Kyoji Shinsaka

Electron–ion recombination rate constants in gaseous, liquid, and solid argon

Electron drift mobilities μe and electron–ion recombination rate constants kr in gaseous, liquid, and solid argon have been measured by the analysis of transient current induced with the irradiation of an x‐ray pulse as functions of temperature and external electric field strength except solid. The effect of external dc electric field on kr up to ∼3×10−3 Td (1 Td=10−17 V cm2/molecule) was examined in gaseous and liquid phases. The observed kr values in both liquid and gas phases were found to be much smaller than those calculated by the reduced Debye equation. The deviation, which is bigger in gas than in liquid, has been compared with recent theoretical studies1–5 which were presented to explain our previous data on the kr values for methane.6 It has been concluded that the recombination also in liquid and gaseous argon is not a usual diffusion‐controlled reaction. In the solid phase the observed kr values were almost in agreement with those calculated by the reduced Debye equation. The electron–ion reco...

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.

Electron mobilities and electron–ion recombination rate constants in solid, liquid, and gaseous methane

Electron mobilities and electron–ion recombination rate constants have been measured in methane by an x‐ray pulse conductivity technique over the density region from 2×1020 to 1.9×1022 molecules/cm3 including the critical region and the liquid–solid phase change. The observed electron mobility abruptly increases on the transition from liquid to solid, which is probably due to the change of isothermal compressibility of methane on the phase change. The observed electron–ion recombination rate constant is in good agreement with the value calculated by the reduced Debye equation in solid methane but not in liquid and gaseous methane. The deviation from the reduced Debye equation is larger in the gas phase than in the liquid phase, which is probably caused by the difference in the efficiency of excess‐energy loss of electrons within the reaction radius of electron–ion recombination.

Photoabsorption cross section of H2S

The photoabsorption cross sections for H2S were measured in the 47–102 nm region using synchrotron radiation. The absorption cross section in the range of λ > 90 nm showed many small peaks assigned to the 5a1 → 6sa1 Rydberg transition. and in the 77–90 nm region the 2b2 → nda1 (n = 4.5) Rydberg series were observed. At the energies of 18.2. 19.5 and 21.1 eV, small broad peaks were recognized and tentatively assigned to the vertical ionization potentials of the 14A2, 4B1 and 24A2 electronic states of the H2S+ ion, respectively, by comparing with the recent ab initio calculations for the H2S+ energy diagram.

Free-ion yields, electron mobilities and electron-ion recombination rate constants in liquid and solid isooctane and several other nonpolar compounds

Abstract Free-ion yields (Gfi0) in solid isooctane were determined by a new simple method which was a combination of the electric current measurement during irradiation of very low dose of 60Co γ-rays and the continuous measurement of thermostimulated current after the irradiation by applying d.c. high voltage to the solid isooctane. Electron mobilities (μe) in the solid isooctane were determined by the free-ion yields and the measurement of peak electric current upon the X-ray pulse irradiation of a few nanoseconds width. In the liquid phase, however, Gfi0 and μe were also measured by the usual charge clearing method and the decay curve analysis or peak current method, respectively. The effects of temperature from 26 to – 158°C and of phase change from liquid to solid on Gfi0, μe and b (the most probable electron-thermalization length) are compared with those for neopentane, tetramethysilane, and cyclohexane. Some values of the Gfi0, μe and b determined in liquid and solid isooctane are as follows: in liquid Gfi0=0.13 at -101°C, μe=1.6cm2V-1s-1 at -99°C, b=103 A at -101°C; in solid, Gfi0=0.15 at -118°C, μe=0cm2V-1s-1 at -118°C, b=104 A at -116°C. The values of apparent activation energy of μe obtained by the Arrhenius plot are 0.049 eV in the liquid and 0.077 eV in the solid isooctane, respectively. As the results, it is concluded that in liquid and solid isooctane localized electrons play a main role in electron transport with somewhat larger contribution of quasi-free electrons than in liquid and solid cyclohexane and that the rather larger contribution of quasi-free electrons than in cyclohexane seems to relate to the variation in Gfi0 and b value with temperature and phase change. Electron-ion recombination reaction in some organic liquids and solids is also discussed.

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.

Temperature dependence of the deexcitation of He(2 3S) by atoms and molecules as studied by pulse radiolysis method

The temperature dependence of the rate constants for deexcitation of He(2 3S) by Ar, Kr, N2, O2, CO, NO, CO2, and C2H4 has been measured in the temperature range from 133 to 300 K by a pulse radiolysis method, and thus the collisional energy dependence of the deexcitation cross sections is obtained. The results are interpreted in terms of an electron exchange mechanism. The value of α/β, the ratio of the exponential factor of the potential width Γ(R)=A exp(−αR) to that of the interaction potential V*(R)=B exp(−βR), is obtained for each target molecule. It is found that the value of α/β decreases with the increase of the deexcitation probability.

Electron Mobilities and Ranges in Liquid Hydrocarbons: Cyclic and Polycyclic, Saturated and Unsaturated Compounds

Penetration ranges bGp of secondary electrons into 21 X-irradiated liquids were estimated from measured free ion yields. The density normalized ranges bGpd are independent of temperature. Increasing the molecular symmetry by creating one or more cycles in the molecule causes the normalized range to increase, provided that the amount of bond distortion due to strain remains small. Ranges are smaller in compounds that contain strained rings. Energy transfer from the secondary electrons to the molecules is enhanced by the presence of distorted bonds in the molecules. The ranges in olefins are affected by the same factors as those in saturated hydrocarbons, with the addition of a contribution of transient negative ion states to the energy transfer processes. Shielding the double bond by methyl groups is not effective; the effect of the added methyl groups on the molecular symmetry is a more important factor. The magnitude of the energy transfer interaction is an inverse function of molecular symmetry.The gene...

Deexcitation cross sections of He(2 1P) by atoms and molecules

The deexcitation cross sections of He(2 1P) by M (M=Kr, Xe, H2, D2, N2, CO, O2, NO, CO2, and CH4) have been obtained by a pulse radiolysis method at a mean collisional energy corresponding to room temperature. The cross sections have been explained fairly well by the Penning ionization process with a long‐range dipole–dipole interaction. Bent trajectories appear to be important in the deexcitation by spherical atoms and molecules. An electron exchange process is also suggested to give an important contribution to the total decay width of the transient collision complex in highly excited autoionizing states of (HeM)*.

Temperature dependence of the rate constants for deexcitation of He(2 1P) by SF6 and Ar

The temperature dependence of the rate constants for deexcitation of He(2 1P) by SF6 and Ar has been measured in the temperature range from 133 to 300 K using a pulse radiolysis method, thus obtaining the collisional energy dependence of the deexcitation cross sections. The results agree well with the calculations of the Penning ionization cross sections based on a long‐range dipole–dipole interaction. It is also suggested that bent trajectory due to an attractive potential is important, which influences greatly the absolute value and the collisional energy dependence of the cross sections.