Single-hole one-electron superexcited states and doubly-excited states of molecules as studied by coincident electron-energy-loss spectroscopy

Single-hole one-electron superexcited states and doubly-excited states of H2, D2, N2 and O2 have been investigated by means of the coincident electron-energy-loss spectroscopy that we developed. In this method electron-energy-loss spectra tagged with the vacuum ultraviolet fluorescences emitted by the neutral fragments produced from superexcited molecules are measured by means of electron-photon coincidence technique. The contribution from ionization in this sort of electron-energy-loss spectra is suppressed to a large extent, and thus the structures attributed to the superexcited states of molecules become highlighted. The comparison with the photoexcitation experiments by means of the oscillator strengths give us clear discrimination between allowed and forbidden superexcited states. As to H2, D2 and N2 the doubly-excited states including those found in the present experiment have been investigated in terms of both their energies and dynamical behaviour. A new possibility of the coincident electron-energy-loss spectroscopy has been established in investigating the single-hole one-electron superexcited states of O2; the time-resolved coincident electron-energy-loss spectrum has been measured to distinguish between the direct process producing excited oxygen atoms in indirect one due to cascade transition. It has been turned out that the coincident electron-energy -loss spectroscopy is a key tool for investigating superexcited molecules. (author)