Spontaneous Ionization in Slow Collisions

Niehaus, A.

doi:10.1002/9780470142646.ch5

Cited by 190 publications

(41 citation statements)

References 64 publications

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“…A wealth of data exists on the simplest type of PI where A * is an electronically excited rare gas atom and B an atom in the ground electronic state. 9,24 In this case, experiment and theory are far advanced, and often exhibit excellent agreement. Theory is less advanced when B is a molecule and the intermolecular potential becomes a highly dimensional surface.…”

Section: Introductionsupporting

confidence: 74%

“…9,24 The model operates only on four potential curves; the excited A * + B curves, corresponding to two different electronic orbitals which may take part in the process, the N-H bond or the lone pair centered on nitrogen atom, and the ionic A + B + curves describing the reaction products. Coupling between the initial and final states can be modelled by the complex part of the V * potential…”

Section: Theorymentioning

confidence: 99%

“…Calculations carried out for F + H 2 , H + HCl, and Li + HF reactions, for example (see Tables 3.1 and 3.3 of Ref. 9), confirm a non-zero rate constant at T = 0 K. Verification of these predictions remains an experimental challenge.…”

Section: Introductionmentioning

confidence: 99%

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Dynamics of gas phase Ne* + NH3 and Ne* + ND3 Penning ionisation at low temperatures

Jankunas

Bertsche

Jachymski

et al. 2014

The Journal of Chemical Physics

155

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Analytical potential energy surface for the NH3+HNH2+H2 reaction: Application of variational transition-state theory and analysis of the equilibrium constants and kinetic isotope effects using curvilinear and rectilinear coordinates J. Chem. Phys. 106, 4013 (1997) product ions, respectively. The cross sections for both reactions were found to increase with decreasing collision energy, E coll , in the range 8 μeV < E coll < 20 meV. The measured rate constant exhibits a curvature in a log(k)-log(E coll ) plot from which it is concluded that the Langevin capture model does not properly describe the Ne * + NH 3 reaction in the entire range of collision energies covered here. Calculations based on multichannel quantum defect theory were performed to reproduce and interpret the experimental results. Good agreement was obtained by including long range van der Waals interactions combined with a 6-12 Lennard-Jones potential. The branching ratio between the two reactive channels, =, is relatively constant, ≈ 0.3, in the entire collision energy range studied here. Possible reasons for this observation are discussed and rationalized in terms of relative time scales of the reactant approach and the molecular rotation. Isotopic differences between the Ne * + NH 3 and Ne * + ND 3 reactions are small, as suggested by nearly equal branching ratios and cross sections for the two reactions. © 2014 AIP Publishing LLC.

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Section: Introductionsupporting

confidence: 74%

Section: Theorymentioning

confidence: 99%

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Dynamics of gas phase Ne* + NH3 and Ne* + ND3 Penning ionisation at low temperatures

Jankunas

Bertsche

Jachymski

et al. 2014

The Journal of Chemical Physics

155

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“…4 At low energies, this leads to the observed relation σ(E coll ) ≈ E −1/3 coll . 21 When E coll becomes comparable with the potential well depth, short range repulsive terms become important and the behavior of the cross section changes. Two maxima are observed in the reaction cross sections, at E coll = 1.8 meV and 7.3 meV.…”

Section: Resultsmentioning

confidence: 99%

Observation of orbiting resonances in He(3S1) + NH3 Penning ionization

Jankunas

Jachymski

Hapka

et al. 2015

The Journal of Chemical Physics

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Resonances are among the clearest quantum mechanical signatures of scattering processes. Previously, shape resonances and Feshbach resonances have been observed in inelastic and reactive collisions involving atoms or diatomic molecules. Structure in the integral cross section has been observed in a handful of elastic collisions involving polyatomic molecules. The present paper presents the observation of shape resonances in the reactive scattering of a polyatomic molecule, NH3. A merged-beam study of the gas phase He((3)S1) + NH3 Penning ionization reaction dynamics is described in the collision energy range 3.3 μeV < Ecoll < 10 meV. In this energy range, the reaction rate is governed by long-range attraction. Peaks in the integral cross section are observed at collision energies of 1.8 meV and 7.3 meV and are assigned to ℓ = 15,16 and ℓ = 20,21 partial wave resonances, respectively. The experimental results are well reproduced by theoretical calculations with the short-range reaction probability Psr = 0.035. No clear signature of the orbiting resonances is visible in the branching ratio between NH3 (+) and NH2 (+) formation.

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“…Our main requirements were low translational temperature (since this ultimately limits the minimum average collision energy achievable in this experiment), tuneable collision energy that includes zero Kelvin, low internal temperature of both reactants, high density, and a maximum selection of accessible reactions, given the limitations imposed by the other criteria. For our first steps into the territory of merged neutral beams we decided to study a particular class of reactions that would alleviate some of the initial technical difficulties, be ideal systems to characterise the technique, and give access to several fundamentally interesting aspects of reactive collisions: Penning ionisation [60,61].…”

Section: General Experimental Considerationsmentioning

confidence: 94%

Merged neutral beams

Osterwalder

2015

EPJ Techn Instrum

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A detailed description of a merged beam apparatus for the study of low energy molecular scattering is given. This review is intended to guide any scientist who plans to construct a similar experiment, and to provide some inspiration in describing the approach we chose to our goal. In our experiment a supersonic expansion of paramagnetic particles is merged with one of polar molecules. A magnetic and an electric multipole guide are used to bend the two beams onto the same axis. We here describe in detail how the apparatus is designed, characterised, and operated.Keywords: Cold molecules; Low energy scattering; Molecular beams; Cold chemistry ReviewIn the last 15 years several methods have been developed to prepare molecular samples at temperatures well below 1 K and to completely control their translational degrees of freedom [1][2][3][4]. A particular area where this is of interest is the study of molecular collisions, and cold chemistry [4][5][6][7][8][9][10][11]. Ultracold molecules, produced by joining two atoms at very low temperature, enabled the investigation of molecular collisions at temperatures as low as 1 μK [12][13][14]. These methods are ideally suited to study alkali dimers, but in order to access broader classes of molecules and a larger range of temperatures a different approach was necessary. The present paper describes such an approach, namely the merging of two molecular beams with controlled velocity.One of the most important developments for reaction dynamics studies in the twentieth century was the crossed-beam technique, for which a Nobel prize was awarded in 1986. In this experiment two molecular beams [15] are crossed to enable a highly detailed investigation of gas-phase molecular scattering, and in particular the measurement of state-to-state differential cross sections [16,17]. The high velocities of supersonic expansions and the crossing angle of usually 90 degrees make the crossed beam technique ideal for studies at high collision energies (E/k B 100 K, where k B is the Boltzmann constant). Lower energies are not accessible without special modifications, thus making the method blind to some of the most fundamental quantum mechanical effects in molecular scattering.The collision energy in a crossed beam experiment depends on the relative velocity of the two reactants. In any molecular collision event the energetics are given only by the motion in the molecular frame of reference (MFR), but not that of the centre of mass (CM). This is illustrated in Fig. 1. Panel A shows the position vectors for reactants 1 and 2

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Spontaneous Ionization in Slow Collisions

Cited by 190 publications

References 64 publications

Dynamics of gas phase Ne* + NH3 and Ne* + ND3 Penning ionisation at low temperatures

Dynamics of gas phase Ne* + NH3 and Ne* + ND3 Penning ionisation at low temperatures

Observation of orbiting resonances in He(3S1) + NH3 Penning ionization

Merged neutral beams

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