(A) Calculated singlet-yield, ΦS(0), at zero field as a function of D for J = 0 (solid line) and J = 500 μT (dashed line). (B) Calculated change in the spherically averaged singlet-yield [〈ΦS(B0)〉 − ΦS(0)] produced by an external 50 μT magnetic field as a function of D for J = 0 (dashed line), J = 50 μT (solid line), and J = 500 μT (dash-dot line). In both cases: a = 500 μT, α = 0, and k = 106 s−1.