Radical-pair: Φ_S = k_S/(k_S + k_T) under equilibrated S/T mixing

In the fast-spin-mixing limit where hyperfine + Zeeman dynamics equilibrate the singlet (S, population 1/4) and triplet (T: T_+, T_0, T_−, combined population 3/4) manifolds far faster than chemical recombination, the singlet yield reduces…