Plasma-chemical reaction frequency

Collision frequencies, mean free paths, and cross sections. Collision processes are central to the description of chemical reactions in the plasma. The mean free path /Ia/b of particle A is the mean distance this particle travels before encountering particle B. AA/b is related to the mean velocity of particle A and to the mean collision frequency between A and B. 

More recently, the parent system has been investigated under radio frequency plasma conditions. The plasma generated by an electric discharge provides an unusual medium for reactions in the gas phase. The free electrons that are present in the plasma are responsible for the chemical reactions that take place. The electrons are accelerated by an applied electric field and collide with the molecule, thereby activating it for the reaction. Most procedures employ radio frequency (RF) discharges so as to keep the pressure, temperature and energy of the system relatively low. The substrate is usually volatilized into the plasma zone with residence times of the order of a second. Because there is no need of chromo-phores, the technique has been used to activate those cases that are inert to the normal photoexcitation in solution, e.g. the parent 1,4-pentadiene. ... 

The chemical reaction rate in this case is determined by frequency of W relaxation and by non-perturbed vibrational distribution f ° E). The rate (3-179) is not sensitive to the characteristics of elementary chemical reactions. Such a situation in practical plasma chemistry takes place, for example, in CO2 and H2O monomolecular dissociation processes (see Chapter 5). 

Treatment of certain polymeric surfaces with excited inert gases greatly improves the bond strength of adhesive joints prepared from these materials. With this technique, called plasma treatment, a low-pressure inert gas is activated by an electrode-less radio-frequency discharge or microwave excitation to produce metastable species which react with the polymeric surface. The type of plasma gas can be selected to initiate a wide assortment of chemical reactions. In the case of polyethylene, plasma treatment produces a strong, wettable, cross-linked skin. Commercial instruments are available that can treat polymeric materials in this manner. Table 7.10 presents bond strength of various plastic joints pretreated with activated gas and bonded with an epoxy adhesive. 

New data concerning the interaction of ions and hydrocarbon radicals allow to have a quite complete mapping of the relevant reaction rates in an Ar/CH4 plasma by now and it is possible to investigate the effect of the chemical kinetics in such a system and eventually to identify the gaseous precursor of the chemical species incorporated in the deposited film. We have modeled the gas-phase chemistry of a typical radio frequency CH4/ Ar plasma used for the deposition of diamond and diamond-like carbon films, with the aim of understanding the effects of the chemical kinetics of argon ions and metastables. 

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