NQR

Nuclear quadrupole resonance (NQR) is a radiofrequency spectroscopic technique that probes the interaction between a nucleus with an electric quadrupole moment and the electric field gradient at the nucleus in a solid. It is sensitive to nuclei with spin greater than 1/2, such as 14N, 35Cl, and 63Cu. In the absence (or with only a small presence) of an external magnetic field, the quadrupole interaction splits the nuclear spin energy levels, producing resonances at characteristic frequencies determined by the quadrupole coupling constant and, when applicable, the asymmetry of the local electric field gradient.

Experimental measurements typically apply radiofrequency pulses and detect the resulting spin-echo or relaxation signals. Because NQR does not rely on Zeeman splitting from a strong magnetic field, it complements conventional NMR. However, NQR signals tend to be weak and highly sensitive to factors such as sample quality, temperature, and relaxation times, making measurements challenging in some materials.

Applications of NQR span science and security. In security contexts, NQR is used to detect explosives and narcotics by identifying signals from specific quadrupolar nuclei present in target compounds, notably nitrogen-14. In materials science and chemistry, NQR provides information about local structure, phase transitions, and molecular dynamics by revealing details of the electric field gradient at sites with quadrupolar nuclei. It is a non-destructive technique and does not require ionizing radiation or strong magnetic fields.

Limitations include long acquisition times, potential spectral overlap in complex mixtures, and the need for suitable quadrupolar nuclei with resolvable transitions. Advances focus on increasing sensitivity, improving signal processing, and extending applicability to more materials.