Reaction cross-sections /crossed molecular beams


PhD, 2021-2024 :
WaterStars, from observation to experiments on water in space.

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PhD Waterstars details
Thesis 2021-2024, ISM-LAB, Astrid Bergeat & Fabrice Herpin

The aim of our Crossed Molecular Beam (CMB) experiment is to get microscopic information SUCH as integral and differential cross sections on bimolecular reactive collisions in the very low collision energy regime prevailing in the interstellar medium. Experiments are performed with a crossed molecular beam apparatus which associates pulsed nozzles and pulsed lasers to create a beam of highly reactive transient species and to probe reactants and products by resonance-enhanced multiphoton ionisation.
Furthermore, the beam intersection angle is continuously adjustable between 90° and 22.5°, which is a key factor in obtaining very low relative translational energies.

This experiment has allowed us to obtain integral and differential cross sections of reactions between unsaturated hydrocarbons with atomic carbon C(3PJ), abundant species in cold interstellar clouds, down to a relative translational energy of reactants ET = 0,4 kJ mol-1, which corresponds to a mean thermal energy (ET = 3/2 RT) of 30 K.
It has been unambiguously demonstrated that such reactions are characterized by an integral cross section which increases when the relative translational energy decreases and are thus without any reaction barrier.

Ongoing research is now focused on inelastic collisions with H2 and He. Experiments conducted with our crossed, molecular beam apparatus allowed us to determine the integral cross sections as a function of the relative translational energy down to a few wavenumbers, i.e. below the thresholds of the (j = 0 - 1) CO transition at 3.85 cm-1 and the ((N = 1 ; j = 0) - (N = 1 ; j = 1)) O2 transition at 3.96 cm-1. The experimental results, which exhibit behaviours characteristic of quantum resonances, are compared to QM scattering calculations. The close agreement between experimental and theoretical (QM scattering) integral cross sections allows for complete assignment of the resonances observed and the results obtained for O2 + H2 show that this ((N = 1 ; j = 0) - (N = 1 ; j = 1)) collision-induced transition occurs exclusively in a pure quantum regime via shape and Feshbach resonances arising from definite partial waves.

Text book :
Low energy scattering in crossed molecular beams.
C. Naulin and A. Bergeat,
in Cold Chemistry : Molecular Scattering and Reactivity Near Absolute Zero, Editors : Olivier Dulieu, Andreas Osterwalder,
Royal Society of Chemistry (2017), Chap. 3, 92.


Reactive Collisions :
Dynamics of the S(1D2) + HD(j=0) Reaction at Collision Energies Approaching the Cold Regime : A Stringent Test for Theory.
M. Lara, S. Chefdeville, K. M. Hickson, A. Bergeat, C. Naulin, J.-M. Launay, M. Costes
Phys. Rev. Lett. , 2012, 109, 133201.

Kinetics and Dynamics of the S(1D2) + H2 → SH + H Reaction at Very Low Temperatures and Collision Energies.
C. Berteloite, M. Lara, A. Bergeat, S.D. Le Picard, F. Dayou, K.M. Hickson, A. Canosa, C. Naulin, JM. Launay, I.R. Sims, M. Costes
Phys. Rev. Lett. , 2010, 105, 203201.

Inelastic Collisions :
Understanding the quantum nature of low-energy C(3Pj) + He inelastic collisions.
A. Bergeat, S. Chefdeville, M. Costes, S. B. Morales, C. Naulin, U. Even, J. Klos, F. Lique
Nature Chemistry, 2018, 10, 519.

Quantum dynamical resonances in low-energy CO(j = 0) + He inelastic collisions.
Astrid Bergeat, Jolijn Onvlee, Christian Naulin, Ad van der Avoird and Michel Costes
Nature Chemistry, 2015, 7, 349.

Experimental and theoretical analysis of low-energy CO + H2 inelastic collisions.
Simon Chefdeville, Thierry Stoecklin, Christian Naulin, Piotr Jankowski, Krzysztof Szalewicz, Alexandre Faure, Michel Costes, Astrid Bergeat
Ap. J. Lett., 2015, 79, L9.

Observation of Partial Wave Resonances in Low-Energy O2 – H2 Inelastic Collisions.
Simon Chefdeville, Yulia Kalugina, Sebastiaan Y. T. van de Meerakker, Christian Naulin, François Lique, Michel Costes
Science, 2013, 341, 1094.





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