Penning Ionization

Abstract A review is presented of recent experimental investigations dealing with the ionization of various atoms and molecules in collisions with metastable rare gas atoms. These investigations include absolute cross section measurements, mass spectrometric analysis of the ions formed, and electron spectroscopic analysis of the electrons ejected. The results are discussed in terms of a potential curve model for Penning ionization, which essentially is equivalent to a description in Born‐Oppenheimer approximation of the process, with the motion of the colliding heavy particles treated classically. The approximate validity of this description is shown to be supported by the experimental results. The discussion of the experimental data in this frame yields detailed in formation on the physical quantities governing the Penning ionization process. In the case of the ionization of molecules it is shown that the Penning process is vertical and in simple cases leads to a population of vibrational states of the ion in accord with the Franck‐Condon transition probabilities. Observed deviations from this behaviour, and other complications, due to competing processes, arising in the case of molecules are discussed.