Electron-impact dissociation, rate determination

In general, the substrate temperature will remain unchanged, while pressure, power, and gas flow rates have to be adjusted so that the plasma chemistry is not affected significantly. Grill [117] conceptualizes plasma processing as two consecutive processes the formation of reactive species, and the mass transport of these species to surfaces to be processed. If the dissociation of precursor molecules can be described by a single electron collision process, the electron impact reaction rates depend only on the ratio of electric field to pressure, E/p, because the electron temperature is determined mainly by this ratio. 

The capillary plasma reactor consists of a Pyrex glass body and mounted electrodes which are not in direct contact with the gas flow in order to eliminate the influence of the cathode and anode region on CO2 decomposition. Analysis of downscaling effects on the plasma chemistry and discharge characteristics showed that the carbon dioxide conversion rate is mainly determined by electron impact dissociation and gas-phase reverse reactions in the capillary microreactor. The extremely high CO2 conversion rate was attributed to an increased current density rather than to surface reactions or an increased electric field. 

For the conditions shown in Fig. 8, it was estimated that the electron density was 8x 10 cm" . This means that kj = 2.5 x 10" cm /s, a value in good agreement with measured rate coefficients for dissociation of small molecules by electron impact . The gas phase propagation rate coefficient kp was also found to be in very good agreement with values determined for conventional butadiene polymerization. The agreement of the adjusted parameter values with those measured independently lends further support to the validity of the proposed model of plasma polymerization. 

Direct determination of non-Boltzmann distributions of the vibrational levels of the ground N2(X1S ) state has recently been performed by a new diagnostic technique, coherent anti-Stokes Raman spectroscopy21 with results consistent with calculated distributions. Kinetic data on N2 dissociation amenable to a comparison with theoretical predictions are scanty. Data from Ref.223 can however be quoted and are reported in Fig. 26. The dashed line has been calculated on the assumption that dissociation takes place via predissociated electronic states excited by direct electron impact. Observed dissociation rates are higher and a much better agreement has been claimed with dissociation rates calculated on the basis of a pure vibrational mechanism223 22b. ... 

Discharge Parameters and Plasma-ChemiealProeesses in CO2 Lasers. Using equation (11-53), determine the typical values of electron density in conventional stationary low-pressure discharge CO2 laser systems. Based on the values of electron density and the relevant reaction rate coefficient, estimate the time of CO2 dissociation in the system through dissociative attachment and electronic excitation by direct electron impact. 

Chemical kinetics of methane and acetylene dissociation and other gas phase reactions are studied for him coating applications under atmospheric pressure plasma conditions. In order to determine the plasma parameters, OES, V-I measurement, micro-photography and numerical simulations are used. From the determined EVDF and n, electron impact plasma chemical reaction rates are determined. On the basis of rate of different possible reaction. 

If we move the chemisorbed molecule closer to the surface, it will feel a strong repulsion and the energy rises. However, if the molecule can respond by changing its electron structure in the interaction with the surface, it may dissociate into two chemisorbed atoms. Again the potential is much more complicated than drawn in Fig. 6.34, since it depends very much on the orientation of the molecule with respect to the atoms in the surface. For a diatomic molecule, we expect the molecule in the transition state for dissociation to bind parallel to the surface. The barriers between the physisorption, associative and dissociative chemisorption are activation barriers for the reaction from gas phase molecule to dissociated atoms and all subsequent reactions. It is important to be able to determine and predict the behavior of these barriers since they have a key impact on if and how and at what rate the reaction proceeds. 

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