Dynamical coupling effects in the vibrational excitation ofH2sandN2scolliding with positrons

The coupling between angular momenta during low-energy collisions of slow positrons as projectiles on simple diatomics like ${\mathrm{H}}_2$ and ${\mathrm{N}}_2$ is analyzed from the point of view of its effect on vibrationally inelastic processes and on possible decoupling schemes that can help to simplify calculations. It is found that the strong decoupling between rotational and vibrational degrees of freedom implied by the well-known infinite-order-sudden approximation (here used within the vibrational close-coupling-rotational-infinite-order-sudden scheme discussed below) is not valid for positron scattering at low collision energies in spite of its low excitation efficiency for the vibrational degrees of freedom of the target molecules.