Coherent Raman beat EPR without pre-excitation pulses

Coherent Raman beat electron paramagnetic resonance (EPR) spectra without application of one or more excitation pulses have been observed in the EPR spectrum of the tetramethyl-1,4-benzosemiquinone anion radical in a frozen mixture (50:50) of protonated and fully deuterated isopropyl alcohol at 4.2 K. The free nuclear transition frequencies of hydrogen and deuterium and the small quadrupole coupling of 146 kHz of the deuterium could be measured with high sensitivity and precision (line width ≈40 kHz). The modulation amplitudes of the deuterium matrix lines at 2.2 MHz, as a function of the observer power, reveal a distinct resonance behavior coinciding with the frequency of the transient nutations, that is, when the Rabi frequency is approximately equal to the nuclear precession frequency. These observations suggest that the mechanism of coherence transfer between the electron and nuclear spin systems is related to the Hartmann-Hahn effect.