Angular dependence of vibronic excitation in He⁺+CO, N₂collisions at medium energies: evidence for non-Franck-Condon and non-Poisson vibrational distributions

The vibrational populations of CO(X 1 Sigma + and a 3 Pi ) and N 2 (X 1 Sigma g + and B 3 Pi g ) produced in collisions of He + with CO(X) and N 2 (X) in the E=100-1000 eV energy range are studied, using a high-resolution energy loss spectrometer, as a function of the scattering angle theta of the He + particles. The results for CO(a 3 Pi ) show dramatic deviations from the Franck-Condon distribution and exhibit contrasting behaviours as a function of both E and theta . The measured vibrational distributions for CO(X) and N 2 (X) deviate strongly from the often invoked Poisson distribution and show peculiar dependences as functions of E and theta . The possible bond-length-dependent phenomena that could influence the vibronic excitation into CO(a, nu ') are discussed. A simple model is set up to explain the main effects that govern the dependence of the vibrational distribution in the electronic ground states of CO and N 2 .