S. Axelrod et Pn. Sen, Nuclear magnetic resonance spin echoes for restricted diffusion in an inhomogeneous field: Methods and asymptotic regimes, J CHEM PHYS, 114(15), 2001, pp. 6878-6895
We develop systematic formulations for calculating the magnetization of spi
ns diffusing in a bounded region in the presence of the surface relaxation
and magnetic field inhomogeneity and compute explicitly the relaxation expo
nent for the Carr-Purcell-Meiboom-Gill spin echoes. The results depend on t
he echo number n, and three dimensionless parameters: L-rho/L-S, (D) over t
ilde (0) =(L-D/L-S)(2), the dimensionless diffusion constant, and <(<gamma>
)over tilde>=(LDLS)-L-2/L-G(3)=Delta omega tau, the dimensionless gyromagne
tic ratio, where the restriction is characterized by a size L-S, the magnet
ic field inhomogeneity by a dephasing length, L-G, the diffusion length dur
ing half-echo time by L-D, and a length L-rho characterizes the surface rel
axation. Here Delta omega is the line broadening and 2 tau is the echo peri
od. Depending on the length scales, three main regimes of decay have been i
dentified: short-time, localization, and motionally averaging regimes (MAv)
. The short-time and the MAv regimes are described well by the cumulant exp
ansion in terms of powers of the "small" parameter <(<gamma>)over tilde>. W
e give simple formulas for decay rates in these two asymptotic regimes. We
show that the Gaussian phase approximation (GPA), i.e., the exponent up to
the second order in <(<gamma>)over tilde>(2) in terms of a full eigenmode e
xpansion interpolates well between these two regimes. In the localization r
egime, the decay exponent depends on a fractional power, <(<gamma>)over til
de>(2/3), denoting a breakdown of the GPA and a breakdown of the cumulant e
xpansion in terms of <(<gamma>)over tilde>. (C) 2001 American Institute of
Physics.