The concentration dependence of free radicals (TEMPO) of enhanced absorptive chemically induced dynamic electron polarization (CIDEP) spectra was studied quantitatively to clarify the mechanism of CIDEP generation in excited singlet state-radical systems. It was experimentally demonstrated that the CIDEP intensity on the free radical was in proportion to the concentration of the spin-polarized free radical. The rate constant of the CIDEP generation was determined to be (6.3 +/- 0.2) X 10(9) M(-1) s(-1) from the Stern-Volmer plots of CIDEP intensity in the coronene-TEMPO system in benzene. The absorptive CIDEP spectra were attributed to the free radicals which enhanced the S-1-T-1 intersystem crossing and were interpreted by introducing the stochastic-Liouville equation into the radical-triplet pair mechanism with doublet precursor.