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Understanding the quantum nature of low-energy C(3Pj) + He inelastic collisions


Inelastic collisions that occur between open-shell atoms and other atoms or molecules, and that promote a spin–orbit transition, involve multiple interaction potentials. They are non-adiabatic by nature and cannot be described within the Born–Oppenheimer approximation; in particular, their theoretical modelling becomes very challenging when the collision energies have values comparable to the spin–orbit splitting. Here we study inelastic collisions between carbon in its ground state C(3Pj=0) and helium atoms—at collision energies in the vicinity of spin–orbit excitation thresholds (~0.2 and 0.5 kJ mol−1)—that result in spin–orbit excitation to C(3Pj=1) and C(3Pj=2). State-to-state integral cross-sections are obtained from crossed-beam experiments with a beam source that provides an almost pure beam of C(3Pj=0) . We observe very good agreement between experimental and theoretical results (acquired using newly calculated potential energy curves), which validates our characterization of the quantum dynamical resonances that are observed. Rate coefficients at very low temperatures suitable for chemical modelling of the interstellar medium are also calculated.

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Fig. 1: Comparison of experimental and theoretical cross-sections for C(3P0) + He → C(3P1) + He inelastic collisions.
Fig. 2: Comparison of experimental and theoretical cross-sections for C(3P0) + He → C(3P2) + He inelastic collisions.
Fig. 3: Adiabatic bender curves with total angular momentum J = 4 and 5 that correlate with the C(3P1) state.


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A.B., M.C., F.L., S.B.M. and C.N. acknowledge financial support from the Programme National ‘Physique et Chimie du Milieu Interstellaire’ (PCMI) of CNRS/INSU with INC/INP, co-funded by CEA and CNES, and the Agence Nationale de la Recherche, contract Hydrides (ANR-12-B505-0011-02). J.K. and F.L. acknowledge financial support from the US National Science Foundation grant no. CHE-1565872. The authors thank N. Lavie for constructing the DBD pulsed valve source, and M. Alexander for support and discussions about the quantum nature of resonances.

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A.B., S.C., M.C., S.B.M. and C.N. carried out the experimental measurements and data analysis, with the contribution of U.E. for the development of the carbon source. J.K. and F.L. performed the theoretical calculations. All authors discussed the results and contributed to the manuscript.

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Correspondence to Astrid Bergeat or François Lique.

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The authors declare no competing interests.

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Supplementary Information

Supplementary Methods, Figures 1–7 and Table 1

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Bergeat, A., Chefdeville, S., Costes, M. et al. Understanding the quantum nature of low-energy C(3Pj) + He inelastic collisions. Nature Chem 10, 519–522 (2018).

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