Ž . Solid State Nuclear Magnetic Resonance 16 2000 199–202 www.elsevier.nlrlocatersolmag Spin diffusion of dipolar energy in NQR Gregory B. Furman ) , Shaul D. Goren Department of Physics, Ben-Gurion UniÕersity, Be’er-SheÕa, Israel Received 12 February 2000; accepted 21 March 2000 Abstract The theory of spin diffusion was extended to the case of nuclear dipolar order in solids containing paramagnetic impurities and nuclei with spin I )1r2 having nuclear quadrupole moment. We show that spin diffusion process of dipolar order takes place in solids containing paramagnetic impurities. At the start of relaxation process, the direct relaxation regime is realized with non-exponential time dependence. Then the relaxation regime will be changed to diffusion-limited one. Using obtained expressions for the spin lattice relaxation times for these two relaxation regimes, the diffusion coefficient of the dipolar order in nuclear quadrupole resonance can be estimated from experimental data. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Spin diffusion; Dipole–dipole interaction; NQR 1. Introduction A study of the spin lattice relaxation in the local field gives an important information about atomic and molecular motions that are impossible to obtain from investigation of the relaxation processes in a strong field. To obtain the spin lattice relaxation times in a local field in nuclear magnetic resonance Ž . NMR spin systems with equidistant energy spectra and to study slow molecular motion, two experimen- tal techniques have been applied: the method of Ž . adiabatic demagnetization in rotating frame ADRF wx 1 and a pair of phase-shifted radiofrequency pulses wx 2 , which allow to transfer Zeeman order to the ) Corresponding author. Ž . E-mail address: gregoryf@bgumail.bgu.ac.il G.B. Furman . dipolar reservoir. Then these methods were applied Ž . to cases in pure nuclear quadrupole resonance NQR where electric nuclear quadrupole relaxation was dominant and therefore, the influence of paramag- w x netic impurities was avoided 3,4 . Here we consider the spin lattice relaxation and the spin diffusion of dipolar order in the case of the spin system with non-equidistant energy spectra in solids containing paramagnetic impurities. 2. Theory Let us consider a spin system consisting of nu- clear I ) 1r2 and paramagnetic S s 1r2 spins, lo- calized at positions r and r , respectively. Here the m j Greek indices indicate the nuclei and the Latin the impurities. 0926-2040r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0926-2040 00 00082-5