Quantal calculations on the rotational excitation of NH₃ and OH in collisions with H₂
Results are presented for quantal close coupled calculations of the rotational excitation of NH₃ and OH in collisions with both ortho and para-H₂. For the latter, these are the first calculations to include the rotational structure of the H₂ molecule, whilst for the former, previous NH₃ - ortho-H₂ 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-H₂ (j = 1) replaces ground state para-H₂ (j = 0) as the collision partner. In particular, cross-sections which were very small for NH₃ - para-H₂ collisions can be of a comparable magnitude with the other rotationally inelastic cross-sections for NH₃ - ortho-H₂ 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-H₂ calculations it is found that the propensities towards preferential excitation of a given component of the A doublets are reduced in strength when ortho-H₂ replaces ground state para-H₂ as the collision partner, similarly when (j = 2) para-H₂ replaces ground state para-H₂ the propensities are weakened. In both cases, the results are discussed in the context of crossed beam measurements at energies of 605cm⁻¹ (NH₃-H₂) and 680cm⁻¹ (OH-H₂). It is found that discrepancies between the experimental results and theoretical calculations for ground state para-H₂ collisions can be explained, at least in part, by the neglect of the (j > 0) H₂ rotational states in the latter.