The theory of time-resolved EPR spectroscopy does not differ in the first place from the general theory of magnetic resonance. Only the occurrence of lines with enhanced absorptive or emissive polarization, the so-called electron spin polarization (ESP), requires an additional explanation.
Together with the far more widespread nuclear magnetic resonance (NMR) spectroscopy, EPR spectroscopy belongs to the methods of magnetic resonance.
Both methods are based on the alignment of the spin (electron or nuclear spin) in an external magnetic field and the possibility of inducing transitions between these two states by electromagnetic radiation of corresponding wavelength (microwave in the EPR, radio frequency in the NMR).
Time-resolved EPR spectroscopy is a direct method for observing spin dynamics and electron spin polarization (ESP) during the formation of spin-polarised paramagnetic species, e.g. radical pairs or triplet states. The measured quantity is the temporal development of transient magnetization. The ESP determines the sign (absorption or emission) of the EPR signal.