The rotational excitation of methanol by helium at low temperatures

We report new calculations of the CH3OH-He interaction potential. The resul ts of these computations have been fitted by an expansion in terms of the c oordinates of the atom with respect to a coordinate system fixed in the mol ecule, and the internal rotation angle of the methyl radical. The potential was then used to determine rotational excitation cross sections by means o f the quantum mechanical, coupled states method. It was assumed (i) that th e 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 temperatu res T = 10 and 20 K. The 'propensity rules' governing the collisional. tran sitions were examined and compared with the results of microwave double-res onance experiments. Finally, the rate coefficients have been used to comput e the excitation temperature of the 12.18 GHz transition of interstellar me thanol, which has been observed in absorption against the 2.7 K cosmic micr owave background radiation.