Solution
= Solution
\b[Almost every absorbed photon generates a triplet], within the stated kinetic scheme. The branching fraction from the excited singlet is
$$
\phi_{\rm ISC}=\frac{k_{\rm ISC}}{k_F+k_{\rm ISC}}\simeq1,
$$
so $k_{\rm ISC}[S_1]\simeq I_{\rm abs}$ under steady illumination. This statement concerns the triplet formation yield; it does not mean that the triplet population or the phosphorescence yield is also one. The triplet can subsequently deactivate or be quenched.