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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

Calculated quenching rates.(a) Comparison between the vibrational and rotational quenching rate coefficients for BaCl+ in the initial states (ν=1, j=0,1,2,3,4). The first and second numbers designate, respectively, the initial vibrational and rotational quantum number of BaCl+. (b) Comparison between the vibrational quenching rate coefficients of several excited rovibrational levels (ν, j) of BaCl+ resulting from collisions with Ca with the experimental results and with the Langevin law. The first and second numbers designate, respectively, the initial vibrational and rotational quantum number of BaCl+. (c) Comparison between the vibrational and rotational quenching rate coefficients for BaCl+ in the initial states (ν=2, j=0,1,2,5). The first and second numbers designate, respectively, the initial vibrational and rotational quantum number of BaCl+. (d) Comparison between the vibrational and rotational quenching rate coefficients for BaCl+ in the initial state (ν=3, j=2). The label of each curve designates the final vibrational level.
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f2: Calculated quenching rates.(a) Comparison between the vibrational and rotational quenching rate coefficients for BaCl+ in the initial states (ν=1, j=0,1,2,3,4). The first and second numbers designate, respectively, the initial vibrational and rotational quantum number of BaCl+. (b) Comparison between the vibrational quenching rate coefficients of several excited rovibrational levels (ν, j) of BaCl+ resulting from collisions with Ca with the experimental results and with the Langevin law. The first and second numbers designate, respectively, the initial vibrational and rotational quantum number of BaCl+. (c) Comparison between the vibrational and rotational quenching rate coefficients for BaCl+ in the initial states (ν=2, j=0,1,2,5). The first and second numbers designate, respectively, the initial vibrational and rotational quantum number of BaCl+. (d) Comparison between the vibrational and rotational quenching rate coefficients for BaCl+ in the initial state (ν=3, j=2). The label of each curve designates the final vibrational level.

Mentions: Figure 2b shows the calculated vibrational quenching rate coefficients for a selected set of rovibrational states of BaCl+. A thick horizontal line represents the experimental measurement4 of the population averaged vibrational relaxation rate for v=1 and v=2, where the length of the line is representative of the energy range in the experiment. The ion-induced-dipole Langevin law9 is shown as a dashed line. The calculated values compare very well with the experiment, whereas the Langevin law is roughly double the experimental value. The close-coupling rate follows the Langevin law in the temperature domain of the experiment and departs from it at lower temperature—as discussed later, this departure is not due to quantum suppression10 as one might expect. In Figures 2a, 2c, 2d, the vibrational and rotational quenching are compared for several initial rotational levels belonging, respectively, to the vibrational levels ν=1, 2 and 3. The vibrational quenching is always larger than the rotational quenching. This very unusual result is due to the low value of the vibrational frequency of BaCl+ and the deep potential well, together yielding a strong coupling between many vibrational levels. This is in contrast with previously studied atom-diatom van der Waal neutral or ionic complexes111213, where the potential well is usually not deep enough to couple even two different vibrational levels of the diatom. The only other possibility to obtain vibrational quenching comparable to rotational quenching is when the bond length of the complex is smaller than expected with a pure Van der Waals interaction, indicating the rise of chemical bonding induced by electron sharing between monomers. This is, for example, the case of the He-CH+ complex14.


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)

Calculated quenching rates.(a) Comparison between the vibrational and rotational quenching rate coefficients for BaCl+ in the initial states (ν=1, j=0,1,2,3,4). The first and second numbers designate, respectively, the initial vibrational and rotational quantum number of BaCl+. (b) Comparison between the vibrational quenching rate coefficients of several excited rovibrational levels (ν, j) of BaCl+ resulting from collisions with Ca with the experimental results and with the Langevin law. The first and second numbers designate, respectively, the initial vibrational and rotational quantum number of BaCl+. (c) Comparison between the vibrational and rotational quenching rate coefficients for BaCl+ in the initial states (ν=2, j=0,1,2,5). The first and second numbers designate, respectively, the initial vibrational and rotational quantum number of BaCl+. (d) Comparison between the vibrational and rotational quenching rate coefficients for BaCl+ in the initial state (ν=3, j=2). The label of each curve designates the final vibrational level.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f2: Calculated quenching rates.(a) Comparison between the vibrational and rotational quenching rate coefficients for BaCl+ in the initial states (ν=1, j=0,1,2,3,4). The first and second numbers designate, respectively, the initial vibrational and rotational quantum number of BaCl+. (b) Comparison between the vibrational quenching rate coefficients of several excited rovibrational levels (ν, j) of BaCl+ resulting from collisions with Ca with the experimental results and with the Langevin law. The first and second numbers designate, respectively, the initial vibrational and rotational quantum number of BaCl+. (c) Comparison between the vibrational and rotational quenching rate coefficients for BaCl+ in the initial states (ν=2, j=0,1,2,5). The first and second numbers designate, respectively, the initial vibrational and rotational quantum number of BaCl+. (d) Comparison between the vibrational and rotational quenching rate coefficients for BaCl+ in the initial state (ν=3, j=2). The label of each curve designates the final vibrational level.
Mentions: Figure 2b shows the calculated vibrational quenching rate coefficients for a selected set of rovibrational states of BaCl+. A thick horizontal line represents the experimental measurement4 of the population averaged vibrational relaxation rate for v=1 and v=2, where the length of the line is representative of the energy range in the experiment. The ion-induced-dipole Langevin law9 is shown as a dashed line. The calculated values compare very well with the experiment, whereas the Langevin law is roughly double the experimental value. The close-coupling rate follows the Langevin law in the temperature domain of the experiment and departs from it at lower temperature—as discussed later, this departure is not due to quantum suppression10 as one might expect. In Figures 2a, 2c, 2d, the vibrational and rotational quenching are compared for several initial rotational levels belonging, respectively, to the vibrational levels ν=1, 2 and 3. The vibrational quenching is always larger than the rotational quenching. This very unusual result is due to the low value of the vibrational frequency of BaCl+ and the deep potential well, together yielding a strong coupling between many vibrational levels. This is in contrast with previously studied atom-diatom van der Waal neutral or ionic complexes111213, where the potential well is usually not deep enough to couple even two different vibrational levels of the diatom. The only other possibility to obtain vibrational quenching comparable to rotational quenching is when the bond length of the complex is smaller than expected with a pure Van der Waals interaction, indicating the rise of chemical bonding induced by electron sharing between monomers. This is, for example, the case of the He-CH+ complex14.

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