Formation and decay of superexcited molecules as studied by synchrotron-radiation photon and electron collisions

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1.Doubly excited molecules as studied by synchrotron-radiation photon collisionsWe have measured the absolute values of the cross sections for the emission of fluorescences in the ultraviolet and visible ranges from neutral fragments in the photoexcitation of methane, ammonia, water and ethylene as a function of incident photon energy in the range 10-40 eV. Interestingly, the oscillator strengths for Balmer fluorescences from doubly excited methane, ammonia and water in the inner valence range amount to the magnitudes comparable to those from nearby single-hole one-electron superexcited states of these molecules, which is a strong indication of the breakdown of the independent electron model. An important role of the doubly excited ethylene has been found in the cross section curves for Balmer fluorescences.2.Superexcited molecules as studied by the coincident electron-energy-loss spectroscopyThe coincident electron-energy-loss spectroscopy is an excellent means for investigating the s … More uperexcited molecules, which was developed by our group a few years ago. The electron energy-loss spectra are measured in coincidence with the emission of the vacuum-ultraviolet fluorescences from neutral fragments in electron collisions with molecules, in which the structures attributed to the superexcited states are shown up. We have measured the coincident electron-energy-loss spectra of molecular nitrogen, molecular oxygen, and methane in the valence range at several incident electron energies and scattering angles. Interestingly the optically-forbidden doubly excited states of nitrogen give larger contribution than the allowed doubly excited states. The time-resolved coincident electron-energy-loss spectra of oxygen have been measured to study the superexcited oxygen molecules in detail. The intense peaks attributed to the doubly excited methane have been found in the range above 25 eV. The second-generation system equipped with a position-sensitive detector for electrons has been developed to obtain the spectra much more efficiently Less