Takeyoshi Sunagawa

Charge-carrier dynamics in polythiophene films studied by in-situ measurement of flash-photolysis time-resolved microwave conductivity (FP-TRMC) and transient optical spectroscopy (TOS)

An in-situ measurement system for flash-photolysis time-resolved microwave conductivity (FP-TRMC) and transient optical spectroscopy (TOS) has been developed to perform simultaneous measurements of photo-induced changes in conductivity and charge-carrier density in an organic thin film. The electric field in the resonant cavity designed for the present system was analysed by electrostatic simulation. Using the present system and the simulated electric field, the photoconductivity and transient absorption in a regioregular poly(3-hexyl thiophene) film were measured using one particular geometry under photon excitation energies of 6.39, 4.98, 3.48, and 2.34 eV. The dynamics of photogenerated charge carriers is discussed in terms of the excitation energy and incident photon intensity. The transient absorption spectrum induced by 3.48 eV light is presented and compared with the TRMC transient.

Low-energy electron attachment to molecules studied by pulse-radiolysis microwave-cavity technique combined with microwave heating

A new experimental technique has been developed to study electron‐energy dependence of the electron attachment to molecules. Time dependence of electron density is measured by conventional pulse‐radiolysis microwave‐cavity method, and a microwave heating technique is additionally applied in order to vary the mean electron energy from thermal to several electronvolts. The calibration of the mean electron energy is made by analyzing the time profile of microwave conductivity signals for thermalizing electrons produced by pulsed x rays in gaseous Xe which shows the Ramsauer minimum in the momentum‐transfer cross sections in collisions with electrons. Presented are rate constants for electron attachment to SF6, CCl4, CHCl3, CFCl3, CF3I, CF3Br, 1,1,1‐C2F3Cl3, and 1,1,1,‐C2H3Cl3 measured in the electron‐energy range from thermal to about 2 eV. The data are discussed in conjunction with previous data obtained by different experimental methods.

Electron-energy dependence of electron attachment to c-C7F14, CH3I and CH2Br2 studied by the pulse-radiolysis microwave-cavity technique combined with microwave heating

A new experimental method has been developed for measurements of the rate constants of electron attachment as a function of the mean electron energy. The conventional pulse-radiolysis microwave-cavity method has been modified by applying the microwave heating technique to elevate the mean electron energy from thermal to about 1 eV. This technique has been applied to the electron attachment to c-C7F14, CH3I and CH2Br2 in Xe buffer gas. The results are discussed in comparison with existing data.

Low-energy electron attachment to brominated methanes

The rate constants as a function of the mean electron energy from thermal to about 2 eV at room temperature have been measured for electron attachment to CBr4, CHBr3, CFBr3, CF2Br2, CH2BrCl, CHBr2Cl, and CBrCl3 using the pulse-radiolysis microwave-cavity method combined with microwave heating. The electron attachment cross sections, derived from the rate constant data, all show maximum at zero energy with no noticeable peak at higher electron energies. Based on the differences observed in the absolute magnitude of the cross sections among the brominated compounds as well as those between brominated and the corresponding chlorinated methanes, a model for the dissociative attachment to brominated methanes has been presented.

Thermal electron attachment to C6F5X and C6H5X (X=I, Br, Cl, and F)

Rate constants have been measured for thermal electron attachment to C6F5X (X=I, Br, Cl, F, and H) and C6H5X (X=I, Br, Cl, and F) at room temperature in N2 buffer gas (1–100 Torr) using the pulse‐radiolysis microwave cavity method. For all the compounds studied, the rate constants are of the two‐body type. Unexpectedly, the values for C6F5X except C6F5H are all the same (∼2×10−7 cm3 molecule−1 s−1), which are higher than most of the previous values, while that for C6F5H, measured in Xe and Ar buffer gases, is very low (7×10−12 cm3 molecule−1 s−1). For C6H5X, the value decreases dramatically with varying X from I to Br to Cl as 1.0×10−8 to 6.5×10−12 to 3×10−14 cm3 molecule−1 s−1, and that for C6H5F must be much lower than 10−13 cm3 molecule−1 s−1. These results for the magnitude of the rate constant are rationalized by the variation in the energy of a transient negative‐ion state of each molecule, which results from a combination of the electron affinities of constituents (halogen atom X and C6F5 radical) ...

