Doubly excited states of water as studied by electron energy loss spectroscopy in coincidence with detecting Lyman-alpha photons

The electron energy loss spectrum of H2O in coincidence with detecting Lyman-? photons (CoEELS) has been measured at the incident electron energy of 100 eV and electron scattering angle of 8? in the inner valence range in order to investigate the formation and decay of the doubly excited states. The present CoEELS has been compared with that at the infinite incident electron energy and 0? electron scattering angle, which was derived from the density of the dipole oscillator strength of H2O for the emission of the Lyman-? photons against the incident photon energy (Nakano et al 2010 J. Phys. B: At. Mol. Opt. Phys. 43 215206). It is remarkable that there exists a large difference in shape between these CoEELSs. This difference has turned out to be attributed to the noticeable contribution of the forbidden doubly excited states at 100 eV incident electron energy and 8? scattering angle. They lie at 25.0 and 27.4 eV and have been found out in this study. The differential cross sections for the excitation to the superexcited states resulting in H(2p) formation have been obtained at 100 eV and 8? and compared with those at the infinite energy and 0?. The electron collisions at 100 eV and 8? enhance the dissociative double excitation against the dissociative single excitation as compared with the electron collision at the infinite energy and 0?.