Spectral density functions for quadrupolar nuclear spin relaxation due to translational diffusion

The spectral density functions for the relaxation of nuclear spins due to time-dependent electric quadrupole interactions are studied for the diffusion of atoms on a crystal structure. It is shown that, for an arbitrary concentration c of diffusing atoms, the spectral density functions and consequent relaxation rates scale with concentration as c(1-c). The detailed forms of the functions can be obtained simply from analysis of the low spin concentration limit, unlike the analogous case for magnetic dipolar relaxation. Some applications to metal-hydrogen systems are described.