FIGURE 1.

The reaction scheme in the Randall-Wilkins picture. C₁ is the charge-separated state (i.e., D⁺PA⁻, where P is the primary pigment donor, D is a secondary electron donor, and A an electron acceptor). It is in rapid equilibrium with the excited state C* (i.e., DP*A), from which recombination to the ground state C₀ (DPA) occurs with a limiting rate constant κ. ΔG, ΔH, and ΔS are the activation free energy, enthalpy, and entropy, respectively. A fixed fraction Φ of the recombination process is radiative, giving rise to the luminescence emission.