Excitation of the fine-structure transitions of C in collisions with ortho- and para-H2

The authors present the results of ab initio (SCF and CEPA) calculations of the interaction potentials for the C( 3 P)-H 2 ( 1 Sigma g + ) system. The five potential curves corresponding to collinear ( 3 Sigma - , 3 Pi ) and orthogonal ( 3 A 2 , 3 B 1 , 3 B 2 ) approaches of the C atom to the midpoint of the H 2 molecule have been computed, as well as the curve of 3 A' symmetry for an intermediate geometry. These results are analysed and used in quantum mechanical (close-coupling) calculations of the cross sections for excitation of the fine structure transitions in C( 3 P J ) by ortho- and para-H 2 . The rate coefficients for these transitions are evaluated for a range of temperatures encompassing those relevant to studies of the interstellar medium.