Radiofrequency polarization effects in low-field electron paramagnetic resonance

Low-field optically detected EPR spectra of photochemically formed transient radical ion pairs are reported for weak circularly and linearly polarized radiofrequency (RF) fields. The spectra are found to be strongly dependent on the polarization and frequency of the RF field and on the angle between the static magnetic field and the plane containing the RF field. The spectra are discussed in terms of resonances arising from Zeeman and hyperfine interactions; the conditions for validity of the rotating frame approximation are determined. Knowledge of the latter is important when using low-field EPR as a diagnostic test for the operation of the radical pair mechanism.