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Explanation of efficient quenching of molecular ion vibrational motion by ultracold atoms.

Stoecklin T, Halvick P, Gannouni MA, Hochlaf M, Kotochigova S, Hudson ER - Nat Commun (2016)

Bottom Line: 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.

View Article: PubMed Central - PubMed

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

Quantum defect theory (QDT) of the Ca+BaCl+ collision.(a) Loss rate coefficients based on the QDT as functions of collision energy. The black solid line corresponds to the loss rate coefficient assuming universal or completely absorbing short-range boundary conditions. Coloured lines correspond to loss rates contributions from individual partial waves and its projections with l≤3. Finally, the dashed line is the loss rate coefficient found by the Langevin capture theory. (b) Loss rate coefficients Kloss as functions of collision energy or temperature for a partial-wave dependent QDT optimized to agree with the coupled-channels BaCl+(v=0, j=1)+Ca vibrational quenching rate coefficient. The blue curve shows Kloss(E) as a function of collision energy, whereas the black curve shows the corresponding thermally averaged rate coefficient. The red curve corresponds to the close-coupling results ‘Vib 1 0' shown in Fig. 2. Shape resonances in Kloss(E) are assigned by their partial wave l. The inset shows the short-range amplitude with parameters η0=0.9916, η1=−0.003 and ηmin=0.6. The short-range phase is δℓm(E)=0.34π.
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f3: Quantum defect theory (QDT) of the Ca+BaCl+ collision.(a) Loss rate coefficients based on the QDT as functions of collision energy. The black solid line corresponds to the loss rate coefficient assuming universal or completely absorbing short-range boundary conditions. Coloured lines correspond to loss rates contributions from individual partial waves and its projections with l≤3. Finally, the dashed line is the loss rate coefficient found by the Langevin capture theory. (b) Loss rate coefficients Kloss as functions of collision energy or temperature for a partial-wave dependent QDT optimized to agree with the coupled-channels BaCl+(v=0, j=1)+Ca vibrational quenching rate coefficient. The blue curve shows Kloss(E) as a function of collision energy, whereas the black curve shows the corresponding thermally averaged rate coefficient. The red curve corresponds to the close-coupling results ‘Vib 1 0' shown in Fig. 2. Shape resonances in Kloss(E) are assigned by their partial wave l. The inset shows the short-range amplitude with parameters η0=0.9916, η1=−0.003 and ηmin=0.6. The short-range phase is δℓm(E)=0.34π.

Mentions: Figure 3a shows the quenching rate coefficient in the so-called universal limit, where all collisions reaching short range lead to quenching, that is, ηℓm(E)=0. This rate agrees reasonably well with experiment and theory at the experimentally relevant energies, but dramatically overestimates the quenching rate at low energies. We thus conclude that the reduction in quenching rate at low energy is not due to quantum suppression as would be expected in systems with shorter ranged potentials8. Therefore, we match the QDT result to the close-coupling calculation by parameterizing the short-range boundary condition as δℓm(E)=δ0 and ηℓm(E)=η0+η1l(l+1) when 0≤ηmin≤ηℓm(E)≤1 and ηℓm(E)=ηmin or 1 otherwise, where η0,ηmin, η1 and δ0 are determined by least-squares fitting (see the Methods for details). Figure 3b shows the good agreement between this fit and the close-coupling calculation and indicates that the suppression of the rate coefficient at low energies comes from lower quenching probabilities for small partial waves, as might be expected since the vibrational quenching is driven by asymmetry in the interaction potential.


Explanation of efficient quenching of molecular ion vibrational motion by ultracold atoms.

Stoecklin T, Halvick P, Gannouni MA, Hochlaf M, Kotochigova S, Hudson ER - Nat Commun (2016)

Quantum defect theory (QDT) of the Ca+BaCl+ collision.(a) Loss rate coefficients based on the QDT as functions of collision energy. The black solid line corresponds to the loss rate coefficient assuming universal or completely absorbing short-range boundary conditions. Coloured lines correspond to loss rates contributions from individual partial waves and its projections with l≤3. Finally, the dashed line is the loss rate coefficient found by the Langevin capture theory. (b) Loss rate coefficients Kloss as functions of collision energy or temperature for a partial-wave dependent QDT optimized to agree with the coupled-channels BaCl+(v=0, j=1)+Ca vibrational quenching rate coefficient. The blue curve shows Kloss(E) as a function of collision energy, whereas the black curve shows the corresponding thermally averaged rate coefficient. The red curve corresponds to the close-coupling results ‘Vib 1 0' shown in Fig. 2. Shape resonances in Kloss(E) are assigned by their partial wave l. The inset shows the short-range amplitude with parameters η0=0.9916, η1=−0.003 and ηmin=0.6. The short-range phase is δℓm(E)=0.34π.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4837476&req=5

f3: Quantum defect theory (QDT) of the Ca+BaCl+ collision.(a) Loss rate coefficients based on the QDT as functions of collision energy. The black solid line corresponds to the loss rate coefficient assuming universal or completely absorbing short-range boundary conditions. Coloured lines correspond to loss rates contributions from individual partial waves and its projections with l≤3. Finally, the dashed line is the loss rate coefficient found by the Langevin capture theory. (b) Loss rate coefficients Kloss as functions of collision energy or temperature for a partial-wave dependent QDT optimized to agree with the coupled-channels BaCl+(v=0, j=1)+Ca vibrational quenching rate coefficient. The blue curve shows Kloss(E) as a function of collision energy, whereas the black curve shows the corresponding thermally averaged rate coefficient. The red curve corresponds to the close-coupling results ‘Vib 1 0' shown in Fig. 2. Shape resonances in Kloss(E) are assigned by their partial wave l. The inset shows the short-range amplitude with parameters η0=0.9916, η1=−0.003 and ηmin=0.6. The short-range phase is δℓm(E)=0.34π.
Mentions: Figure 3a shows the quenching rate coefficient in the so-called universal limit, where all collisions reaching short range lead to quenching, that is, ηℓm(E)=0. This rate agrees reasonably well with experiment and theory at the experimentally relevant energies, but dramatically overestimates the quenching rate at low energies. We thus conclude that the reduction in quenching rate at low energy is not due to quantum suppression as would be expected in systems with shorter ranged potentials8. Therefore, we match the QDT result to the close-coupling calculation by parameterizing the short-range boundary condition as δℓm(E)=δ0 and ηℓm(E)=η0+η1l(l+1) when 0≤ηmin≤ηℓm(E)≤1 and ηℓm(E)=ηmin or 1 otherwise, where η0,ηmin, η1 and δ0 are determined by least-squares fitting (see the Methods for details). Figure 3b shows the good agreement between this fit and the close-coupling calculation and indicates that the suppression of the rate coefficient at low energies comes from lower quenching probabilities for small partial waves, as might be expected since the vibrational quenching is driven by asymmetry in the interaction potential.

Bottom Line: 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.

View Article: PubMed Central - PubMed

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