Noriyuki Kouchi

Photoabsorption, photoionization, and neutral-dissociation cross sections of simple hydrocarbons in the vacuum ultraviolet range

Abstract The absolute photoabsorption cross sections, σt, of CH4 and n-C4H10 have been measured in the photon energy range of the valence electrons using a double ionization chamber equipped with a metallic thin film window and synchrotron radiation as a continuous-wavelength light source. The absolute photoionization quantum yields, η, of CH4 have been also measured, from which the photoionization cross sections, σi, and neutral-dissociation cross sections, σd, are obtained. The values of σt and η of C1–C4 normal alkanes including our previous results for C2H6 and C3H8 have been compared in detail with those by the dipole-simulation method using the virtual photons. The gross features of the σt and η values of these alkanes are discussed in terms of those as functions of the photon energy and the number of C atoms in an alkane molecule.

Ionizing and nonionizing decays of superexcited acetylene molecules in the extreme-ultraviolet region

Absolute measurements of the photoionization and photodissociation cross sections of C2H2 have been made using continuum monochromatized synchrotron radiation in the 53–93 nm region. The absolute photoabsorption cross section and photoionization quantum yield of C2H2 have also been measured. The excitation spectra of C2(d 3Πg→a 3Πu), C2(C 1Πg→A 1Πu), CH(A 2Δ→X 2Π), and H(Lyman‐α) fluorescence have also been obtained. The obtained results have been compared with theoretical calculations. An unresolved problem of the spectral interpretation concentrated on the σ * and π* shape resonances has been clarified by the straightforward demonstration of the photoionization quantum yield. The ionizing and nonionizing decay processes of the superexcited C2H2 molecules have been discussed in view of the strong competition of autoionization and neutral dissociation. An overlapping nature of Rydberg states with the shape resonance is found to be important.

A high‐flux 3‐M normal incidence monochromator at beamline 20A of the Photon Factory

A high‐flux 3‐m normal incidence monochromator (3‐m NIM) has been installed at beam line 20A of the Photon Factory. The 3‐m NIM can supply a high photon flux in the wavelength region 30–200 nm, primarily because of a large horizontal acceptance angle of 28 mrad for the synchrotron radiation (SR) beam from bending magnet 20. The photon flux of the 3‐m NIM was estimated to be ≊1012 photons/s with Δλ≊0.1 nm at ≊50 nm by measuring the photoemission current from a gold‐coated surface. The resolving power with a 2400 grooves/mm grating was found to be greater than 25 000 at 90 nm from the autoionization profile of the Xe nd’ states converging to Xe+ 2P1/2.

The effect of an external electric field on diffusion‐controlled bulk ion recombination

The effect of an external electric field on the rate of diffusion‐controlled ion recombination was investigated. The Brownian motion of positive and negative ions under the influence of the mutual Coulomb attractive potential and an external electric field was described by using the diffusion equation. Both fully and partially diffusion‐controlled recombinations were considered. Analytical expressions for the rate constant were obtained for low electric fields. For high electric fields, the rate constant was calculated numerically. For fully diffusion‐controlled recombination, the rate constant always increases with increasing field strength. However, for partially diffusion‐controlled recombination, the rate constant decreases with increasing field strength if the intrinsic rate at the encounter distance is very slow. Possible physical mechanisms were proposed to explain these results. Finally, these results were compared with available experimental data on the electric field effect on electron–ion recombination.

PHOTOABSORPTION PHOTOIONIZATION, AND NEUTRAL-DISSOCIATION CROSS SECTIONS OF C2H6 AND C3H8 IN THE EXTREME-UV REGION

Abstract The absolute photoabsorption cross sections (σt) and photoionization quantum yields (η) of alkanes (C2H6 and C3H8) have been measured in the 50 – 120 nm region using synchrotron radiation as a light source and an ionization chamber combined with metallic thin film windows. We have evaluated the absolute photoionization cross sections (σi) and the total cross sections for non-ionizing processes, i.e., the neutral-dissociation cross sections (σd) following superexcitation. Several superexcited states have been revealed in these spectra.

