T1 ϱ in nuclear quadrupole resonance: a theoretical study

Abstract

A new NQR method of measuring the spectral density of slow motions in solids is proposed. It is shown that also in NQR a 90 ° phase shift of a resonant rf magnetic field following a 90 ° pulse locks the nuclear magnetization in a ‘rotating frame’ similarly as in NMR. The spin-lattice relaxation time T 1 ϱ of the locked magnetization is calculated in general for an arbitrary spin. It is assumed that the fluctuations of the EFG tensor dominate the spin-lattice relaxation. The calculations show that T 1 ϱ depends on the spectral density J( ω) of the electric quadrupole fluctuations at the NQR frequencies, and also at a low frequency Ω. Here Ω ≈ γ B 1 kHz depends on the orientation of the rf magnetic field in the principal-axis system of the EFG tensor. The term containing J(Ω) in the expression for T 1 ϱ −1 depends on the orientation of the rf magnetic field in the principal-axis system of the EFG tensor, only through the orientation dependence of Ω. This term vanishes when the electric quadrupole fluctuations do not modulate the frequency of the NQR transition excited by the rf magnetic field. Two particular examples: I = 1 and I = 3 2 are worked out in details.