Plasma-chemical reaction rate coefficient

The coefficient a is a key parameter describing the irtiiuence of plasma excitation of molecules on their chemical reaction rates (2-205). Ntrmeric values of the a-coefficients for some specific chemical reactions are presented in Table 2-26 (Levitsky, Macheret, Fridman, 1983 Levitsky et al., 1983c Rusanov Fridman, 1984). 

Contribution of Methyl Radicals (CH3) and Acetylene (C2H2) in Non-Equilibrium Plasma-Chemical Deposition of Diamond Films. Taking into accormt the reaction rate coefficients of attachment of methyl radicals (9-117) and acetylene (9-118) to the diamond fihn surface as well as typical molar fractions of CH3 radicals and C2H2 (see Table 9-23), calculate the relative contribution of these major active carbon-containing chemical species to the deposition of diamond films from a H2-CH4 mixture in non-equilibrium plasma conditions. 

Here Xe is the coefficient of anharmonicity and B is the normalizing factor. Comparison of the parabolic-exponential Treanor distribution with the linear-exponential Boltzmann distribution is illustrated in Fig. 3-3. A population of highly vibrationally excited levels at TV > To can be many orders of magnitude higher than that predicted by the Boltzmaim distribution even at vibrational temperature. The Treanor distribntion resnlts in very high rates and energy efficiencies of chemical reactions stimulated by vibrational excitation in plasma. 

The Treanor effect (3-165) can be applied for isotope separation in plasma (Belenov et al., 1973). The ratio of rate coefficients of chemical reactions for two different vibrationally excited isotopes (v = V ), called the coefficient of selectivity, can be expressed as... 

The formula can be applied in the case of IXeEaT /hcoTo < 1, where hco and Xe are the vibrational quantum and anharmonicity coefficient of N2 molecules. Compare the rate coefficient (6-34) for NO synthesis stimulated by vibrational excitation with that of CO2 dissociation (see Section 5.2). Because NO synthesis represents the slow reaction, the rate coefficient (6-34) includes detailed characteristics of the elementary process in contrast to the case of fast reaction in plasma-chemical CO2 dissociation. For calculations of the rate coefficient using equation (6-34) at typical non-equilibrium plasma conditions (T 3000 K, To < 1000 K), one can take the pre-exponential factor Ao = 10 -10 cm°/s. 

Zeldovich Chain Mechanism of Plasma-Chemical NO Synthesis. Based on expressions (6-39) for the stationary concentration of atomic oxygen and (6-34) for the rate coefficient of limiting reaction of chain propagation, derive a formula for calculating the chain length for the Zeldovich mechanism of NO synthesis. Analyze the dependence of the chain length on electron temperature vibrational temperature (, and translational temperature. 

The balance equations [APP 64] enable us to estabhsh the entropy production rate, from which we deduce the form of the phenomenological relations by way of linearized TIP. Note that with regard to the chemical reactions, there is no need to linearize the stem, because the hterature generally gives us nonlinear chemical kinetic laws with known coefficients (the specific reaction rates). In addition, the balance laws give us the system of equations to be solved in order to determine the fields of variables characterizing the evolution of the plasma in space and time, by means of the knowledge of the physical coefficients. Here, we shall assume steady-state one-dimensional evolution. 

---