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

Abstract

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. The coupling between the nuclear quadrupole moment and the electric-field-gradient tensor is, besides th e Zeeman effect, the most important interaction of nuclear spins I > + with their environment.’ Since this coupling depends strongly on the electronic environment of the nucleus, it is a sensitive probe for the structure of solids2 and, through its effect on spin relaxation, also of motional processes. Determination of the nuclear quadrupole coupling tensor has, therefore, often been an important tool in magnetic resonance investigations of crystalline, powdered, or amorphous materials. Comparison of the measured values with theoretically calculated data can serve as a check for electronic-structure calculations or as a tool for measuring atomic polarizabilities.3 The main experimental tool for the determination of quadrupole coupling constants is nuclear magnetic resonance (NMR) in high magnetic fields 4 and its low-field relative, nuclear quadrupole resonance (NQR). These methods can provide very precise data on the magnitude of the quadrupole coupling constants; however, they are insensitive to its sign; multiplication of the quadrupole coupling Hamiltonian with - 1 has no effect on the observed magnetic resonance spectrum obtained by any combination of static and oscillatory magnetic fields, as long as th e hightemperature approximation for the nuclear-spin system is valid.5 It is, therefore, necessary to reduce the spin temperature to T ’ In favorable cases, it is possible to measure the sign of the coupling constant relative to other quantities. As an example, it was shown that the two different sites of Li in Li3N have opposite signs;’ the absolute sign, however, is still un