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 el ectron spectra of van der Waals clusters of rare gas atoms in collisions wi th metastable atoms, He*(2(3)S) and Ne*(3(3)P). Unusually fast electrons wi th extra kinetic energies of 1-2 eV as compared to those expected for verti cal ionization have been detected for Ar and Kr van der Waals clusters in c ollisions with He*(2(3)S). These fast electrons are ascribed to autoionizat ion into stable ionic structures via superexcited states embedded in the io nization continua, and the unusual electrons can be considered to be relate d to an adiabatically ionized state to which a direct ionization process is almost forbidden because of a very small Franck-Condon factor. (C) 2000 El sevier Science B.V. All rights reserved.