Dissociation and Recombination

The excitation of internal degrees of freedom of molecules happens through “collisions”. If a molecule receives “too much” energy by collisions, it will dissociate. Recombination is the reverse phenomenon in which two atoms (re)combine to a molecule. However, a “third body” is needed to carry away the excess energy, i. e., the dissociation energy, which is released during the recombination process. The third body can be an atom or a molecule, but also the vehicle surface, if it is finitely or fully catalytic (catalytic surface recombination). At very high temperatures also ionization occurs. It is the species NO, which is ionized first, because it needs the lowest ionization energy of all species, Fig. 2.5.

Dissociation and recombination are chemical reactions, which alter the composition of the gas, and which bind or release heat [1, 7]. In equilibrium, Section 5.4, the five species N2, N, O2, O, and NO, are basically products of the three reactions

N2 ^ N + N,

O2 ^ O + O, (5.20)

NO ^ N + O.

If ionization occurs, we get for the first appearing product

NO ^ NO+ + eM (5.21)

We distinguish between homogeneous and heterogeneous reactions. The former are given, if only the gas constituents are involved. The latter, if also the vehicle surface plays a role (catalytic surface recombination). In the following Section 5.4 we discuss the basics of homogeneous reactions, and in Section 5.6 those of heterogeneous reactions.