The rotational excitation by helium of methanol in its ground and first excited torsional states

We have improved and extended our previous calculations of cross sections for the rotational excitation of methanol by helium. In the case of methanol in its torsional ground state, we extended our coupled states computations of the cross sections to higher collision energies and larger rotational basis sets. The accuracy of the rotational energy levels and eigenfunctions has been reviewed and improved for A-type methanol. The calculations have been extended to A- and E-type methanol in their first excited torsional states by averaging the CH3OH-He interaction potential over the excited state torsional eigenfunctions. Thermal rate coefficients have been calculated at low temperatures. Predictions are made of line intensity ratios which are sensitive to the density of the He perturber and which lend themselves to the determination of the perturber densities in the dark molecular clouds of the interstellar medium.