AUTOIONIZATION PROCESS IN PENNING IONIZATION OF THE CO MOLECULE BY HELIUM-ATOMS IN TRIPLET METASTABLE STATE

In this paper, we investigate the Penning ionization of the CO molecul e by helium atoms in metastable states. The energy distribution spectr um of electrons related to the CO+(X (2) Sigma(+)) ion state shows a d eviation of the vibrational population from the predicted Franck-Condo n factors. The angular distribution measurements exhibit an intensity increasing towards the backward direction for the v = 0 level, while t he corresponding angular distributions of the higher vibrational level s (v = 2, 3 and 4) are isotropic. To explain these results, an excitat ion transfer process between the colliding particles is suggested. Thi s gives rise to a CO molecule either in a doubly excited Rydberg state converging to CO+(D-2 Pi) or in the sigma quasi-bound state (shape r esonance). Autoionization of the doubly excited state (CO*) leads to the population of vibrational levels which are forbidden by the Franck -Condon principle in the case of a direct processes.