Gaseous Phase and Surface Processes

Zusammenfassung

Consider the following reactions:

$${\mathrm{a)}}\quad 2\,\mathrm{U}+3\,\mathrm{V}\rightarrow\mathrm{W}+2\,\mathrm{Z}$$

$${\mathrm{b)}}\quad \mathrm{U}+\mathrm{V}\rightarrow\mathrm{W}+\mathrm{Z}$$

$${\mathrm{c)}}\quad \mathrm{U}\rightarrow\mathrm{W}+\mathrm{Z}@,,$$

(5.1)

where U, V, W and Z are molecules. The reactions are denominated elementarily, if they occur in one step. This means all molecules in a), b) must collide at the same time, and/or in c) the decomposition “in a step” must take place. Reaction a) is in all probability not an elementary reaction, since five molecules must collide at the same time. For the reactions b) 2; an elementary reaction is conceivable:

$$\mathrm{O}^{+}+\mathrm{O}_{2}\rightarrow\mathrm{O}+\mathrm{O}_{2}^{+}\,;$$

likewise for reaction c):

$$\mathrm{A}^{\ast}\rightarrow\mathrm{A}+{h}\nu\,.$$

On the other hand, it is well known that the following reaction is not elementary:

$$\mathrm{Cl}_{2}+{\mathrm{H}}_{2}\rightarrow 2\,{\mathrm{HCl}}\,.$$

This reaction corresponds to b). From this it follows that on the basis of the stoichiometric equations a)–c) it cannot decided whether a reaction is elementary or not. However, chemical reactions can be divided into different basic steps:

• 1. Unimolecular reactions:

  $$\mathrm{U}\rightarrow\text{final products}.$$

• 2. Bimolecular reactions:

  $$\mathrm{U}+\mathrm{V}\rightarrow\text{final products}.$$

At high pressure triple molecular reactions can also occur:

$$\mathrm{U}+\mathrm{V}+\mathrm{W}\rightarrow\text{final products}.$$

In low pressure plasmas the reactions are usually complex. By means of the so-called stoichiometric coefficients the reaction rates can be defined. The reaction rates for reactants (U, V in Eq. (5.1)) are negative, for products positive:

$$R=\frac{1}{\alpha_{j}}\frac{\mathrm{d}n_{j}}{\mathrm{d}t}\,,$$

(5.2)

here

n _j :

is volume density of molecules at the jth component.