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arXiv:0710.4892 · The Journal of Chemical Physics

Collisional quenching at ultralow energies: Controlling efficiency with internal state selection

S. Bovino, Enrico Bodo, Franco Antonio Gianturco

Published Dec 11, 2007• 4 citations
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Abstract

Calculations have been carried out for the vibrational quenching of excited H(2) molecules which collide with Li(+) ions at ultralow energies. The dynamics has been treated exactly using the well-known quantum coupled-channel expansions over different initial vibrational levels. The overall interaction potential has been obtained from the calculations carried out earlier by our group using highly correlated ab initio methods. The results indicate that specific features of the scattering observables, e.g., the appearance of Ramsauer-Townsend minima in elastic channel cross sections and the marked increase of the cooling rates from specific initial states, can be linked to potential properties at vanishing energies (sign and size of scattering lengths) and to the presence of either virtual states or bound states. The suggestion is made such that by selecting the initial state preparation of the molecular partners, the ionic interactions would be amenable to controlling quenching efficiency at ultralow energies.

DOI
10.1063/1.2800658

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Calculations have been carried out for the vibrational quenching of excited H(2) molecules which collide with Li(+) ions at ultralow energies. The dynamics has been treated exactly using the well-known quantum coupled-channel expansions over different initial vibrational levels. The overall interaction potential has been obtained from the calculations carried out earlier by our group using highly correlated ab initio methods. The results indicate that specific features of the scattering observables, e.g., the appearance of Ramsauer-Townsend minima in elastic channel cross sections and the marked increase of the cooling rates from specific initial states, can be linked to potential properties at vanishing energies (sign and size of scattering lengths) and to the presence of either virtual states or bound states. The suggestion is made such that by selecting the initial state preparation of the molecular partners, the ionic interactions would be amenable to controlling quenching efficiency at ultralow energies.
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 H2+He{\mathrm{H}}_{2}+\mathrm{He}H2​+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 Cs2{\mathrm{Cs}}_{2}Cs2​ 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 H3{\mathrm{H}}_{3}H3​ reaction strongly suggest a possible explanation for such differences.
Inducing low-energy p -wave resonances by an additional external electric field has been shown recently to be a possible way of controlling ultra-low energy atomic collisions (Krems R. V., Phys. Rev. Lett. , 96 (2006) 123202). We show here by calculation that also in molecular systems with long-range interactions there exist internal states for which a low energy p -wave resonance is clearly contributing to the scattering even at ultra-low energies, and that for this collisional regime the scattering length turns out to be positive. We thus present results for the vibrational relaxation cross-sections of H 2 , prepared in different vibrational states, in collision with Li + in that energy regime and examine the changes in scattering attributes, both in the elastic and inelastic channels, when the internal vibrational state of H 2 is modified.
Quantum-scattering calculations at ultralow (close to ${10}^{\ensuremath{-}6}$ cm${}^{\ensuremath{-}1}$) collision energies are carried out for the Cs dimer in its spin-stretched triplet state, interacting with helium. An ab initio potential energy surface is computed and employed, while the target molecule is kept in its ground vibrational state and several excitated initial rotational states are considered in the quantum dynamics. The highly anisotropic interaction is seen to cause, in spite of its weakness, internal energy quenching rates comparable with the efficiency of the collisional cooling of relative kinetic energies. The rates of spin-flip processes are also analyzed and compared with pure rotational quenching events.
The interaction between the triplet state of the lithium dimer, (7)Li(2), with (4)He is obtained from accurate ab initio calculations where the vibrational dependence of the potential is newly computed. Vibrational quenching dynamics within a coupled-channel quantum treatment is carried out at ultralow energies, and large differences in efficiency as a function of the initial vibrational state of the targets are found as one compares the triplet results with those of the singlet state of the same target.
A new three-dimensional potential energy surface (PES), which describes the interaction between the OH(-)(X (1)Sigma(+)) anionic molecule and the Rb((2)S) atomic gas has been obtained by using a highly correlated ab initio model of the electronic structure and a large atomic basis set supplemented with a set of midbond Gaussian functions. This accurate PES is being employed to evaluate the OH(-)(X (1)Sigma(+)) vibrational quenching efficiency induced by collisions with Rb((2)S) at low and ultralow energies. Calculations employ the coupled-channel ansatz to describe the nuclear quantum dynamics and finally show a very marked dependence of the inelastic cross sections and rates, at vanishing collision energies, on the initial vibrational energy content of the partner molecules. The present findings allow us to make specific suggestions on the likely outcomes from energy interplay between the title species within a mixed MOT/Paul Trap environment.
Quantum coupled-channel scattering calculations have been carried out at ultralow energies (down to 10(-5) K) for rotational quenching of ionic and spin-stretched states of the lithium dimer in collision with He atoms. Marked cross section differences, to be related to changes in their interactions with He, have been observed with respect to the singlet case while little changes in the collisional behavior are seen to occur upon ionization when the spin-stretched target is considered. Both effects originate from an interplay of structural and dynamical features of the colliding partners which are analyzed in detail.