L. G. Christophorou

Low energy electron attachment to SF6 in N2, Ar, and Xe buffer gases

The electron attachment rate constants ka for SF6 have been measured in dilute mixtures of SF6 in high pressure (>1 atm) N2, Ar, and Xe buffer gases at room temperature (T≊300 K) over a wide E/N range (electric field strength to gas number density ratio), corresponding to mean electron energies 〈e〉 from near thermal electron energies (≊0.04 eV) to 〈e〉≊4.3 eV. Particular attention has been paid to the effects of space charge distortion, molecular impurities, and changes in the electron energy distribution function on the measured electron attachment rate constant values at the lower E/N values in these mixtures. The present measured thermal electron attachment rate constants in SF6/N2 and SF6/Xe gas mixtures are in excellent agreement with recent accurate measurements of these parameters in several SF6/buffer gas mixtures. At higher 〈e〉 values, the present SF6/N2 measurements are in fair agreement with previous measurements, while no previous measurements using Ar and Xe buffer gases have been published. T...

'Excimer' Fluorescence. IV. Solution Spectra of Polycyclic Hydrocarbons

The fluorescence spectra of solutions of the following compounds which show excimer fluorescence are recorded: 5-ethyl, 5-butyl, 5-propyl, 5-amyl and 6-isopropyl 1:2-benzanthracene; 1:2:3:4-dibenzanthracene, perylene, 1:12-benzperylene and anthanthrene; and 2,5-diphenyloxazole. The spectra of these and other compounds showing excimer fluorescence are analyzed and discussed in terms of the potential energy of the dimer in its excited and ground states. A consideration of the molecular structure and energy levels of further compounds which do not show excimer fluorescence in solution indicates various factors which may inhibit excimer formation, namely (i) chemical combination, (ii) steric hindrance, and (iii) weakness of interaction.

Electron transport studies of gas mixtures for use in e-beam controlled diffuse discharge switches

Gas mixtures for possible use in diffuse discharge switching applications require both high dielectric strength and specific electron transport properties in the conducting and the opening stages of the operation of the switch. In the conducting stage, the electron drift velocity must be large, and the electron loss processes (e.g., due to electron attachment and recombination) must be low so as to maximize the current in the discharge while maintaining low‐discharge impedance. In the opening stage, strong electron attachment along with high dielectric strength is required of the gas mixtures in order to extinguish the discharge as quickly as possible (and, thus, achieve a fast opening time) and to prevent arcing occurring between the switch electrodes due to the high voltages induced across the switch in the opening phase. Measurements of the electron drift velocity, attachment, ionization and transverse diffusion coefficients, and high‐voltage dielectric strengths of several gas mixtures proposed as can...

Electron transport measurements in methane using an improved pulsed Townsend technique

An improved pulsed Townsend technique for the measurement of electron transport parameters in gases is described. The accuracy and sensitivity of the technique have been investigated by performing, respectively, electron attachment coefficient measurements in pure O2 over a wide range of E/N at selected O2 pressures and by determining the electron attachment and ionization coefficients and electron drift velocity in CH4 over a wide E/N range. Good agreement has been obtained between the present and the previously published electron attachment coefficients in O2 and for the drift velocity measurements in CH4. The data on the electron attachment coefficient in CH4 (measured for the first time) showed that with the present improved pulsed Townsend method, electron attachment coefficients up to 10 times smaller than the ionization coefficients at a given E/N value can be accurately measured. Our measurements of the electron attachment and ionization coefficients in CH4 are in good agreement with a Boltzmann e...

Electron attachment to chlorofluoromethanes using the electron-swarm method

Electron attachment rate constants, as a function of pressure‐reduced electric field, were measured in mixtures with nitrogen for CCl3F, CCl2F2, and CClF3 using the electron swarm method. From these data total electron attachment cross sections σa(e) as a function of electron energy e were determined for the chlorofluoromethanes using the electron‐swarm unfolding technique and a new set of electron energy distribution functions for N2. For CCl3F and CCl2F2 three maxima in σa(e) were found at thermal energy, 0.25 and 0.75 eV and at 0.07, 0.30, and 0.93 eV, respectively. Only one pronounced maximum in σa(e) for CClF3 was observed at 1.4 eV. The thermal values of the attachment rate constant for CCl3F, CCl2F2, and CClF3 are, respectively, 3.90×108, 4.00×107, and 6.05×103 sec−1u2009Torr−1. Additionally, comparisons are made between the σa(e) calculated for these and other molecules using both the new electron energy distribution functions for N2 and those determined earlier. Substituent effects on the number and ...

