G. Sinou

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.

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.

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.

Excitation Transfer Process in Penning Ionization of the CO Molecule by Helium Atoms in Singlet Metastable State

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.