We present results of quantum mechanical scattering calculations including full rovibrational coupling on the relaxation of vibrationally excited ${\mathrm{H}}_{2}$ molecules in collision with ultracold ${}^{3}\mathrm{He}$ and ${}^{4}\mathrm{He}$ atoms. The exothermic vibrational deactivation cross sections exhibit minima at translational temperatures near 10 K and show an inverse velocity dependence in accordance with Wigner's threshold law in the limit of zero kinetic energy. The corresponding rate coefficients exhibit minima before attaining constant values below ${10}^{\ensuremath{-}3}$ K. The rate coefficients increase by 3 orders of magnitude when the initial vibrational level of the molecule is raised from $v\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$ to $v\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}10$ . The results suggest that quasibound resonance states of ${}^{3}{\mathrm{HeH}}_{2}$ and ${}^{4}{\mathrm{HeH}}_{2}$ exist, associated with each excited vibrational state of ${\mathrm{H}}_{2}$ , and that both the triatomic species ${}^{3}{\mathrm{HeH}}_{2}$ and ${}^{4}{\mathrm{HeH}}_{2}$ have one bound vibrational state.We present results of quantum mechanical scattering calculations including full rovibrational coupling on the relaxation of vibrationally excited ${\mathrm{H}}_{2}$ molecules in collision with ultracold ${}^{3}\mathrm{He}$ and ${}^{4}\mathrm{He}$ atoms. The exothermic vibrational deactivation cross sections exhibit minima at translational temperatures near 10 K and show an inverse velocity dependence in accordance with Wigner's threshold law in the limit of zero kinetic energy. The corresponding rate coefficients exhibit minima before attaining constant values below ${10}^{\ensuremath{-}3}$ K. The rate coefficients increase by 3 orders of magnitude when the initial vibrational level of the molecule is raised from $v\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$ to $v\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}10$. The results suggest that quasibound resonance states of ${}^{3}{\mathrm{HeH}}_{2}$ and ${}^{4}{\mathrm{HeH}}_{2}$ exist, associated with each excited vibrational state of ${\mathrm{H}}_{2}$, and that both the triatomic species ${}^{3}{\mathrm{HeH}}_{2}$ and ${}^{4}{\mathrm{HeH}}_{2}$ have one bound vibrational state.

Physical Review LettersApr 13, 1998166 citations

Reduced dimensional model of ultracold molecule-molecule scattering

Christopher Ticknor, Brian K. Kendrick, Jeffery A. Leiding

Bulletin of the American Physical SocietyJun 8, 20170 citations

Vibrational quenching at ultralow energies: Calculations of the Li2(Σg+1;ν⪢0)+He superelastic scattering cross sections

Enrico Bodo, Franco Antonio Gianturco, Ersin Yurtsever

Accurate quantum calculations have been carried out at ultralow energies (from ${10}^{\ensuremath{-}2}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}{10}^{\ensuremath{-}6}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$) for the vibrational deexcitation of ${\mathrm{Li}}_{2}(^{1}\ensuremath{\Sigma}_{g}^{+})$ by collisions with He, starting from a broad range of initial highly excited vibrational levels. The results indicate the clear dominance of a few transitions with the smallest $\ensuremath{\Delta}\ensuremath{\nu}$ changes and show the overall deexcitation cross sections to markedly depend on the initial vibrational state of the molecule, in line with earlier results on ${\mathrm{H}}_{2}+\mathrm{He}$ [Balakrishan et al. Phys. Rev. Lett. 80, 3224 (1998)] vibrational quenching. A connection is made with very recent measurements on the vibrational quenching of ultracold ${\mathrm{Cs}}_{2}$ molecules in optical traps that were instead found to behave in a very different manner. Numerical experiments on the present system as well as on the ${\mathrm{H}}_{3}$ reaction strongly suggest a possible explanation for such differences.Accurate quantum calculations have been carried out at ultralow energies (from ${10}^{\ensuremath{-}2}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}{10}^{\ensuremath{-}6}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$) for the vibrational deexcitation of ${\mathrm{Li}}_{2}(^{1}\ensuremath{\Sigma}_{g}^{+})$ by collisions with He, starting from a broad range of initial highly excited vibrational levels. The results indicate the clear dominance of a few transitions with the smallest $\ensuremath{\Delta}\ensuremath{\nu}$ changes and show the overall deexcitation cross sections to markedly depend on the initial vibrational state of the molecule, in line with earlier results on ${\mathrm{H}}_{2}+\mathrm{He}$ [Balakrishan et al. Phys. Rev. Lett. 80, 3224 (1998)] vibrational quenching. A connection is made with very recent measurements on the vibrational quenching of ultracold ${\mathrm{Cs}}_{2}$ molecules in optical traps that were instead found to behave in a very different manner. Numerical experiments on the present system as well as on the ${\mathrm{H}}_{3}$ reaction strongly suggest a possible explanation for such differences.

Physical Review AMay 31, 200623 citations

Cold and ultracold chemical reactions of F+HCl and F+DCl

Goulven Quéméner, N. Balakrishnan

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 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.

The Journal of Chemical PhysicsJun 10, 200840 citations

We present results of quantum mechanical scattering calculations including full rovibrational coupling on the relaxation of vibrationally excited

{\mathrm{H}}_{2}

molecules in collision with ultracold

{}^{3}\mathrm{He}

and

{}^{4}\mathrm{He}

atoms. The exothermic vibrational deactivation cross sections exhibit minima at translational temperatures near 10 K and show an inverse velocity dependence in accordance with Wigner's threshold law in the limit of zero kinetic energy. The corresponding rate coefficients exhibit minima before attaining constant values below ${10}^{\ensuremath{-}3}$ K. The rate coefficients increase by 3 orders of magnitude when the initial vibrational level of the molecule is raised from

v\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1

v\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}10

. The results suggest that quasibound resonance states of

{}^{3}{\mathrm{HeH}}_{2}

and

{}^{4}{\mathrm{HeH}}_{2}

exist, associated with each excited vibrational state of

{\mathrm{H}}_{2}

, and that both the triatomic species

{}^{3}{\mathrm{HeH}}_{2}

and

{}^{4}{\mathrm{HeH}}_{2}

have one bound vibrational state.

{\mathrm{H}}_{2}+\mathrm{He}

[Balakrishan et al. Phys. Rev. Lett. 80, 3224 (1998)] vibrational quenching. A connection is made with very recent measurements on the vibrational quenching of ultracold

{\mathrm{Cs}}_{2}

molecules in optical traps that were instead found to behave in a very different manner. Numerical experiments on the present system as well as on the

{\mathrm{H}}_{3}

reaction strongly suggest a possible explanation for such differences.