I. Theory of the relaxation by collision of molecular multipole moments: impact approximation with long range electrostatic interactions

A theoretical treatment of the relaxation and transfer of molecular multipole moments due to intermolecular collisions is presented assuming the impact approximation. General expressions for the collision cross section are obtained for any multipole-multipole interaction, to first and second order of perturbation assuming a long range electrostatic potential. The generalization to short range potential is discussed. Effects of fine and hyperfine structure in Hund's cases a and b are sketched