Penning ionization electron spectroscopy of CO2 clusters in collision with metastable rare gas atoms

Penning ionization electron spectra (PIES) of CO2 clusters have been observed for the first time. An unusually fast electron band with excess kinetic energies of 1.4–2.9 eV with respect to the monomer band for the ionic X state was observed for CO2 clusters in collision with He*(23S) atoms. While for PIES with Ne*(33P), no such unusual band was observed. The unusual band is ascribed to autoionization into stable structures of ionic clusters to which direct ionization processes are almost impossible due to very small Franck–Condon overlaps associated with a very large geometry difference between the ionic and neutral clusters.