Rotational excitation of HCN by para- and ortho-H2

Rotational excitation of the hydrogen cyanide (HCN) molecule by collisions with para-H-2(j = 0, 2) and ortho-H-2(j = 1) is investigated at low temperatures using a quantum time independent approach. Both molecules are treated as rigid rotors. The scattering calculations are based on a highly correlated ab initio 4-dimensional (4D) potential energy surface recently published. Rotationally inelastic cross sections among the 13 first rotational levels of HCN were obtained using a pure quantum close coupling approach for total energies up to 1200 cm(-1). The corresponding thermal rate coefficients were computed for temperatures ranging from 5 to 100 K. The HCN rate coefficients are strongly dependent on the rotational level of the H-2 molecule. In particular, the rate coefficients for collisions with para-H-2(j = 0) are significantly lower than those for collisions with ortho-H-2(j = 1) and para-H2(j = 2). Propensity rules in favor of even Delta j transitions were found for HCN in collisions with para-H-2(j = 0) whereas propensity rules in favor of odd Delta j transitions were found for HCN in collisions with H-2(j >= 1). The new rate coefficients were compared with previously published HCN-para-H-2(j = 0) rate coefficients. Significant differences were found due the inclusion of the H-2 rotational structure in the scattering calculations. These new rate coefficients will be crucial to improve the estimation of the HCN abundance in the interstellar medium. (C) 2014 AIP Publishing LLC.