AUTHOR=Bergeat Astrid , Morales Sébastien B. , Naulin Christian , Kłos Jacek , Lique François 

TITLE=Quantum Behavior of Spin-Orbit Inelastic Scattering of C-Atoms by D2 at Low Energy

JOURNAL=Frontiers in Chemistry

VOLUME=7

YEAR=2019

URL=https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2019.00164

DOI=10.3389/fchem.2019.00164

ISSN=2296-2646

ABSTRACT=<p>Fine-structure populations and collision–induced energy transfer in atoms are of interest for many fields, from combustion to astrophysics. In particular, neutral carbon atoms are known to play a role in interstellar media, either as probes of physical conditions (ground state <sup>3</sup>P<sub><italic>j</italic></sub> spin-orbit populations), or as cooling agent (collisional excitation followed by radiative decay). This work aims at investigating the spin-orbit excitation of atomic carbon in its ground electronic state due to collisions with molecular deuterium, an isotopic variant of H<sub>2</sub>, the most abundant molecule in the interstellar medium. Spin-orbit excitations of C(<sup>3</sup>P<sub><italic>j</italic></sub>) by H<sub>2</sub> or D<sub>2</sub> are governed by non-adiabatic and spin-orbit couplings, which make the theoretical treatment challenging, since the Born-Oppenheimer approximation no longer holds. Inelastic collisional cross-sections were determined for the C(<sup>3</sup>P<sub>0</sub>) + D<sub>2</sub> → C(<sup>3</sup>P<sub><italic>j</italic></sub>) + D<sub>2</sub> (with <italic>j</italic> = 1 and 2) excitation process. Experimental data were acquired in a crossed beam experiment at low collision energies, down to the excitation thresholds (at 16.42 and 43.41 cm<sup>−1</sup>, respectively). C-atoms were produced mainly in their ground spin-orbit state, <sup>3</sup>P<sub>0</sub>, by dissociation of CO in a dielectric discharge through an Even-Lavie pulsed valve. The C-atom beam was crossed with a D<sub>2</sub> beam from a second valve. The state-to-state cross-sections were derived from the C(<sup>3</sup>P<sub><italic>j</italic></sub>) (<italic>j</italic> = 1 or 2) signal measured as a function of the beam crossing angle, i.e., as a function of the collision energy. The results show different quantum behaviors for excitation to C(<sup>3</sup>P<sub>1</sub>) or C(<sup>3</sup>P<sub>2</sub>) when C(<sup>3</sup>P<sub>0</sub>) collides with <italic>ortho</italic>-D<sub>2</sub> or <italic>normal</italic>-D<sub>2</sub>. These experimental results are analyzed and discussed in the light of highly accurate quantum calculations. A good agreement between experimental and theoretical results is found. The present data are compared with those obtained for the C-He and C-H<sub>2</sub> collisional systems to get new insights into the dynamics of collision induced spin-orbit excitation/relaxation of atomic carbon.</p>