Explanation of efficient quenching of molecular ion vibrational motion by ultracold atoms.
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Here, we theoretically explain the recently observed exception to this rule: efficient vibrational cooling of BaCl(+) by a laser-cooled Ca buffer gas.We perform intense close-coupling calculations that agree with the experimental result, and use both quantum defect theory and a statistical capture model to provide an intuitive understanding of the system.This result establishes that, in contrast to the commonly held opinion, there exists a large class of systems that exhibit efficient vibrational cooling and therefore supports a new route to realize the long-sought opportunities offered by molecular structure.
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Affiliation: Université de Bordeaux, Institut des Sciences Moléculaires, UMR 5255 CNRS, 33405 Talence, France.
ABSTRACT
Buffer gas cooling of molecules to cold and ultracold temperatures is a promising technique for realizing a host of scientific and technological opportunities. Unfortunately, experiments using cryogenic buffer gases have found that although the molecular motion and rotation are quickly cooled, the molecular vibration relaxes at impractically long timescales. Here, we theoretically explain the recently observed exception to this rule: efficient vibrational cooling of BaCl(+) by a laser-cooled Ca buffer gas. We perform intense close-coupling calculations that agree with the experimental result, and use both quantum defect theory and a statistical capture model to provide an intuitive understanding of the system. This result establishes that, in contrast to the commonly held opinion, there exists a large class of systems that exhibit efficient vibrational cooling and therefore supports a new route to realize the long-sought opportunities offered by molecular structure. No MeSH data available. Related in: MedlinePlus |
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Mentions: The location and origin of the series of resonances explains much of our successful fit. They are shape resonances behind the even partial-wave centrifugal barriers. Analysis of the analytic E=0 solution of the −C4/R4 potential with short-range amplitude ηℓm(E=0)=1 and l-independent phase δℓm(E=0) has shown that if an E=0 bound state occurs for partial wave l then such bound state also exist for the ..., l-4, l-2, l+2, l+4, ... partial waves. This condition is only satisfied for two values of the short-range phase δ0, one corresponding to all even partial waves, one to the odd waves. For different values of δ0 as well as ηℓm(E)<1, these bound states become shape resonances where the inelastic loss is resonantly enhanced. For BaCl++Ca collisions with its ηℓm(E)≈η0≈1 for small partial waves, we find that near the optimal δ0 resonances occur for odd partial waves and, in particular, near E/k=10 μK the rate coefficient is sensitive to the location of the l=3, f-wave resonance. For E/k>0.5 mK, where l≥8 wave collisions become prominent, the amplitude ηℓm(E) deviates sufficiently from one, such that resonances from even partial wave collisions are observed. Finally, for the optimal conditions, the absolute value of the elastic scattering length a for s-wave BaCl++Ca collisions is larger than the natural length scale of the C4 potential. In this case, the finite Wigner-threshold prediction for the loss rate coefficient is only reached for collision energies below E/k=1 nK not shown in Fig. 3b. Figure 5 predicts the ratio of the QDT elastic and loss rate coefficient as a function of temperature. Ratios much larger than one indicate that the kinetic energy in the centre-of-mass motion of the molecular ion is more efficiently relaxed than its vibrational energy. For example, for BaCl+ ions at T=0.1 K, the ratio is close to one. We anticipate two cooling stages for BaCl+ ions in collisions with ultracold and highly polarizable Ca atoms. In the first stage cooling or relaxation of the rovibrational states occurs, as the small ratio Kelastic/Kloss implies that translational cooling in this region will be inefficient. Only when the molecules are in the lowest rovibrational state, the second stage, will elastic collisions cool the external motion of the molecules. |
View Article: PubMed Central - PubMed
Affiliation: Université de Bordeaux, Institut des Sciences Moléculaires, UMR 5255 CNRS, 33405 Talence, France.
No MeSH data available.