Simulating rotationally inelastic collisions using a direct simulation Monte Carlo method

Schullian, Otto; Loreau, Jérôme; Vaeck, Nathalie; Avoird, Ad van der; Heazlewood, Brianna R.; Rennick, Chris

doi:10.1080/00268976.2015.1098740

Cited by 17 publications

(16 citation statements)

References 23 publications

(29 reference statements)

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“…For example, the rotational cooling of ammonia seeded into a helium supersonic jet was recently modeled by taking into account rotationally inelastic NH 3 -He collisions. 30 The recent report of the experimental trapping of cold ground state argon atoms 31 also suggests that the sympathetic cooling of molecules such as ammonia might be realized by thermalizing collisions with rare gases. The possibility of sympathetically cooling NH 3 using laser-cooled alkali atoms has been investigated previously 32 but the potential energy surfaces for these systems were deemed too anisotropic.…”

Section: Introductionmentioning

confidence: 99%

Scattering of NH3 and ND3 with rare gas atoms at low collision energy

Loreau

Avoird²

2015

The Journal of Chemical Physics

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We present a theoretical study of elastic and rotationally inelastic collisions of NH 3 and ND 3 with rare gas atoms (He, Ne, Ar, Kr, Xe) at low energy. Quantum close-coupling calculations have been performed for energies between 0.001 and 300 cm −1 . We focus on collisions in which NH 3 is initially in the upper state of the inversion doublet with j = 1, k = 1, which is the most relevant in an experimental context as it can be trapped electrostatically and Stark-decelerated. We discuss the presence of resonances in the elastic and inelastic cross sections, as well as the trends in the inelastic cross sections along the rare gas series and the differences between NH 3 and ND 3 as a colliding partner. We also demonstrate the importance of explicitly taking into account the umbrella (inversion) motion of NH 3 in order to obtain accurate scattering cross sections at low collision energy. Finally, we investigate the possibility of sympathetic cooling of ammonia using cold or ultracold rare gas atoms. We show that some systems exhibit a large ratio of elastic to inelastic cross sections in the cold regime, which is promising for sympathetic cooling experiments. The close-coupling calculations are based on previously reported ab initio potential energy surfaces for NH 3 -He and NH 3 -Ar, as well as on new, four-dimensional, potential energy surfaces for the interaction of ammonia with Ne, Kr, and Xe, which were computed using the coupled-cluster method and large basis sets. We compare the properties of the potential energy surfaces corresponding to the interaction of ammonia with the various rare gas atoms. C 2015 AIP Publishing LLC. [http://dx

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Section: Introductionmentioning

confidence: 99%

Scattering of NH3 and ND3 with rare gas atoms at low collision energy

Loreau

Avoird²

2015

The Journal of Chemical Physics

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“…While this can lead to statistical scatter, it is only problematic in situations where there are very low densities and thus the simulated particles (which each represent 10 11 physical He or NH 3 particles) cannot adequately represent the physical conditions. As detailed in a previous publication, 10 the DSMC method can successfully describe a supersonic beam expansion. A similar approach is adopted here, where the system of interest-a buffer gas cell-is divided into subcell regions of finite size.…”

Section: A Dsmc Modelmentioning

confidence: 99%

“…The principle of detailed balance is satisfied, and the total energy is conserved at each step, as described in previous work. 10 Collisions are only considered between pairs of particles within the same simulated subcell. The probability P coll of a collision between two simulated particles (which each represent F N real particles) during time interval ∆t is equal to the ratio of the volume swept out by the total collision cross section at the relative velocity of the collision, divided by the volume of the subcell (V cell ),…”

Section: B Rotational State-changing Collision Cross Sectionsmentioning

confidence: 99%

“…The convergence properties of the scattering calculations are described elsewhere. 10,11 C. NH 3 -buffer gas collisions…”

Section: B Rotational State-changing Collision Cross Sectionsmentioning

confidence: 99%

“…When a collision is deemed to have occurred between a NH 3 molecule and a He buffer gas atom, the collision energy is calculated and the appropriate rotationally inelastic cross section is selected. 10 Post-collision velocities are randomly oriented, with the magnitude of the velocity determined by the energy transferred in the collision. Calculating collisions between all M pairs of simulated particles in a subcell is inefficient, given the frequently large number of simulated particles and the low collision probability for any given pair.…”

Section: B Rotational State-changing Collision Cross Sectionsmentioning

confidence: 99%

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Using a direct simulation Monte Carlo approach to model collisions in a buffer gas cell

Doppelbauer

Schullian

Loreau

et al. 2017

The Journal of Chemical Physics

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A direct simulation Monte Carlo (DSMC) method is applied to model collisions between He buffer gas atoms and ammonia molecules within a buffer gas cell. State-to-state cross sections, calculated as a function of the collision energy, enable the inelastic collisions between He and NH 3 to be considered explicitly. The inclusion of rotational-state-changing collisions affects the translational temperature of the beam, indicating that elastic and inelastic processes should not be considered in isolation. The properties of the cold molecular beam exiting the cell are examined as a function of the cell parameters and operating conditions; the rotational and translational energy distributions are in accord with experimental measurements. The DSMC calculations show that thermalisation occurs well within the typical 10-20 mm length of many buffer gas cells, suggesting that shorter cells could be employed in many instances-yielding a higher flux of cold molecules.

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New Method to Study Ion–Molecule Reactions at Low Temperatures and Application to the Reaction

et al. 2016

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Studies of ion-molecule reactions at low temperatures are difficult because stray electric fields in the reaction volume affect the kinetic energy of charged reaction partners. We describe a new experimental approach to study ion-molecule reactions at low temperatures and present, as example, a measurement of the H ion prepared in a single rovibrational state at collision energies in the range E col /k B = 5-60 K. To reach such low collision energies, we use a merged-beam approach and observe the reaction within the orbit of a Rydberg electron, which shields the ions from stray fields.The first beam is a supersonic beam of pure ground-state H 2 molecules and the second is a supersonic beam of H 2 molecules excited to Rydberg-Stark states of principal quantum number n selected in the range 20-40. Initially, the two beams propagate along axes separated by an angle of 10• . To merge the two beams, the Rydberg molecules in the latter beam are deflected using a surface-electrode Rydberg-Stark deflector. The collision energies of the merged beams are determined by measuring the velocity distributions of the two beams and they are adjusted by changing the temperature of the pulsed valve used to generate the ground-state H 2 beam and by adapting the electric-potential functions to the electrodes of the deflector. The collision energy is varied down to below E col /k B = 10 K, i.e., below

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Simulating rotationally inelastic collisions using a direct simulation Monte Carlo method

Cited by 17 publications

References 23 publications

Scattering of NH3 and ND3 with rare gas atoms at low collision energy

Scattering of NH3 and ND3 with rare gas atoms at low collision energy

Using a direct simulation Monte Carlo approach to model collisions in a buffer gas cell

New Method to Study Ion–Molecule Reactions at Low Temperatures and Application to the Reaction

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