The relaxation processes occurring in a chemically reacting gas irradiated by a monochromatic light source are investigated. For the case of a bimolecular isomerization, the decay of the progress variable of the reaction to the steady state produced by the illumination is studied. A macroscopic equation for this quantity is derived from the quantum mechanical Boltzmann equation for this system by projection operator methods. The derivation also provides microscopic expressions for the rate coefficients which depend on time (or frequency). The rate coefficients which characterize the one way fluxes in the steady state are compared with the observed rate coefficients. Calculations for specific models of the isomerization indicate that significant differences between these quantities occur at all values of the light intensity. In addition, the time evolution of the part of the rate coefficient which depends on the coupling between the chemical and internal relaxation processes is studied and compared with the decay of the progress variable.
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