The rotational excitation of methanol by helium atlow temperatures

Abstract

We report new calculations of the CH3OH-He interactionpotential. The results of these computations have been fittedby an expansion in terms of the coordinates of the atom withrespect to a coordinate system fixed in the molecule, and theinternal rotation angle of the methyl radical. The potentialwas then used to determine rotational excitation cross sectionsby 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 interactionpotential was an average with respect to the ground statetorsional eigenfunctions of A- and E-type methanol. From thecross sections, thermally averaged rate coefficients werecalculated at kinetic temperatures T = 10 and 20 K. The`propensity rules' governing the collisional transitions wereexamined and compared with the results of microwave double-resonance experiments. Finally, the rate coefficients have beenused to compute the excitation temperature of the 12.18 GHztransition of interstellar methanol, which has been observed inabsorption against the 2.7 K cosmic microwave backgroundradiation.