Translational spectroscopy of electron-impact dissociative excitation of H2O and D2O by doppler profile measurements of Balmer-α emission

Abstract The Doppler profiles of Balmer-α emission by electron impact on H 2 O and D 2 O have been measured through varying the impact electron energy using an etalon-grating monochromator with a high-resolving power. The Doppler profiles consist of the two components, i.e., the narrow component and the broad one. This result implies that there are two kinds of precursors leading to H* (3) or D* (3) contributing to the Balmer-α emission. The average kinetic energies of H* (3) are (0.4 ± 0.2) eV and (4 ± 1) eV for the narrow component and the broad component, respectively, and are the same as those of D* (3) within experimental uncertainties. Discussions are given on the precursors of H* (3) and D* (3) contributing to the Balmer-α emission, the isotope effect on the kinetic energies of H* (3) and D* (3), and the ratio of the radiation intensity of the broad component to that of the narrow component.

Photoabsorption, photoionization, and neutral‐dissociation cross sections of SiF4, SiCl4, and Si(CH3)4 in the extreme‐ultraviolet range

The absolute photoabsorption cross sections (σt) and photoionization quantum yields (η) of SiF4, SiCl4, and Si(CH3)4 have been measured in the 50–100 nm range, from which we have evaluated the absolute photoionization cross sections (σi) and the total cross sections for nonionizing processes, i.e., the neutral‐dissociation cross sections (σd) following superexcitation. In addition to the broad structures in the σt curve for SiF4 around 95, 83, 78, 74.5, and 70 nm, oscillating structures which have not been found previously are revealed around 65 nm. These structures, which appear also in the present σi curve, are tentatively ascribed to the vibrational progressions of Rydberg states converging to the (4t2)−1C ionic state. Other oscillating structures in the 57–62 nm range are ascribed to the Rydberg states converging to the (5a1)−1D ionic state. Broad peaks around 95, 92, 75, and 58 nm are observed in the σt and σi curves of SiCl4. Rydberg structures converging to the 7a1−1 limit are observed in the σt ...

Absolute measurements of photoabsorption cross sections, photoionization cross sections, and photoionization quantum yields of silane in the 13–40 eV region

The absolute values of the photoabsorption cross section of silane have been measured in the energy region between 13 and 40 eV. The photoionization quantum yields have also been measured in the 13–22 eV region. Using these values, we have evaluated the absolute photoionization cross sections and the total cross sections for nonionizing processes, i.e., neutral fragmentations. The photoabsorption cross sections show a broad peak at around 14.6 eV and a vibrational structure between 16 and 18 eV. These structures are superimposed on the spectra of direct transitions to the ionization continuum. The decaying features of the superexcited states corresponding to these two bands are shown to be very different from each other. Corresponding to the broad absorption peak located at around 14.6 eV, considerable deviations from unity in the photoionization quantum yields are observed. This means that the superexcited states at this energy decay through fast neutral fragmentation rather than autoionization. On the other hand, the superexcited states, which appear as the vibrational structure between 16 and 18 eV, decay predominantly through autoionization over the neutral fragmentation.

Dissociation dynamics of superexcited molecular hydrogen

The dissociation dynamics of superexcited molecular hydrogen and its isotopes has been discussed from an experimental viewpoint. Recently, more attention has been paid to doubly excited states since new experiments on ionization and dissociation via doubly excited states have started due to the large progress in synchrotron radiation light source and electron spectroscopy. This paper reviews the experimental techniques and results obtained and indicates that the competition between neutral dissociation and autoionization is a key to investigating the molecular superexcited states.

Photoionization of hydrogen and deuterium

Cross sections for the formation of slow molecular and energetic fragment ions in the photoionization of hydrogen and deuterium have been determined using synchrotron radiation within the energy range 25–40 eV. Superexcited states have been shown to play an important role in the photoionization process but recently predicted resonance effects have not been observed.