoretical description of chemical reactions at low temperatures. In particular, we discuss the crucial role played by the van der Waals interaction potential in quantum-mechanical scattering calculations of atom–diatom collisions in the cold and ultracold regimes, where abstraction reactions proceed by tunnelling. The importance of zero-energy resonances in enhancing the reactivity in the zero-temperature limit is assessed. The impact of Feshbach resonances associated with the decay of metastable states of van der Waals complexes on reactive scattering is addressed through a series of examples. Finally, we discuss the sensitivity of the reaction dynamics to the topology of the van der Waals well. Importance of long-range interactions in chemical reactions at cold and ultracold temperaturesAll authorsPhilippe F. Weck & N. Balakrishnanhttps://doi.org/10.1080/01442350600791894Published online:22 February 2007Table Download CSVDisplay Table