Initial-state preparation effects in time-resolved electron paramagnetic resonance experiments

We explain a recent experimental observation that the time-resolved electron paramagnetic resonance spectra of an organic molecule for optical excitation within a highly absorbing region of the molecule has similar intensities to the spectra for optical excitation in a nonabsorbing region [D. L. Meyer et al., J. Phys. Chem. Lett. 8, 1677 (2017)]. We demonstrate that this phenomenon is due to an initial-state preparation effect of photoexcitation that leads to similar initial populations of triplet states for both optical excitation regions. Due to the low intersystem crossing rates, the initial triplet populations are not perturbed on the time scale of the experiment, so they determine the relative intensities of the paramagnetic resonance spectra. The effect is surprising given the weak spin-orbit interactions of organic molecules. Such initial-state preparation effects are likely to occur in systems where the intersystem crossing time scales are long compared to the time scale of the experiment.