Gas-phase EPR linewidths and intermolecular potentials. I. Theory

A theoretical investigation is presented which relates gas-phase EPR linewidths to the elements of the scattering matrix describing binary collisions. Under the assumption of full rotational symmetry, the EPR linewidths are also related to multipole decay constants and population transfer coefficients. General expressions are developed which are valid for any angular momentum (J) state in which the observed atom may be. Results are explicitly tabulated for all J values less than or equal to 2. As an example, a model van der Waals second-order dipole-dipole potential is presumed to exist between a 2PJ atom and an S-state atom. The EPR linewidths predicted from this model yield differing broadening cross sections for different 2P3/2 transitions. Furthermore a zero cross section is predicted for the 2P1/2 state.