Determination of the absolute sign of nuclear quadrupole interactions by laser radio-frequency double-resonance experiments.

The interaction between the quadrupole moment of nuclear spins I > + with the electric-field-gradient (EFG) tensor leads to a splitting of the energy of the nuclear spin states. We show how the combination of laser and radio-frequency irradiation allows measurements of nuclear spin transitions in quadrupolar systems that are, in contrast to purely magnetic experiments, sensitive to the absolute sign of the quadrupole interaction. This determination of the sign is essential for comparison with calculated EFG tensors.