Electron impact reactions

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. 

Acronym Electron impact reaction Threshold energy (eV Reference ) Comment... 

FIG. 15. Electron impact reaction rales as a function of the average electron energy. A I 1 mixture of SiHa and H2 was used, at a total pressure of 83 Pa. (a) Reaction rates for SiHa, (b) reaction rates for Si2H6 (dotted lines) and H2 (solid lines). Abbreviations are ion. ionization dis, dissociation vib, vibrational excitation att, attachment. See Table II for details and references. (Adapted from G. J. Nienhuis, Ph.D. Thesis. Universileit Utrecht. Utrecht, the Netherlands. 1998.)... 

The high-energy electrons generated in the plasma mainly initiate the chemical reactions by reactions with the background gas molecules (see Table 12.1). Direct electron impact reactions with NO are usually not important for NO decomposition, as in real flue gas, as well as in experiments in simulated gas, the concentrations of NO are very low (some hundreds of ppm), and therefore, the probability of electron collisions is also low. 

Electron impact reactions occur at a rate determined by the concentrations of both electrons (az ) and a particular reactant (N) species (27)... 

A second type of gas phase collision is that occurring between the various (heavy) species generated by electron impact reactions, as well as between these species and the unreacted gas-phase molecules (25,2d). Again, dissociation and ionization processes occur, but in addition, recombination and molecular rearrangements are prevalent. Similar rate expressions to that of Equation 2 can be written for these collisions (27). In this case, the concentration of each chemical species, along with the collision cross section, and the species energy distribution function must be known if k is to be calculated. Clearly, much of this information is presently unknown. 

As noted earlier, electron impact reactions in the plasma produce a variety of species owing to processes such as... 

Although ion-molecule processes would be possible in some electron impact reactions, this process would not lead to the formation of dimeric products formed in the present experiments (20). Ethylene was the major gaseous product. Ethylene and butadiene may be produced from the reverse Diels-Alder reaction of cyclohexene (10) since they were also... 

Photoassisted CVD. In addition to thermal energy and electron-impact reactions, photons (e.g., UV light) can also drive CVD reactions... 

Reaction (66a) is a dissociative resonant capture. Reaction (66b) is shown as an ion-pair formation process induced by electron impact Reaction (66c) is electron capture by a molecule or a neutral fragment produced by a previous dissociative process. If the excited state of Tr lies significantly above the ground state,... 

XK m(K) cm. i sec. 1 Rate coefficient for the electron-impact reaction specified by the suffix with respect to the concentrations of the species X and of electrons... 

When fx differs from unity so slightly that all processes involving electron-impact reaction with species other than X occur at negligible rates,... 

When an apparently acceptable identification has been made of an electron-impact reaction of the type Rl, or of its ion-impact counterpart, it remains to trace the subsequent reactions that lead to the formation of the particular final product, and for that reaction mechanism to derive values of the loss-cum-stoichiometric factor 4>xk,fp that pertain to the conditions explored by experiment. Procedures for dealing with problems of that kind are outlined in discussing the detailed interpretation of the results of Kirkby s experiments. 

The Interpretation of Kirkby s Data Electron-Impact Reaction. 

In seeking a quantitative interpretation of the rate coefficient for the formation of water, Pwa/n, Kirby invoked the theory of electron-impact reaction formulated first by Townsend (85, 86). In terms of the account given earlier in this paper, the formal statement of that early theory can be seen to approach closely Formulae 2 and 12 when the following qualifications are made ... 

It is convenient to assume provisionally that the total water formation in the positive column, at the volume average rate Rwa m(H20) cm." sec."1, is initiated by positive ion reaction. The volume average value of a+wa would then be practically the same as that of for electron-impact reaction because n+ — nc, and for any specified value of X/n may be computed from... 

It is worth noting that the rate of reactions involving electrons depends on the relative velocity between the electron and the projectile. Since the electron velocity is much higher than the neutral velocity, the latter is neglected. Hence the electron-impact reaction rate coefficients are a function of the electron temperature rather than the gas temperature. An exception is electron attachment (R18). The corresponding reaction coefficient depends on the gas temperature because the latter affects the rovibrational populations of the molecules and the attachment coefficient can be sensitive to the rovibrational state [45]. 

Using this normalization, a Maxwellian distribution (Eq. 9) would be a straight line or a semi-logarithmic plot of /o (e, t) vs. e. Once the EEDF has been determined, the reaction rate coefficient for electron impact reaction i can be calculated by... 

The Damkholer number shows the relative importance of species production (or loss) by gas-phase mechanisms relative to their ability to diffuse away from the source (or sink). When Da > 1, large concentration gradients may be expected. The Da number in these systems varies depending on the power level (which affects electron density and degree of dissociation) and pressure (which affects neutral density, diffusivity, and the rate coefficient for electron impact reactions). 

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