Collisions between sodium atoms and nitrogen molecules: rotational excitation and fine-structure-changing collisions

The formalism describing low-energy collisions between a linear molecule and an atom with fine-structure levels is set forth. It is shown that the resulting equations are rendered more tractable by making either the fixed-nucleus or averaged-field approximations. These are usually complementary approximations, but it is shown by physical arguments and numerical calculation that they are in good agreement for Na-N2 collisions. Using the surfaces calculated in an earlier paper, rotational excitation and fine-structure-changing cross sections for Na(2S, 2P)-N2(j) collisions at energies between 0.004 and 0.05 eV are presented, the molecule being in its ground vibrational state. The results are in agreement with the single available measurement.