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Physical Review A

Cold collisions involving rotationally hot oxygen molecules

K. Tilford, M. Hoster, P. M. Florian, Robert C. Forrey

Published May 12, 2004• 38 citations
Publisher

Abstract

Cold and ultracold collisions involving rotationally hot oxygen 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 oxygen ``super rotors'' against collisional decay is compared to previous investigations involving hydrogen molecules where the rotational inertia was significantly smaller. In contrast to hydrogen, all possible states of rotationally hot oxygen are quenched very rapidly during a collision with a buffer gas helium atom, and the quenching efficiency is always dominated by pure rotational transitions.

DOI
10.1103/physreva.69.052705

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Physica Scripta•Nov 1, 2008•2 citations
Cold and ultracold collisions involving rotationally hot oxygen 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 oxygen ``super rotors'' against collisional decay is compared to previous investigations involving hydrogen molecules where the rotational inertia was significantly smaller. In contrast to hydrogen, all possible states of rotationally hot oxygen are quenched very rapidly during a collision with a buffer gas helium atom, and the quenching efficiency is always dominated by pure rotational transitions.
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.
Cold collisions involving highly rotationally excited diatomic molecules are investigated. As the translational energy is lowered, the total inelastic cross section decreases sharply for specific channels where quasiresonant transitions are no longer energetically allowed. Rate coefficients are given for collisional quenching of rotationally excited H2,D2,T2,{\mathrm{H}}_{2},{\mathrm{D}}_{2},{\mathrm{T}}_{2},H2​,D2​,T2​, and O2{\mathrm{O}}_{2}O2​ at zero temperature. The specific rotational states that are stable against collisional relaxation would be interesting prospects for cooling and trapping. The application of collisional cooling methods together with recently developed schemes for producing rotationally hot molecules may allow high densities of ultracold ``super rotors'' to be achieved.
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.
Building on recent advances in ultrafast lasers and methods to slow molecules, an experiment is proposed to produce translationally cold CO2{}_{2}2​ super-rotors ($j\ensuremath{\sim}200$) by combining an optical centrifuge with helium-buffer-gas cooling. Quantum mechanical calculations of the complex scattering length for He-CO2{}_{2}2​ collisions demonstrate that the efficiency of rotational quenching decreases rapidly with increasing rotational excitation jjj in the ultracold regime. Extrapolating to helium cryogenic temperatures, rotational quenching is predicted to remain inefficient up to $\ensuremath{\sim}$1 K, allowing for the possible creation of a beam of translationally cold, rotationally hot molecules.
Collision cross sections between oxygen molecules and helium atoms are computed at translational energies between 0.1 K and 10.0 K, motivated by the recently demonstrated cooling of molecules by a helium buffer gas. Detailed calculations based on a rigid-rotor model demonstrate the differences among various isotopic combinations of oxygen atoms and also serve to illustrate resonant features unique to cold molecules. Comparisons of elastic versus spin-changing inelastic collision rates suggest that buffer-gas cooling is likely to be a widely applicable tool for producing cold molecular samples.
Quantum mechanical methods are employed to obtain superelastic cross sections involving H 2 and N 2 molecules, in excited rotational states, colliding with electrons at the very low collision energies which are encountered in cold trap experiments. This computational analysis intends to explore the feasibility of cold electrons for the collisional quenching of molecular gases down to the nanokelvin regimes. The results are obtained using rigorous coupled-channel (CC) calculations in the laboratory frame of reference which allows one to correctly describe the cross section behaviour at ultralow energies. The results are analysed down to the ultracold region of validity of Wigner's law, where it is found that electron-driven collisions exhibit substantial efficiency for the quenching of rotational populations in molecular gases involving the title systems.