Mass-action law for highly excited dimers

Dimer partition functions and the equilibrium constant for dissociation to monomers are investigated in the classical approximation taking into account the limited number of bound and quasibound pair states. It is shown how the dimerization mass action law should be modified when thermal excitation is not negligible in comparison to the dissociation energy from the ground state. The results are applicable to a variety of weakly bound species near room temperature as well as to conventional diatomics affected by extreme nonequilibrium heating.