Penning ionization electron spectroscopy of van der Waals clusters

A magnetic bottle electron spectrometer with retarding electrodes has been applied to observation of Penning ionization electron spectra (PIES) of van der Waals clusters. Considerably increased efficiency of the spectrometer by 1000 times over the conventional PIES apparatus enabled us to measure electron spectra of van der Waals clusters of rare gas atoms in collisions with metastable atoms, He*(2³S) and Ne*(3³P). Unusually fast electrons with extra kinetic energies of 1-2 eV as compared to those expected for vertical ionization have been detected for Ar and Kr van der Waals clusters in collisions with He*(2³S). These fast electrons are ascribed to autoionization into stable ionic structures via superexcited states embedded in the ionization continua, and the unusual electrons can be considered to be related to an adiabatically ionized state to which a direct ionization process is almost forbidden because of a very small Franck-Condon factor.