The rotational excitation of methanol by helium at low temperatures

We report new calculations of the CH(3)OH-He interaction potential. The results of these computations have been fitted by an expansion in terms of the coordinates of the atom with respect to a coordinate system fixed in the molecule, and the internal rotation angle of the methyl radical. The potential was then used to determine rotational excitation cross sections by means of the quantum mechanical, coupled states method. It was assumed (i) that the methyl radical was fixed at its minimum energy conformation, or (ii) that the interaction potential was an average with respect to the ground state torsional eigenfunctions of A- and E-type methanol. From the cross sections, thermally averaged rate coefficients were calculated at kinetic temperatures T = 10 and 20 K. The 'propensity rules' governing the collisional. transitions were examined and compared with the results of microwave double-resonance experiments. Finally, the rate coefficients have been used to compute the excitation temperature of the 12.18 GHz transition of interstellar methanol, which has been observed in absorption against the 2.7 K cosmic microwave background radiation.