Peculiarities in Rabi oscillations for fast-relaxing electron spins

Rabi oscillations (transient nutations) are a phenomenon that has proven itself well in EPR for identifying electron spin quantum numbers and electron-spin transitions. They are successfully applied when the Rabi frequency significantly exceeds the spin relaxation rates and therefore does not depend on these rates. However, the short transverse relaxation time, being comparable to or even shorter than the dead time of EPR spectrometers, makes it difficult to observe Rabi oscillations and their frequency depends not only on the intensity of the short microwave pulse, but also on its shape and relaxation rates. Two techniques are considered that are suitable for this case, in which Rabi oscillations are detected by monitoring the FID amplitude as a function of pulse duration or microwave field amplitude. We describe the FID-detected Rabi oscillations analytically or numerically for rectangular or shaped pulses, respectively. The description is confirmed by EPR experiments using DPPH as a model sample.