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The principle of chemical equilibrium is found to govern formation of negatively charged excitons (X^-) by excitons (X) and free electrons (e^-). An optical spectroscopy study of undoped CdTe/Cd₀. ₈₈Mn₀. ₁₂Te double quantum wells (QW's) revealed formation of a negatively charged exciton state in the thicker QW due to the interwell tunneling of photoexcited electrons. A peculiar, nonmonotonic behavior of the ratio of X to X^- densities as a function of the excitation density evidenced that the chemical law of action of masses corresponding to the reaction X+e^-X^- is fulfilled.
Siviniant et al. (Fri,) studied this question.