EFFECTS OF FINITE GRADIENT-PULSE WIDTHS IN PULSED-FIELD-GRADIENT DIFFUSION MEASUREMENTS

The effects of finite gradient-pulse widths on NMR diffusion measurements for fluids in restricting geometries are studied, It is shown that the echo amplitude is the spatial Fourier transform of a ''center-of-mass'' propagator, which reduces to the usual diffusion propagator in the limit of zero pulse widths, A finite gradient-pulse width delta effectively changes the pore shape, making isolated pores appear smaller than their actual size. The diffraction analogy still holds for long diffusion times, with the fluid density rho(r) being replaced by p(cm)(r, delta). This quantity, the ''center-of-mass distribution function,'' is the spatial probability distribution of the center of mass of Brownian trajectories of duration delta in the pore space, For a periodic pore space, ''Bragg'' peaks still appear in the amplitude at the reciprocal lattice vectors. The heights of these peaks are enhanced for small delta but reduced for large delta, A number of results valid for small delta and piecewise smooth pore surfaces are presented. (C) 1995 Academic Press, Inc.