Dynamics of gas phase Ne∗^* + NH3_3 and Ne∗^* + ND3_3 Penning ionization at low temperatures

Two isotopic chemical reactions, $\mathrm{Ne}^*$ + NH$_3$, and $\mathrm{Ne}^*$ + ND$_3$, have been studied at low collision energies by means of a merged beams technique. Partial cross sections have been recorded for the two reactive channels, namely $\mathrm{Ne}^*$ + NH$_3$ $\rightarrow$ Ne + NH$_3^+$ + $e^-$, and $\mathrm{Ne}^*$ + NH$_3$ $\rightarrow$ Ne + NH$_2^+$ + H + $e^-$, by detecting the NH$_3^+$ and NH$_2^+$ product ions, respectively. The cross sections for both reactions were found to increase with decreasing collision energy, $E_{coll}$, in the range 8 $\mu$eV$<E_{coll}<$ 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 $\mathrm{Ne}^*$ + 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, $\Gamma = \frac{[NH_2^+]}{[NH_2^+]+[NH_3^+]}$, is relatively constant, $\Gamma\approx 0.3$, in the entire collision energy range studied here. Possible reasons for this observation are discussed and rationalised in terms of relative time scales of the reactant approach and the molecular rotation. Isotopic differences between the $\mathrm{Ne}^*$ + NH$_3$ and $\mathrm{Ne}^*$ + ND$_3$ reactions are small, as suggested by nearly equal branching ratios and cross sections for the two reactions

Cold and Controlled Molecular Beams: Production and Applications

The field of cold molecules has become an important source of new insight in fundamental chemistry and molecular physics. High-resolution spectroscopy benefits from translationally and internally cold molecules by increased interaction times and reduced spectral congestion. Completely new effects in scattering dynamics become accessible with cold and controlled molecules. Many of these experiments use molecular beams as a starting point for the generation of molecular samples. This review gives an overview of methods to produce beams of cold molecules, starting from supersonic expansions or effusive sources, and provides examples of applications in spectroscopy and molecular dynamics studies

Low-temperature Collisions between Neutral Molecules in Merged Molecular Beams

We have developed an experiment for the investigation of neutral molecular collisions in the gas phase at temperatures as low as 100 mK. These low temperatures are obtained by merging two supersonic expansions, using an electric and a magnetic guide, and by matching the velocities of the beams. Since the energy available for the collisions, or the temperature, is determined only by the relative velocity of the reaction partners this enables the study of chemical processes at very low temperatures without the need to prepare slow molecules in the laboratory frame of reference. This paper describes the method and presents results on the Ne(P-3(2))+NH3 Penning ionization

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

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(S-3(1)) + NH3 Penning ionization reaction dynamics is described in the collision energy range 3.3 mu eV < E-coll < 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 P-sr = 0.035. No clear signature of the orbiting resonances is visible in the branching ratio between NH3+ and NH2+ formation. (C) 2015 AIP Publishing LLC

Importance of rotationally inelastic processes in low-energy Penning ionization of CHF3

Low energy reaction dynamics can strongly depend on the internal structure of the reactants. The role of rotationally inelastic processes in cold collisions involving polyatomic molecules has not been explored so far. Here we address this problem performing a merged-beam study of the He*+CHF$_3$ Penning ionization reaction in a range of collision energies $E/k_B$=0.5--120 K. The experimental cross sections are compared with total reaction cross sections calculated within the framework of the quantum defect theory. We find that the broad range of collision energies combined with the relatively small rotational constants of \chfs makes rotationally inelastic collisions a crucial player in the total reaction dynamics. Quantitative agreement between theory and experiment is only obtained if the energy-dependent probability for rotational excitation is included in the calculations, in stark contrast to previous experiments where classical scaling laws were able to describe the results

Experimental and Theoretical Studies of Low-Energy Penning Ionization of NH3, CH3F, and CHF3

We present results from a joint theoretical and experimental study of the low-energy Penning ionization of NH3, CH3F, and CHF3 by metastable Ne(P-3(2)) and He(S-3(1)) atoms. We combine the merged neutral beams experiment, covering a range of collision energies between 0.1-150K, with multichannel quantum defect theory calculations based on interaction potentials from symmetry-adapted perturbation theory. The three symmetric tops provide several distinct properties that make them interesting targets for cold chemistry studies. Of these three, only NH3 has a lone electron pair that leads to a strong binding with rare gas atoms. The CHF3 molecule has much smaller rotational constants than both NH3 and CH3F, and thus has a considerably higher density of rotational states already at low energies. Their presence opens inelastic collision channels that reduce the observed reactive cross section. We show that this effect dominates the total rate coefficient in heavy molecules already at relatively low collision energies but is much less prominent for lighter molecule

Oriented O(3P2), Ne(3P2), and He(3S1) atoms emerging from a bent magnetic guide

We describe our observation of strongly oriented total electronic angular momentum J in O(P-3(2)), Ne(P-3(2)), and He(S-3(1)) atoms emerging from a bent magnetic multipole guide, as measured by resonant multiphoton ionisation. This was contrary to our expectation because no additional (uniform) magnetic fields were applied to orient the atoms behind the exit of the guide. Two- and three-photon ionisation techniques were employed to determine the degree of J polarisation, from which we infer that atoms become oriented as a result of a combination of weak fringe fields, possible stray fields, and the fact that molecular beam packets do not oscillate around the geometric center of the bent multipole guide. We conclude that similar effects may exist in other, related experiments and that a detailed characterisation of the degree of orientation is required prior to any study of chemical dynamics or spectroscopy. This paper should serve as a warning for anybody using similar devices not to assume isotropic angular momentum distributions of atoms and molecules emerging from a magnetic guide or a decelerator, particularly when it is bent; whenever possible, the possibility for a J anisotropy should be experimentally checked. [GRAPHICS