Abstract
It is suggested that an interaction not being invariant about the internuclear molecular axis is useful for the description of rotational excitation of diatomic molecules in the electronic Pi -state. In the framework of the helicity-basis of angular momentum a complete scattering theory qualified for numerical use is presented. Collision induced excitation by rare gases, Sigma - and Pi -state molecules, are treated separately. Special attention is paid to polarization phenomena. The connection with the usual angular momentum coupling scheme is given explicitly.