Equilibrium in Molecular Gases

The Saha equation (3-14) presented earlier for ionization equilibrium A+ + e A can be generalized to describe the dissociation equilibrium X + Y XY, which is especially important in thermal plasma chemistiy. The relation between densities Ax of atoms X, Ny of atoms T, and Nxy of the molecules XY in the ground state canbe writtenbased on (3-14) as follows  

In this relation, gx, gy, and gxr are relevant statistical weights /u. is the reduced mass of atomsX and T andD is the dissociation energy of the molecule AT. Most molecules are not in a ground state but rather in excited states at high thermal plasma temperatures. For this reason it is convenient to replace the ground-state concentration Nxy (d = 0, J = 0) in (3-15) by the total Nxy concentration, using (3-13)  

Based on (3-15) and (3-16), the statistical relation for equilibrium dissociation in thermal plasma is 

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