Quantal calculations on the rotational excitation of NH(_3) and OH in collisions with H(_2)

Abstract

Results are presented for quantal close coupled calculations of the rotational excitation of NH(_3) and OH in collisions with both ortho and para-H(_2). For the latter, these are the first calculations to include the rotational structure of the H(_2) molecule, whilst for the former, previous NH(_3) - ortho-H(_2) calculations have been subject to subsidiary approximations. The results from the NH3-H2 calculation show substantial qualitative changes in the cross-sections when ground state ortho-H(_2) (j = 1) replaces ground state para-H(_2) (j = 0) as the collision partner. In particular, cross-sections which were very small for NH(_3) - para-H(_2) collisions can be of a comparable magnitude with the other rotationally inelastic cross-sections for NH(_3) - ortho-H(_2) collisions. The changes in cross-sections are discussed in relation to the collisional pumping scheme for an astrophysical maser in the (jk = 33) inversion lines. From the OH-H(_2) calculations it is found that the propensities towards preferential excitation of a given component of the A doublets are reduced in strength when ortho-H(_2) replaces ground state para-H(_2) as the collision partner, similarly when (j = 2) para-H(_2) replaces ground state para-H(_2) the propensities are weakened. In both cases, the results are discussed in the context of crossed beam measurements at energies of 605cm(^-1)(NH(_3)-H(_2)) and 680cm(^-1) (OH-H(_2)). It is found that discrepancies between the experimental results and theoretical calculations for ground state para-H(_2) collisions can be explained, at least in part, by the neglect of the (j > 0) H(_2) rotational states in the latter.