The formation and decay of doubly excited ammonia produced by photon and electron interactions have been investigated through measuring (i) the cross sections for the emission of the Lyman-α fluorescence in the photoexcitation of NH3 as a function of incident photon energy in the range 15–60 eV and (ii) the electron-energy-loss spectrum of NH3 tagged with the Lyman-α photons at 100 eV incident electron energy and 8° electron scattering angle in the range of the energy loss 15–48 eV. Six superexcited states have been found, four of which are doubly excited states with one being a singly excited state and the other being a state including electron configurations of a single excitation and double excitations. There exist two forbidden doubly excited states whose widths are approximately 1 eV, i.e. much sharper than the allowed superexcited states in the electron-energy-loss spectrum tagged with the Lyman-α photons. It is remarkable that the electron interaction with NH3 at 100 eV incident electron energy and 8° electron scattering angle promotes the ratio of the oscillator strength for the double excitation to that for the single excitation as compared with the photon interaction.

The doubly excited states of methane produced in electron interaction have been investigated by measuring the electron-energy-loss spectra tagged with Lyman-α photons at 80 eV incident electron energy and electron scattering angles of 12 • and 24 • . The triply differential cross sections at both angles have been put on the same relative scale for the first time as a function of energy loss. It turned out that electron interaction with methane at 80 eV incident electron energy increases double excitation against single excitation as compared with photon interaction up to 24 • electron scattering angle.

The differential cross sections for the dissociative single and double excitations resulting in H(2p) formation with the excitation energy of 19–46 eV in electron–CH4 collisions have been measured as a function of electron scattering angle in the range 4°–48° at 80 eV incident electron energy by means of angle-resolved electron energy-loss spectroscopy in coincidence with detecting Lyman-α photons. This is the first measurement of the differential cross sections for the dissociative double excitations as a function of electron scattering angle in electron–molecule collisions. Their fractions have been compared with those at the optical limit calculated from the density of the dipole oscillator strength for the emission of Lyman-α photons previously measured by our group. The dissociative double excitations in 80 eV electron collisions seem to be brought about in a very different way from those at the optical limit where they arise from the electron correlation in a methane molecule. The differential cross sections have also been discussed in terms of momentum transfer, leading to a universal curve.

The electron energy loss spectrum of H2O in coincidence with detecting Lyman-α 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-α 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°.

The differential cross sections of the dissociative double excitations resulting in H(2p) formation in 80 eV electron collisions with H2 have been measured at electron scattering angles of 3, 20, 30 and 40° by means of angle-resolved electron energy loss spectroscopy in coincidence with detecting Lyman-α photons. The doubly excited states of H2 involved in this study are the optically allowed Q1 1Πu(1) and Q2 1Πu(1) states and the forbidden state at 26 eV, which has not been assigned so far and is temporarily referred to as the F state in this study. At a 3° electron scattering angle the differential cross section of the dissociative excitation to the forbidden F state is the largest among those for the three doubly excited states and decreases with increasing the electron scattering angle more rapidly than those for the allowed Q1 1Πu(1) and Q2 1Πu(1) states. The differential cross sections for the forbidden and allowed states do not differ quite so much, which may show that the forbidden F state is a dissociative state rather than an autoionizing state. The effective generalized oscillator strengths for H(2p) formation have been compared with the dipole oscillator strengths according to the limit theorem.