The eigenphase formulation of fine-structure transition cross sections

The eigenphase formulation of Blatt and Biedenharn (1952) is applied to fine-structure transitions in 2 P atoms colliding with 1 S perturbers. Consideration is given to the limit of weak spin-orbit interaction. If the eigenphases are equal to the phaseshifts for elastic scattering by the molecular potentials then the expression for the total cross section reduces to the expression derived in the elastic approximation. However, a numerical comparison for the Li(2p 2 P)+He( 1 S) system shows that the elastic molecular phaseshifts are not good approximations to the eigenphases. Hence the elastic approximation cannot be reliable.