Masatoshi Ukai

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.

Dissociative photoionization of H2 and D2 in the energy region of 25–45 eV

Dissociative photoionization of molecular hydrogen and deuterium has been studied using synchrotron radiation and an electrostatic analyzer with a position sensitive detection system for the threshold region of the doubly excited states, 25–45 eV. The Q1 1Σ+u(1) autoionizing state plays an important role in the production of fast protons below 30 eV. It is found that, for the incident photon energy 30–40 eV, the dominant process for the production of energetic protons is autoionization of the Q2 1Πu(1) state which preferentially autoionizes to the H+22pσu state rather than to the H+21sσg state. The Q2 1Πu(2) and Q2 1Σ+u autoionizing states are also found to produce fast protons in this energy region. Above 42 eV, the kinetic energy distributions are dominated by direct ionization to the 2pσu and 2pπu states of H+2.

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.

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.

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.

Electron–ion recombination rate constants in gaseous and liquid krypton

Electron drift mobilities μe and electron–ion recombination rate constants kr have been measured by the analysis of transient current induced with the irradiation of an x‐ray pulse on gaseous or liquid krypton in a cell as a function of external electric field strength. The effects of an external dc electric field on kr up to 12 mTd (1 Td=10−17 V cm2/molecule) in liquid and 63 mTd in gas have been examined. The observed kr values in both liquid and gas phases have been found to be much smaller than those calculated by the reduced Debye equation. The deviation has been compared with recent theoretical studies, leading to the conclusion that recombination in liquid and gaseous krypton is not the usual diffusion‐controlled reaction. The kr values in the gas phase increase with an external dc electric field up to a critical electric field strength below which μe values are nearly constant, and above which both kr and μe values descrease with further increase in the electric field for both gaseous and liquid p...

Lyman‐α, Lyman‐α coincidence detection in the photodissociation of doubly excited molecular hydrogen into two H(2p) atoms

Photodissociation processes of the doubly excited states of H2 into H(2p)+H(2p) have been studied using a coincidence detection of two Lyman‐α photons. Coincidence spectra have been measured in the energy region of 29.0–36.0 eV. The intensity of the observed coincidence peak corresponding to two Lyman‐α photons increases with increasing energy from its threshold which is about 29 eV. The main precursor of the two H(2p) atoms is assigned to the doubly excited Q2 1Πu state.