Penning ionization of the H a molecule by the He'(2'S) metastable atom is studied. The energy shift is measured for the three observed X, A and B eiectronic bands of the residual HzO' ion. The absence of the vibrational structures is discussed in light of the known interaction potential surface of the He'-t120 complex. More altention is given to the X electronic band characterized by interference structures at high electron energy and an important angular dependence intensity.

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Penning ionization of the CO2 molecule by Ne* (3 3P2,0) metastable atoms is studied by electron spectroscopy. The recorded electron energy spectrum is dominated by an important vibrational progression that is decomposed over the nν1 and nν1+2ν3 vibrational series up to n=5. In addition, we observe a low-energy broad hump extending from 0.5 to 1.5 eV. The presence of this new feature is attributed to a non-van der Waals interaction between the colliding particles. Making the hypothesis that an ion–pair state channel opens during the collision, we are in a position to interpret the entire set of experimental results. We believe that the opening of this channel is responsible for the production of the CO2 molecule in an autoionized Tanaka–Ogawa Rydberg state.

In this paper, we investigate the Penning ionization of the CO molecule by helium atoms in metastable states. The energy distribution spectrum of electrons related to the CO + (X 2 + ) ion state shows a deviation of the vibrational population from the predicted Franck-Condon factors. The angular distribution measurements exhibit an intensity increasing towards the backward direction for the v = 0 level, while the corresponding angular distributions of the higher vibrational levels (v = 2, 3 and 4) are isotropic. To explain these results, an excitation transfer process between the colliding particles is suggested. This gives rise to a CO molecule either in a doubly excited Rydberg state converging to CO + (D 2 ) or in the σ * quasi-bound state (shape resonance). 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.

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