Pulsed ENDOR spectroscopy at large thermal spin polarizations and the absolute sign of the hyperfine interaction

It is shown that in pulsed Mims-type ENDOR experiments performed at 95 GHz and 1.2 K the sign of the ENDOR signal can be positive, corresponding to an increase of the stimulated echo intensity, as well as negative, corresponding to a decrease of the stimulated echo. The positive ''anomalous'' sign is not observed at conventional EPR frequencies. It is explained that the effect arises through spin-lattice relaxation in the situation of large thermal spin polarizations and that it allows the determination of the absolute sign of the hyperfine interaction. (C) 1997 Academic Press.