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Physical Review A

Cold He+H2 collisions near dissociation

Akiko Mack, Tricia K. Clark, Robert C. Forrey, N. Balakrishnan, Teck-Ghee Lee, Phillip C. Stancil

Published Nov 30, 2006• 23 citations
Publisher

Abstract

Cross sections for $\mathrm{He}+{\mathrm{H}}_{2}$ collisions are reported for rovibrational states near dissociation at translational energies less than $1000\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. In contrast to our previously reported cross sections for lower-lying initial rotational states, the excited rotational states near dissociation give rise to shape resonances for energies between 0.001 and $1\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. The emergence of these resonances with increasing rotational level is opposite to the trend found for $\mathrm{He}+\mathrm{C}\mathrm{O}$ where the shape resonance strength decreases with rotational level for low-lying states. Quasiresonant energy transfer and low energy excitation thresholds are also discussed and it is found that there are 11 rovibrationally excited states that are stable against collision at ultracold temperatures.

DOI
10.1103/physreva.74.052718

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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.
Full-dimensional quantum scattering calculations are reported for ro-vibrational transitions in HD+HD collisions using a highly accurate interaction potential for the H2_22​-H2_22​ system. Several near-resonant ro-vibrational transitions are identified that conserve the overall rotational angular momentum and nearly conserve the internal energy of the collision partners. Key anisotropic terms that drive the rotational transitions and angular momentum partial waves that contribute to low energy resonant features in the energy dependence of the cross sections are identified. The computed results are in agreement with total cross sections reported in previous experimental results, including resonant features in the energy dependence of the cross section. In particular, low-energy cross sections show a strong resonant feature associated with an l=3l=3l=3 partial wave in the incident channel. Rate coefficients for several inelastic rotational and ro-vibrational transitions are reported for temperatures ranging from 0.10.10.1 K to 200200200 K and they display a maximum between 111 K-101010 K reflecting the important contributions from the l=3l=3l=3 shape resonance that occurs around 2.5 K.
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