Coupled states cross sections for rotational excitation of H₂CO by He impact at interstellar temperatures

Cross sections for rotational excitation of ortho formaldehyde due to collision with helium are computed following the coupled states (CS) formalism and compared with recent coupled channel (CC) results obtained employing the same ab initio configuration interaction intermolecular potential. The CS equations are integrated at 9 scattering energies between 25 and 95°K using a basis set of 16 ortho H₂CO states (1 ≤ j ≤ 5). The CS procedure with the orbital angular momentum quantum number l set equal to the total angular momentum J yields the correct order of magnitude for scattering cross sections. Qualitative differences are found, however, in the energy dependence of some inelastic transitions.