Weixing Ding

Dissociation of benzene and methylene chloride based on enhanced dissociative electron attachment to highly excited molecules

We present preliminary results on a low-pressure glow-discharge-based process for the destruction of dilute concentrations of volatile organic compounds. Methylene chloride and benzene were studied in dilute mixtures with Ar, Ne and He rare gases. The destruction efficiency was observed to increase with increasing pressure. This and other observations are compatible with molecular dissociation by dissociative electron attachment to high-lying Rydberg states of molecules produced via excitation transfer from the metastable states of the rare gases.

Enhanced electron attachment to Rydberg states in molecular hydrogen volume discharges

We review recent studies on negative ion formation and studies in other areas that are relevant to the role of high-Rydberg states of H2 and H3 in hydrogen negative ion sources. Possible mechanisms for the formation of these excited states are discussed, including the formation of long-lived superexcited (core-excited) Rydberg states. Experimental evidence for negative ion formation via electron attachment to core-excited Rydberg states in a glow discharge apparatus is presented. An expression for the dissociative electron attachment rate constant for Rydberg molecules is derived based on electron capture by a Rydberg molecule due to polarization interaction.

Enhanced formation of negative ions by electron attachment to highly excited molecules in a flowing afterglow plasma

Preliminary evidence for efficient negative-ion formation using a plasma mixing scheme was reported in a recent letter [L. A. Pinnaduwage, W. Ding, and D. L. McCorkle, Appl. Phys. Lett. 71, 3634 (1997)]. In the present article we confirm the negative ion formation using a probe-assisted photodetachment technique and estimate rate constants for electron attachment to electronically excited CH4 and NO in a flowing afterglow plasma. It is shown that enhanced electron attachment to molecules in highly excited states populated via excitation transfer from rare gas metastables is responsible for the observed negative ion formation. Implications for plasma processing and plasma remediation discharges are also discussed.

Temperature dependence of electron attachment to methylene chloride

Temperature dependence of dissociative electron attachment to methylene chloride in the electron energy range of 0–10 eV was studied in a high-temperature electron swarm apparatus. The measurements were made using N2 and Ar as buffer gases. From the measured electron attachment rate constants, the electron attachment cross sections at 300, 400, and 500 K were determined using an unfolding technique. The maximum electron attachment cross sections at 300, 400, and 500 K were ≈3.1×10−18, ≈8.2×10−18, and ≈1.7×10−17 cm2, and occurred at electron energies of ≈0.8, ≈0.65, and ≈0.55 eV, respectively. The increase in electron attachment to methylene chloride with temperature is attributed to the increase in the vibrational energy of the molecule.

Enhanced electron attachment to highly excited molecules using a plasma mixing scheme

We present preliminary results on a glow discharge-based technique to populate highly excited states of molecules using a novel excitation transfer process, and to efficiently produce negative ions via electron attachment to those excited states.

Dissociation of Benzene in a Pulsed Glow Discharge

Destruction of benzene in a benzene/Ar mixture subjected to a pulsed glow discharge was studied. The destruction efficiency was much improved compared to a dc glow discharge, and the destruction efficiency increased with decreasing pulse width at a constant pulse frequency. Diagnostics experiments were conducted to elucidate the destruction mechanisms involved. The results show that excitation transfer from the metastable states of Ar to benzene in the afterglow of the discharge was primarily responsible for the destruction of benzene.

Methodology for Gas Chromatographic-Mass Spectral Analysis of Volatile Organic Compounds Emerging From a Low-Pressure, Flow-Through Reaction Cell

Abstract A methodology has been developed for the analysis of volatile organic compounds (VOCs) emerging from a plasma discharge cell operating at a constant flow-rate under subatmospheric pressure (0.266 to 2.66 kPa). The analytical system consisted of a gas reservoir for trapping a portion of the VOC–rare gas mixture, a sampling loop for cryogenically concentrating the VOC products, and either gas chromatography–mass spectrometry (GC–MS) or gas chromatography–flame ionization detection (GC–FID). The methodology was evaluated for the analysis of methylene chloride, benzene and tetrachloroethylene, using n-octane as the internal standard. Calibration curves were constructed by plotting the pressure ratios of the gas standard relative to the internal standard versus the corresponding peak area ratios. Over a pressure range of 1.133 to 5.32 kPa, the linearity of the calibration curve for each gas standard was determined with correlation coefficients ranging from 0.96 to 0.98. The relative standard deviation for a minimum of triplicate analyses varied from 1.1 to 18.3% for most VOCs. The calibration curves were used to measure the concentration of premixed VOC–rare gas mixtures as a function of energy input of the plasma reactor.

Decomposition of volatile organic compounds in a positive column glow discharge plasma

Summary form only given. Plasma processing has been used for remediation of dilute mixtures of volatile organic compounds (VOCs) in air. Energy efficiency is the main concern of these plasma technologies because large volume emissions have been treated. The destruction mechanisms of VOCs in plasmas greatly affect the economics of the process. We are exploring a novel plasma chemical scheme which is based on a extremely large dissociative electron attachment cross section. In this scheme highly-excited states of VOCs are excited by long-lived rare gas metastable states. In a dissociative electron attachment process a molecule is dissociated into neutral particles (or radicals) and a negative ion fragment. Most molecules have small dissociative attachment cross sections in their ground states. therefore the dissociative process could be weak. However when the molecules are highly excited close to their ionization potential, the dissociative attachment process becomes highly efficient, which enhances the decomposition of VOCs. Measurements were performed in a positive column glow discharge tube with a gas flow-through configuration.

Enhanced radical formation by electron attachment to highly-excited states of molecules in plasmas

Summary form only given. Large number densities of radicals are required in plasma assisted material processing where the formation of radicals is governed by conventional plasma chemical process. Operational pressure in plasma reactors has been greatly reduced to 10/sup -3/-10/sup -4/ torr to achieve the good control of particles transport toward the substrate. However low operational pressure results in the relatively low deposition rate. Therefore new plasma chemical reactions have to be pursued in order to efficiently produce radicals. We present a new experimental technique to obtain enhanced radical formation by electron attachment to highly excited state of molecules.