Formation of Molecular Superexcited States and Their Dissociation Dynamics

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We have established a new experimental method, i.e. the coincident electron-energy-loss spectroscopy, to investigate dissociation dynamics of molecular superexcited states, especially that of optically-forbidden superexcited states. A vuv photon detector and a coincidence counting system are combined with a specially-designed electron-energy-loss-spectroscopy system for gas phase. An electron energy-loss spectrum of HィイD22ィエD2 in coincidence with H(2p) formation, i.e. a coincident energy-loss spectrum, have then successfully been obtained after careful optimization of experimental conditions and developing a unique normalization procedure for coincidence counting rates. By using this method, we measured an electron energy-loss spectrum of doubly excited states of HィイD22ィエD2 for the first time. It should be emphasized that a large peak due to the forbidden doubly-excited states of HィイD22ィエD2 is observed in the coincident energy-loss spectrum whose position and shape are not explained by … More a conventional theory, which has stimulated some experimental as well as theoretical scientists. Isotope effects in the coincident energy-loss spectra of doubly excited states of HィイD22ィエD2 and DィイD22ィエD2 were also investigated. The coincident energy-loss spectrum of highly excited states of HィイD22ィエD2 near the first ionization potential was measured at the lower incident electron energy, i.e. 30eV. The branching ratio of H(2p) formation is found to be higher for optically forbidden states such as the d, k, n and f states than other allowed states. Since 1999, the measurement for NィイD22ィエD2, as an extension to multi-electron systems, has been started.Various experiments by using synchrotron radiation were done to investigate the dissociation dynamics of optically allowed superexcited states : the precise measurement of photoabsorption, photoionization and neutral dissociation cross sections, that of dispersed vuv-fluorescence excitation spectra (2D FES), and ion-fluorescence coincidence measurement. In addition to them, we have recently succeeded in measuring a dispersed uv/visible-fluorescence excitation spectra by position sensitive detection technique to investigate the dissociation dynamics comprehensively.Collisional deexcitation processes of excited rare gas atoms and electron-ion recombination processes, where the superexcited states play an important role as reaction intermediates have also been investigated by the pulse radiolysis technique. Less