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Penning ionization electron spectroscopy of the CO molecule by He*(2 1 S) metastable atoms is studied. Vibrational populations are reported for the three energetically accessible states of the CO + ion (X 2 Σ + , A 2 Π and B 2 Σ + ). An obvious discrepancy is observed between the excited vibrational levels of the X 2 Σ + state and the calculated Franck-Condon factors. This result is attributed to an excitation transfer process via Rydberg vibrational progression converging to the CO + (D 2 Π) ionic state in agreement with the recent observations by fluorescence of neutral dissociate states. The presence of electrons coming from the well-known σ shape resonance is not excluded.

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 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 or in the quasi-bound state (shape resonance). Autoionization of the doubly excited state leads to the population of vibrational levels which are forbidden by the Franck - Condon principle in the case of a direct processes.

Penning ionization electron spectroscopy of the CO molecule by Ne ∗ ( 3 P 2 , 3 P 0 ) metastable atoms is reported. An extensive vibrational excitation of the electronic ground state X 2 Σ + of the CO + ion is observed. The measured angular distribution of the electrons corresponding to the first vibrational states ( ν = 0−1) is anisotropic, while that of the higher levels is isotropic. A quasi-resonant excitation transfer process, leading to excitation of the molecular target into an intermediate autoionizing Rydberg state, is proposed to explain the discrepancy between the Franck-Condon factors and the observed excited vibrational levels of the X 2 Σ + state.

Penning ionization of the H2O molecule by the He*(21S) metastable atom is studied. The energy shift is measured for the three observed X, A and B electronic bands of the residual H2O+ ion. The absence of the vibrational structures is discussed in light of the known interaction potential surface of the He-H2O complex. More attention is given to the X electronic band characterized by interference structures at high electron energy and an important angular dependence intensity.

Nous avons étudié l'ionisation Penning de H2O par des métastables 21S et 23S de l'hélium, ainsi que 3P0 et 3P2 du néon. Nous avons analysé l'énergie cinétique des ions créés au cours de la collision (parents et fragments). Afin d'interpréter certaines particularités expérimentales, l'hypothèse de transferts d'énergie de vibration en énergie cinétique est proposées. Par ailleurs, les caractéristiques des distributions en énergie des ions fragments sont expliquées par la prédissociation de l'état 2B2 de H2O+.

We have measured the kinetic energy of the ions H 2 + , D 2 + , H 3 + produced by the collision of helium on metastable neon, on molecular hydrogen or on deuterium: from measurements on H 2 + and D 2 + it has been possible to confirm the existence of a two-step Penning collision, the second step being a transfer from vibrational to kinetic energy. The measurement of the kinetic energy of H 3 + and D 3 + leads us to conjecture that a certain amount of (H 2 ) 2 and (D 2 ) 2 dimers is present in the target Pour les ions H 2 + et D 2 + , confirmation d'une ionisation de Penning en deux etapes, avec un transfert d'energie vibrationnelle en energie cinetique dans la 2eme etape. Hypothese de la formation de dimeres (H 2 ) 2 et (Δ 2 ) 2 dans les conditions de l'experience pour expliquer la formation de H 3 + et D 3 +

Experimental results for the energy distributions of CO + ions produced by chemi-ionisation of carbon monoxide are fitted using a simple mathematical model. The impact of helium and neon metastables is considered. In both cases the experimental distributions can be reproduced only if a two-step collision process is assumed. The second step involves non-radiative de-excitation of CO + vibrational levels, the vibrational energy being transformed into kinetic energy. For the purpose of comparison, results are also given for chemi-ionisation of argon.

In this paper we try to elucidate the origin of the electron continuum background observed in Penning ionization of molecular hydrogen by helium metastables states 21S and 23S. Using mass-spectrometry studies we verify that this anomaly is not due to the presence of water in our apparatus. However, we found an H+3 ion which we believe is produced by the reaction: H ∗ 2 + H 2 → H + 3 + H + e leading to electronic emission.