A distorted wave impulse approach for atom--diatom collisions

A formalism is derived to include the effects of the long-range attractive part of the interaction potential in the calculation of atom--diatom collision cross sections using the impulse approach (IA). These calculations have, until now, assumed the atom--diatom potential given by a sum of two atom--atom interactions, consequently yielding a poor representation of the long-range attractive part. In the distorted wave impulse approach (DWIA) the long-range attractive part, located at the center of mass of the diatom, is a spherically symmetric potential which ''distorts'' the incoming and outgoing waves. The DWIA formalism is used to calculate differential cross sections for the rotationally inelastic process Li++N2(v=0, j=2)→Li++N2(v'=0, j'), as a function of the final rotational level j', at a relative kinetic energy of 4.23 eV and center of mass scattering angles of 49.2 degree and 37.1 degree. It is shown that differential cross sections calculated using the DWIA formalism are in much better agreement with experimentally measured ones than IA differential cross sections using atom--atom interactions expressed by either hard-core, or exponential repulsive, functions