`Excimer' Fluorescence. I. Solution Spectra of 1:2-Benzanthracene Derivatives

Observations have been made of the concentration dependence of the fluorescence spectra of solutions of 1:2-benzanthracene and fifteen of its hydrocarbon derivatives. All of the compounds, except the 9,10-dim ethyl derivative, exhibit dim er emission at higher concentrations. The lower excited states, 1Lb and 1La, satisfy Förster’s conditions for fluorescent dim er formation. The factors determining the relative quantum yield of excimer fluorescence are discussed. The different types of crystal fluorescence spectra shown by the compounds are explained in terms of excimer formation in the crystal lattice.

Electron attachment to halocarbons of environmental interest: Chloroethanes

Electron attachment to the chlorofluoroethanes 1,1‐C2Cl2F4, 1,2‐C2Cl2F4, 1,1,1‐C2Cl3F3, and 1,1,2‐C2Cl3F3 was investigated in dilute mixtures with nitrogen and argon using the electron swarm method. The 1,1,1‐C2Cl3F3 molecule has a very large electron attachment rate constant at thermal energy (2.8×10−7 cm3 s−1); the rate constant for this molecule decreases monotonically with increasing electron energy above thermal. The other chlorofluoroethanes investigated attach electrons less strongly and show maxima in the rate constant vs mean electron energy at ∼0.5 eV. For all four compounds, the measured rate constants are due primarily to dissociative electron attachment producing Cl−. Electron attachment cross sections were determined for these halocarbons which showed the existence of several negative ion states below ∼2 eV. Additionally, the temperature dependence of electron attachment to 1,1‐C2Cl2H4 was investigated from 323–473 K. The measured rate constants and unfolded cross sections indicated electron...

Ionization coefficients in selected gas mixtures of interest to particle detectors

We have measured and report the total gas‐number‐density normalized effective ionization coefficient, ᾱ/N, as a function of the density‐reduced electric field, E/N for Ar, CO2, CF4, the binary gas mixtures CF4:Ar (20:80), CO2:Ar(20:80), CO2:CH4(20:80) and for the ternary gas mixtures CO2:CF4:Ar (10:10:80) and H2O:CF4:Ar(2:18:80). We also report the limiting value, (E/N)lim of the above gaseous systems. A comparison of our results on the unitary gases shows good agreement with previous measurements. The results on the ternary mixtures show that the addition of CO2 or H2O to the binary gas mixture CF4:Ar lowers the electron energies and increases ᾱ/N(E/N).

Ramsauer-Townsend minima in the electron-scattering cross sections of polyatomic gases: methane, ethane, propane, butane, and neopentane

Electron drift velocities, w, as a function of temperature were measured and on the basis of these data, Ramsauer-Townsend (RT) minima were found in the cross section for momentum transfer, sigma m, for all these gases. The position of the RT minimum and the magnitude of the corresponding sigma m at the energy where the minimum occurs are 0.12, 0.14, 0.14, and 0.25 eV and 1.2, 3.0, 4.6, and 5.8*10-16 cm2 for ethane, propane, butane, and neopentane, respectively. The density dependence of the w for neopentane was also investigated. This latter investigation and similar ones in the literature on methane, ethane, and propane are discussed in connection with the observed RT minima and the mobility of thermal electrons in the low- and high-pressure vapours and the liquids of these compounds.

Effect of temperature on the attachment of slow (≤1 eV) electrons to CH3Br

The electron attachment rate constant ka(〈e〉, T) for CH3Br has been measured in a buffer gas of N2 using a high pressure electron swarm apparatus, within the mean electron energy range from thermal (∼0.046 eV) to 0.87 eV and over the temperature range, T, 300–700 K. At room temperature, CH3Br attaches low energy electrons weakly but as T is raised from 300 to 700 K the total electron attachment cross section increases by more than 2 orders of magnitude. At T=300 K the electron attachment cross section exhibits a peak at 0.38 eV which shifts progressively to lower electron energies as T is increased. The thermal value of ka is 1.08×10−11 cm3u2009s−1 at 300 K and 3.28×10−9 cm3u2009s−1 at 700 K.