F. Tuffin

PENNING IONIZATION OF THE CO2 MOLECULE BY NE* (3 3P2,0) METASTABLE ATOM

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.

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 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.

AUTOIONIZATION PROCESS IN THE PENNING IONIZATION OF THE CO MOLECULE BY NE*(3P0, 3P2) METASTABLE ATOMS

Abstract 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.

Experimental study of the Penning ionization of the H2O molecule by He* (23S, 21S) metastable atoms

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.

Penning and associative ionization of argon atoms by excited metastable neon

The associative-to-Penning ionization cross section ratio is simulated with a classical model for the Ne*(3P2,0)-Ar collision. We used the semi-classical optical potential curve of Gregor and Siska together with calculated ab initio Ne-Ar+ potential curves to describe the entry and exit channels, respectively. This model which had already been adjusted to fit the total cross section failed to reproduce both the experimental associative ionization cross section and the associative-to-Penning ionization cross section ratio. The contrasting results obtained between theory and experimental data are understood to be the result of an unsuitable real part of the potential for the entrance channel. We did not attempt to modify the average semi-classical potential to fit the experimental data, and it is concluded that an accurate ab initio complex potential is needed for a quantitative analysis of all the observable quantities.

An investigation of the kinetic energy of the ions produced by interaction of helium and neon metastables with carbon monoxide

Abstract 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.

Etude de l'energie des electrons ejectes au cours de l'ionisation penning de l'hydrogene moleculaire par les etats metastables 21S et 23S de l'helium

Abstract The authors report experiments on the Penning ionization of molecular hydrogen by metastable helium (21S and 23S states). The vibrational structure of the H2+ ion in the state X2Σg+ is resolved. The relative population of the three first vibrational levels is obtained. The authors also show that a continuum is superposed on the Penning ionization due to the autoionization process.

Ionisation de l'argon au voisinage du seuil par impacts electroniques

Abstract The authors report new ionization processes by electron impact using an electrostatic electron selector.

Penning ionization electron spectroscopy of the C2H2 molecule by Ne∗ (33P2, 33P0) metastable atoms

Abstract The present paper deals with Penning ionization electron spectroscopy (PIES) of C2H2 by Ne∗ (33P0 and 33P2) atoms. The observed vibrational population of the X 2IIu (C2H2+) state is in agreement with the Franck-Condon factors. The measured angular distribution of ejected electrons is found to be isotropic. This behaviour is consistent with the existence of a process competing with the covalent channel. This is also in agreement with results published on the measurement of ionization cross-section as a function of collision energy. An excitation transfer process was put forward to explain the entire experimental results obtained for the Ne∗C2H2 system. An indirect process via Rydberg states converging to the A2Σg+ state leads to a vibrational population that is not of the Franck-Condon type. However, an excitation transfer via the (2σ u −1 → 1π g 1 )π ∗ shape resonance can explain the Franck-Condon vibrational population, the large absolute value and the flattened behaviour of the ionization cross-section with respect to the variation of both collision energy and detection angle.

Etude de l'ionisation penning de l'hydrogene moleculaire mise en evidence d'une reaction secondaire

Abstract 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.