Temperature dependence of low-energy electron attachment to CHCl3

Abstract Dependence of the rate constants for electron attachment to CHCl3 on mean electron energy from thermal to ∼2 eV has been investigated at ambient temperatures between 300 and 600 K. The rate constant at thermal energy increases dramatically with temperature with an activation energy of 0.13 ± 0.01 eV, which is obtained by the Arrhenius plot of the present rate constants, while those at higher mean electron energies increase moderately with temperature. The attachment cross sections obtained by unfolding the rate constants show two peaks at 0 and 0.3 eV at 300 K, but only the zero-energy peak increases dramatically with temperature. The importance of vibrationally excited states in the deformation mode of CHCl3 has been proposed to interpret the cross sections observed at 600 K.

Low-energy electron attachment to C6F5X (X=F, Cl, Br and I)

Abstract Rate constants have been measured for electron attachment to C6F5X (X=F, Cl, Br and I) in Xe buffer gas at room temperature at the mean electron energy between 0.04 and 2 eV using the pulse-radiolysis microwave-cavity method combined with microwave heating. The cross sections as a function of electron energy have been derived by unfolding the obtained rate constants. The rate constants at thermal energy are all the same, around 2 × 10−7 cm3 molecule−1 s−1, for C6F5X, and decrease monotonically with increasing mean energy. The cross sections have a peak at ≈0 eV for each compound. Only C6F6 shows another cross-section maximum at ≈0.75 eV. The discrepancy in the rate constants at thermal energy with those in previous reports is discussed.

Low energy electron attachment to brominated ethanes and ethylenes

Abstract Rate constants have been measured for electron attachment to CHBr 2 CHBr 2 , CHBr 2 CH 2 Br, CHBr 2 CH 3 , CH 2 BrCH 2 Br, CF 2 BrCH 2 Br, CF 2 BrCF 2 Br, CF 3 CH 2 Br, CH 2 BrCH 2 F, CHBrCBr 2 , and CHBrCHBr in Xe buffer gas at room temperature with mean electron energies between 0.04 and 2 eV using the pulse-radiolysis microwave-cavity method combined with microwave heating. The rate constant as a function of the mean electron energy is converted to a cross-section as a function of electron energy by an unfolding procedure. In all the compounds, the derived cross-sections show a peak at 0 eV with no other peaks at higher energies. The effects of molecular structure on the attachment processes are discussed.

Low-energy electron attachment to C6H5X (X = Cl, Br and I)

Abstract Rate constants for the dissociative electron attachment to C6H5X (X = Cl, Br and I) have been measured in Xe buffer gas at room temperature in the mean electron energy region between 0.04 and /2~ 2 eV using the pulse-radiolysis microwave-cavity method combined with microwave heating. Cross sections as a function of electron energy have been derived by unfolding the obtained rate constants. The rate constants for C6H5I decrease with the mean electron energy, while those for C6H5Br and C6H5Cl exhibit drastic increases, by about two and four orders of magnitude, respectively. The cross sections for C6H5I show a maximum at /2~ 0 eV, and those for C6H5Br and C6H5Cl have a peak at 0.3 and 0.7 eV, respectively. These differences are brought about mainly through the difference in the dissociation energy of the C6H5-X bond.

Electron energy loss rates in gaseous argon determined from transient microwave conductivity

Thermalization of high-energy electrons in gaseous Ar at room temperature has been investigated by analyzing the imaginary component of the transient microwave conductivity produced by pulse radiolysis. The conductivity signal amplitude showing a peak due to the Ramsauer minimum has been correlated with the amplitude derived from calculations of the effective collision frequency using Margenau’s formula assuming Maxwellian velocity distribution of electrons. Two approaches, using the peak and the plateau of the signal, for absolute normalization of the conductivity amplitude give results consistent with each other. It has been found that the excess mean electron energy drops very rapidly to about 0.2 eV and then decreases exponentially with a constant relaxation time. The thermalization time for 1 eV electrons to relax to 10% thermal energy has been determined to be 5.8 ms at 1 Torr Ar. Electron energy loss rate coefficients have been derived as a function of the mean electron energy.