Rotational excitation ofN2andCl2molecules by electron impact in the energy range 0.01–1000 eV: Investigation of excitation mechanisms

The rotational excitation of ${\mathrm{N}}_2$ and ${\mathrm{Cl}}_2$ molecules on electron impact is investigated over a wide range of incident electron energies (0.01–1000 eV). Two different excitation mechanisms are reported, the importance of which depends on the impact energy. At low electron energies only a few rotational quanta are exchanged, and the differential cross section decreases exponentially with Δj. At high electron energies the excitation spectrum shows a rotational rainbow, i.e., the differential cross secton has a maximum at a relatively high Δj. The location of this maximum depends on electron energy E and scattering angle θ. For intermediate energies there is an interplay of these two mechanisms. It can be seen that the contributions of both mechanisms can roughly be added to yield the cross section for the observed process.