Distribution of reaction products over internal energy states

Spectroscopic methods naturally focus attention on the distribution of products over internal energy states. It seems clear that virtually every elementary reaction produces products in a non-equilibrium distribution. Even phase space models3, which assume a strong coupling complex in a certain region of phase space, give 

Of course, any molecule not in its lowest electronic, vibrational and rotational state is, strictly speaking, in an excited state. Hence the title includes all chemical reactions. Since we do not intend to discuss all chemical reactions, we have restricted the discussion to gas phase reactions with the reactants distributed in approximately an equilibrium way over their internal energy states. 

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