The rotational excitation of methanol by molecular hydrogen

We have computed cross-sections and rate coefficients for the rotational excitation of A- and E-type methanol by molecular hydrogen. Calculations were performed for rotational transitions within the torsional ground state, ν = 0, and within the first and second excited torsional states, ν = 1 and ν = 2. For collisions of methanol with para-H₂ in its rotational ground state, j₂ = 0, the methanol basis included rotational states j₁ <= 15, thereby extending previous calculations, which included states j₁ <= 9 only. For the first time, calculations have also been performed for ortho-H₂ in its rotational ground state, j₂ = 1, although it was necessary to revert to the smaller methanol basis, j₁ <= 9, owing to the coupling to states of molecular hydrogen with j₂ > 0. The coupled states approximation was used in the production calculations, to generate the thermal rate coefficients at temperatures 10 <= T <= 200 K, but some limited comparisons, at a few collision energies, of cross-sections obtained using the full coupled channels (CC) method have been made. The propensities of the collisions, with respect to changes in the rotational quantum number, j₁, and its projection, K, on the symmetry axis of the methanol molecule, were investigated. There are qualitative differences between the K propensities for collisions with ortho- and para-H₂, which relate to the fact that the inelastic cross-sections tend to be significantly larger when ortho-H₂ is the perturber.