Study of the Ne(3P2) + CH3F Electron-Transfer Reaction below 1 K

Benjamin Bertsche, Justinas Jankunas, Andreas Osterwalder
Amer Chemical Soc, 2014
Article

Résumé

Relatively little is known about the dynamics of electron-transfer reactions at low collision energy. We present a study of Penning ionization of ground-state methyl fluoride molecules by electronically excited neon atoms in the 13 μeV–4.8 meV (150 mK–56 K) collision energy range, using a neutral–neutral merged beam setup. Relative cross sections have been measured for three Ne(3P2) + CH_3F reaction channels by counting the number of CH_3F^+, CH_2F^+, and CH_3^+ product ions as a function of relative velocity between the neon and methyl fluoride molecular beams. Experimental cross sections markedly deviate from the Langevin capture model at collision energies above 20 K. The branching ratios are constant. In other words, the chemical shape of the CH3F molecule, as seen by the Ne(3P2) atom, appears not to change as the collision energy is varied, in contrast to related Ne(3PJ) + CH_3X (X = Cl and Br) reactions at higher collision energies.

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https://infoscience.epfl.ch/record/199989?ln=fr

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

Benjamin Bertsche, Justinas Jankunas, Andreas Osterwalder

Two isotopic chemical reactions, Ne* + NH_3, and 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, Ne* + NH_3 → Ne + NH^+_3 + e −, and Ne* + NH_3 → 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 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, Γ=[NH^+_2]/([NH^+_2]+[NH^+_3]), is relatively constant, Γ ≈ 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.

American Institute of Physics2014

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The chemi-ionization of Ar, Kr, N2, H2, and D2 by Ne(3P2) and of Ar, Kr, and N2 by He(3S1) was studied by electron velocity map imaging (e-VMI) in a crossed molecular beam experiment. A curved magnetic hexapole was used to state-select the metastable species. Collision energies of 60 meV were obtained by individually controlling the beam velocities of both reactants. The chemi-ionization of atoms and molecules can proceed along different channels, among them Penning ionization and associative ionization. The evolution of the reaction is influenced by the internal redistribution of energy, which happens at the first reaction step that involves the emission of an electron. We designed and built an e-VMI spectrometer in order to investigate the electron kinetic energy distribution, which is related to the internal state distribution of the ionic reaction products. The analysis of the electron kinetic energy distributions allows an estimation of the ratio between the two-reaction channel Penning and associative ionization. In the molecular cases the vibrational or electronic excitation enhanced the conversion of internal energy into the translational energy of the forming ions, thus influencing the reaction outcome.

2021

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