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Journal of Physics B Atomic Molecular and Optical Physics

Chemical reactions in the limit of zero kinetic energy: virtual states and Ramsauer minima inF + H2 HF + H

Enrico Bodo, Franco Antonio Gianturco, N. Balakrishnan, A. Dalgarno

Published Sep 15, 2004• 82 citations
Publisher

Abstract

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.

DOI
10.1088/0953-4075/37/18/007

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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.
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.
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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
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The role of van der Waals forces in O((3)P)+H(2)(upsilon=1,j=0) collisions is investigated theoretically at low and ultralow temperatures. Quantum scattering calculations have been performed for zero total angular momentum using the lowest London-Eyring-Polanyi-Sato double-polynomial (3)A(") potential-energy surface reported by [Rogers et al., J. Phys. Chem. A 104, 2308 (2000)] and its recent BMS1 and BMS2 extensions developed by [Brandao et al., J. Chem. Phys. 121, 8861 (2004)] which provide a more accurate treatment of the van der Waals interaction. Our calculations show that van der Waals forces strongly influence chemical reactivity at ultracold translational energies. The presence of a zero-energy resonance for the BMS1 surface is found to enhance reactivity in the ultracold regime and shift the Wigner threshold to lower temperatures.
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.