Takeshi Odagiri

Doubly excited states of molecular hydrogen as studied by coincident electron-energy-loss spectroscopy

The doubly excited states of molecular hydrogen have been studied by means of electron-energy-loss spectroscopy in coincidence with the detection of H(2p) formation at 80 eV incident energy and scattering angle. This is the first observation of doubly excited states of in electron energy-loss spectra. It is shown that a forbidden doubly excited state makes a large contribution as well as allowed states. The measured spectrum has been analysed by theoretical fits based on the reflection approximation.

Forbidden doubly excited states of molecular nitrogen dissociating into two neutral atoms in electron collisions

The ordinary and coincident electron-energy-loss spectra of N2 have been measured in an expected energy range for the formation of doubly excited states to investigate their formation and decay processes in electron-N2 collisions. The present results are compared with those obtained by photoexcitation experiments. Existence of the optically forbidden doubly excited states and their dissociative character have been revealed clearly for the first time.

Doubly excited states of methane produced by photon and electron interactions

The formation and decay of doubly excited methane in 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 CH4 as a function of incident photon energy in the range 18–51 eV and (ii) the electron-energy-loss spectrum of CH4 tagged with the Lyman-α photons at 80 eV incident electron energy and 10° electron scattering angle in the range of the energy loss 20–45 eV. Five superexcited states have been found, three of which are doubly excited states with the others being singly excited states. It has been found that the electron interaction with CH4 at 80 eV incident electron energy and 10° electron scattering angle accelerates the double excitation against the single excitation as compared with the photon interaction.

Electron energy-loss study of superexcited hydrogen molecules with the coincidence detection of the neutral dissociation

The electron energy-loss spectra of H2 have been measured in coincidence with the neutral dissociation leading to H(1s)+H(2p) near the first ionization potential at the electron impact energy of 150 eV and the scattering angles of 2 degrees and 10 degrees , and compared with ordinary electron energy-loss spectra. The two spectra, the coincident and the ordinary ones, are similar to each other in shape below the energy level of H2+(X2 Sigma g+, upsilon i=1), while they are different from each other above this energy level. Such a large change around upsilon i=1 is discussed in terms of the dynamics of superexcited hydrogen molecules, and the availability of this method for such studies is examined.

Multiply excited states of molecular nitrogen in the vacuum ultraviolet range as studied by (γ, 2γ) method

The cross sections for the emission of two photons of N fluorescence in photoexcitation of N2, differential with respect to each solid angle for the two photons, have been measured as a function of incident photon energy in the range of 28–49 eV to investigate the multiply excited states of N2. The resonance peaks attributed to the neutral excited states of N2 have been found at 36, 39, 40–44 and 45 eV in the ionization-free fluorescence cross section curve. It has turned out that these neutral excited states are multiply excited states of N2. It is remarkable that the multiply excited state of N2 at 45 eV embedded in the double ionization continuum makes a contribution comparable to those below the double ionization potential in the ionization-free cross section curve.

(γ, 2γ) studies on doubly excited states of molecular hydrogen

The doubly differential cross sections for the emission of two Lyman-α photons in photoexcitation of H2 have been measured as a function of incident photon energy in the range of 30–44 eV with a photon–photon coincidence technique. A cross section curve that is free from ionization and thus is attributed entirely to the doubly excited states of H2 has been obtained for the first time. A simple theoretical calculation based on the reflection approximation and semiclassical treatment of the decay dynamics in the Q21Πu(1) state of H2 has reproduced well the experimental cross section curve. It has been shown that this method, the (γ, 2γ) method, is an excellent tool for investigating spectroscopy and dynamics of doubly or multiply excited molecules.

The electron-energy-loss spectra of methane tagged with Lyman-α photons in the range of doubly excited states

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.

Multiply excited molecules in electron collisions as probed by energy and angle-resolved coincidence measurements between a scattered electron and a photon from a fragment atom

Doubly excited states of CH4 in electron collisions were investigated for the first time by measuring electron energy-loss spectra tagged with the emission of the Lyman-α photon. It turned out that the collisional cross section resulting in the emission of the Lyman-α photon originating from the doubly excited states are larger than or comparable to those originating from the nearby singly excited states and the electron interaction with molecules promotes the double excitation against the single excitation as compared with the photon interaction.

Generation of a pair of photons through the three-body dissociation of a multiply excited water molecule around the double ionization potential

The cross sections for the generation of a photon-pair from excited fragments in photoexcitation of H2O have been measured as a function of incident photon energy. The multiply excited states of H2O have been observed even above the adiabatic double ionization potential.

Symmetry-resolved spectroscopy by detection of a metastable hydrogen atom for investigating the doubly excited states of molecular hydrogen

Symmetry-resolved spectroscopy for investigating the doubly excited states of molecular hydrogen has been newly developed, where a metastable hydrogen atom dissociating in a direction parallel and perpendicular to the electric vector of the linearly polarized incident light is detected.