Probing Nonadiabatic Effects in Low-Energy C( 3 P j ) + H 2 Collisions.

Nonadiabatic effects are of fundamental interest in collision dynamics. In particular, inelastic collisions between open-shell atoms and molecules, such as the collisional excitation of C( ³ P _j ) by H ₂ , are governed by nonadiabatic and spin-orbit couplings that are the sole responsible of collisional energy transfer. Here, we study collisions between carbon in its ground state C( ³ P _j=0 ) and molecular hydrogen (H ₂ ) at low collision energies that result in spin-orbit excitation to C( ³ P _j=1 ) and C( ³ P _j=2 ). State-to-state integral cross sections are obtained experimentally from crossed-beam experiments with a source of almost pure beam of C( ³ P _j=0 ) and theoretically from highly accurate quantum calculations. We observe very good agreement between experimental and theoretical data that demonstrates our ability to model nonadiabatic dynamics. New rate coefficients at temperatures relevant to astrochemical modeling are also provided. They should lead to an increase of the abundance of atomic C( ³ P) derived from the observations of interstellar clouds and a decrease of the efficiency of the cooling of the interstellar gas due to carbon atoms.