Nuclear relaxation in a quadrupolar spin system has been investigated by selectively exciting nuclei into particular magnetic levels and observing the transient recovery of the spin system toward an equilibrium population distribution. Selective excitation is achieved by correlating the frequency and precessional behavior of nuclei in certain states with applied elliptically and linearly polarized, pulsed radio-frequency fields. A quantum-mechanical analysis is presented to describe the excitation of a quadrupolar spin system produced by a pulsed, elliptically polarized rf field. Using selective excitation techniques, several new modes of longitudinal relaxation are observed. Experiments using the chlorine quadrupole resonance in a single crystal of KCl${\mathrm{O}}_3$ demonstrate how these new relaxation modes are used (1) to study dynamic spin-spin interactions and cross relaxation between overlapping resonance lines, and (2) to determine the individual $\Delta m=\pm 1 \mathrm{and} \pm 2$ quadrupolar spin-lattice relaxation transition probabilities. A method is introduced by which the magnetic dipole-dipole contribution to the resonance linewidth can be determined independently of static quadrupole broadening, by observing the decay of the beat modulation of certain free-induction signals caused by precession in a small magnetic field. The measured magnetic linewidth of ${\mathrm{Cl}}^{35}$ in KCl${\mathrm{O}}_3$ is in good agreement with the value obtained from a second-moment calculation.

• Received 6 May 1960

DOI:https://doi.org/10.1103/PhysRev.120.365