Hiroshi Shimamori

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.

Thermal electron attachment to O2 in H2 and D2

Abstract The attachment of thermal electrons to O 2 in H 2 and D 2 has been investigated using a microwave cavity technique combined with X-ray pulse radiolysis. The results show that the electron attachment processes obey the Bloch-Bradbury mechanism. At 290 K the three-body electron attachment rate constants with O 2 , H 2 and D 2 as third bodies are (2.2 ± 0.2) × 10 −30 , (4.8 ± 0.3) × 10 −31 , (1.40 ± 0.05) × 10 −31 cm 6 s −1 , respectively. The mechanism involved in the collisional stabilization of unstable oxygen molecular negative ions by H 2 and D 2 is briefly discussed.

Rate constants for thermal electron attachment to CF3I, CH3I, C2H5I, 1-C3H7I and 2-C3H7I determined between 250 and 350 K

Abstract Thermal electron attachment rate constants for several iodine-containing compounds have been measured at temperatures between 250 and 350 K using the pulse radiolysis-microwave cavity method. The two-body dissociative electron attachment rate constants and the activation energies are dependent on the molecular structure of alkyl iodides.

Mechanism of thermal electron attachment in O2–C2H4, O2–CO2, and O2–neopentane mixtures

Thermal electron attachment of O2 in O2–C2H4, O2–CO2, and O2–neopentane mixtures has been investigated at room temperature, using a microwave conductivity technique combined with pulse radiolysis. The measurements have been extended to a higher pressure region (∼850 Torr) than previous observations in order to compare them with the results of an electron swarm method at very high pressures. From low pressure data, the values of (2.0±0.3) ×10−30, (3.2±0.3) ×10−30, and (7±1) ×10−30 cm6/molecule2 sec are determined for the overall three body attachment rate constants of O2 with the stabilizing partners C2H4, CO2, and neopentane, respectively. In each case, the effective rate constant continued to increase with increased density and exceeded those predicted by the Bloch–Bradbury mechanism by sizeable amounts. The excess attachment is suggested to involve pre‐existing van der Waals complexes such as (O2⋅C2H4). Some quantitative conclusions which follow from this mechanism are given.

Mechanism of thermal electron attachment to O2: Isotope effect studies with 18O2 in rare gases and some hydrocarbons

Thermal electron attachment to 18O2 has been studied at room temperature for mixtures with rare gases (He, Ne, Ar, Kr, and Xe) and hydrocarbons (CH4, C2H6, C3H8, and neo‐C5H12) as third bodies (M), and the results are compared with those for the corresponding 16O2 systems. The obtained three‐body attachment rate constants (k18M ) for rare gases and CH4 are nearly equal to, or even less than those for the 16O2 systems (k16M ), which strongly suggests that the attachment to van der Waals molecules (or the vdW‐M mechanism) predominates in these systems. On the other hand, for C2H6, C3H8, and neo‐C5H12 k18M ’s are about twice greater than k16M ’s, thus indicating the importance of the Bloch–Bradbury (or the B–B) mechanism. All the isotope‐effect data imply that the well‐known feature of the marked dependence of the three‐body rate constants upon nature of third bodies originates mainly from the B–B mechanism, and also that the vdW‐M mechanism becomes important only when the B–B rate constant is very small, as...

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) ...

Mechanism of thermal electron attachment in O2N2 mixtures

Abstract The mechanism of thermal electron attachment in O 2 N 2 mixtures including air has been investigated using a microwave cavity technique combined with X-ray pulse radiolysis. From the experimental results it has been shown that the simple Bloch—Bradbury mechanism breaks down and the mechanism must include overall four-body electron attachment processes. At 290 K the three-body electron attachment rate constants are (2.4 ± 0.1) × 10 −30 cm 6 /s and (8.5 ± 0.3) × 10 −32 cm 6 /s for O 2 and N 2 as third bodies, respectively, and the four-body attachment rate constant for O 2 as a fourth body is (1.0 ± 0.5) × 10 −49 cm 9 /s.

Examination of the effects of van der Waals molecules on the thermal electron attachment to NO2 at relatively high pressures

Thermal electron attachment to NO2 in He, Ar, Xe, N2, CO2, and n‐C4H10 has been studied at pressures from about 100 Torr to near one atmosphere. With the increase of buffer‐gas pressures the effective two‐body attachment rate constants tend to increase gradually from the value of 1.1×10−10 cm3 molecule−1 s−1 obtained in the previous study as the initial two‐body attachment rate constant (k1) of the two‐step three‐body process; the only exception is He, for which no such increase of the rate constant has been observed. These results suggest that in all the buffer gases except for He the electron attachment to van der Waals molecule (NO2⋅M), where M is a buffer‐gas molecule, appears at higher pressures. But for He as the buffer gas such a mechanism is negligible in the pressure range employed here, because the equilibrium constant for the complex formation is very small in this case. The rate constants for the electron attachment to van der Waals molecules are about two orders of magnitudes larger than the ...