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.

We report the first calculations of the spectrum of methanol, arising in shock waves in molecular outflows. The small grid of shock wave models that we have computed incorporates the results of very recent computations of the rate coefficients for the collisional excitation of methanol by ortho-and para-H 2 and by He. The two strongest transitions, one of A-and the other of E-type methanol, are masers that have been observed in a Class I methanol maser source, which is believed to be related to a molecular outflow. The same collisional propensities that give rise to population inversion and maser action can, in other transitions, lead to population anti-inversion and the lines appearing in absorption against the cosmic background radiation. We attempted to model specifically the outflow source L1157 B1, in which transitions of methanol have been observed recently by means of the Herschel satellite. Comparison with the predictions of the shock wave models is complicated by uncertainty in the value of the beam filling factor that should be adopted.

We have extended and improved previous calculations of cross-sections for the rotational excitation by helium of A-and E-type methanol in their torsional ground states. We increased the maximum value of the rotational quantum number, j, of the methanol molecule from j = 9, used in the previous calculations, to j = 15, thereby ensuring the completeness of the rotational energy levels and the corresponding basis of eigenfunctions up to the first threshold for exciting the internal torsional motion of the molecule. The representation of the rotational-torsional eigenfunctions of E-type methanol has been improved significantly. We compare the results of the present calculations, which were performed in both the coupled channels (CC) and the coupled states (CS) approximations, with the previous results. All the calculations exhibit the same collisional propensities, but there exist quantitative discrepancies, for individual transitions, between both the present and previous determinations and also between the present CC and CS results, notably for E-type methanol. Cross-sections have been generated on an energy grid that is adapted to the evaluation of the thermal rate coefficients at low temperatures (10 ≤ T ≤ 200 K); these data are available online.

Cross-sections and rate coefficients have been computed for rotationally and torsionally inelastic transitions between levels of the torsional manifolds ν = 0, 1 and 2 of methanol; the collision partner was the helium atom. We find the torsional inelasticity to be most pronounced for transitions between the excited states, ν = 1 and 2. The torsionally inelastic cross-sections are smaller, on average, than the cross-sections for pure rotationally inelastic transitions, by typically an order of magnitude. We compared cross-sections calculated using the full CH 3 OH-He interaction potential, which is a function of the torsion angle, with the cross-sections computed for the staggered conformation, i.e. for a fixed value of the torsion angle. The agreement between these two sets of results improves with increasing magnitude of the cross-section. Accordingly, the use of previous results, for the rotational excitation of methanol by He and by H 2 , obtained for the staggered conformation, should not compromise the analysis of observations of methanol in the interstellar medium.

A detailed coupled state quantum mechanical calculation using an adiabatic basis is analysed to investigate the validity of the Langevin model for very low energy charge transfer processes in ion–atom collisions. Taking as an example the N3+/H system, it is shown that the success of the Langevin model in describing the energy variation of the cross section is due to the accuracy of the phase averaging procedure in the double passage through the avoided crossing. On the other hand, the model does not yield the correct isotopic dependence of the cross sections. These can only be accurately determined by a coupled state calculation. At low thermal and sub-thermal energies less than 1 meV, the phase averaging procedure breaks down and quantum tunnelling effects become important. In these conditions, the energy variation of the cross section exhibits an appreciable departure from the Langevin model.