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Physical Review A

Cooling and trapping of molecules in highly excited rotational states

Robert C. Forrey

Published Apr 18, 2001• 43 citations
Publisher

Abstract

Collisional relaxation of highly rotating hydrogen molecules is investigated as a function of energy. Calculations demonstrate that inelastic collisions are dramatically suppressed for specific rotational levels of the molecule as the energy is lowered due to the closing of quasiresonant rotation-vibration channels. It is predicted that a ${}^{3}\mathrm{He}$ buffer gas may be used to load these highly excited molecules into a trap without a significant loss of population. It is further predicted that evaporative cooling may be used to cool the ``super rotors'' to even lower temperatures.

DOI
10.1103/physreva.63.051403

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Collisional relaxation of highly rotating hydrogen molecules is investigated as a function of energy. Calculations demonstrate that inelastic collisions are dramatically suppressed for specific rotational levels of the molecule as the energy is lowered due to the closing of quasiresonant rotation-vibration channels. It is predicted that a 3He{}^{3}\mathrm{He}3He buffer gas may be used to load these highly excited molecules into a trap without a significant loss of population. It is further predicted that evaporative cooling may be used to cool the ``super rotors'' to even lower temperatures.
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.
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.
The collisional behavior of (X)6Li2 molecules in very high rotational levels of v=0 is considered. Highly efficient vibration–rotation transfer is predicted in these “super rotors” particularly when the conditions for quasiresonant transfer are fulfilled. This requires simultaneous near-resonance in energy and in angular momentum. Values of Δj for which quasiresonant vibration–rotation transfer (QRT) occurs become smaller as initial rotor state increases and transfer is likely to become particularly fast for Δj=2, predicted to occur when ji=130. This behavior is contrasted with the inefficiency of pure rotational transfer within the v=0 level for fast-rotating molecules. QRT will take place for quite cold collisions and thus will provide competition for the spinning-up process used to create the super rotors.
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.
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.
The translational and internal level cooling of atoms and molecules in ultracold gases results from a combination of elastic and inelastic collisional processes. While elastic collisions lead to rapid thermalization, exoergic inelastic collisions may lead to heating and trap loss. To date, most collisional studies have targeted low-lying levels of diatomic molecules. Here we investigate inelastic quenching and elastic scattering of rotationally excited linear (H2{}_{2}2​, HD, CO, O2{\mathrm{O}}_{2}O2​, and CO2{}_{2}2​) and nonlinear (H2{}_{2}2​O and NH3{}_{3}3​) molecules in ultracold collisions with He and report the corresponding complex scattering lengths. It has been found that the ratio of the imaginary component $\ensuremath{\beta}$ to the real component $\ensuremath{\alpha}$ of the scattering length generally increases with decreasing rotational constant for linear molecules. With the exception of CO, $\ensuremath{\beta}$ becomes significantly smaller than $\ensuremath{\alpha}$ as the energy gap for rotational transitions increases. In all cases, $\ensuremath{\beta}$ decreases with rotational energy gap for relatively large rotational excitation, allowing for convenient fits to an exponential energy gap formula. Excited rotational levels of H2{\mathrm{H}}_{2}H2​ and HD appear to be collisionally stable due to the very low values of $\ensuremath{\beta}/\ensuremath{\alpha}$. Rotationally excited H2{\mathrm{H}}_{2}H2​O also appears to be a viable candidate for He buffer gas cooling due to relatively small values of $\ensuremath{\beta}$.