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

Abstract

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.