Measurement of spin-exchange rate constants between 129Xe and alkali metals

By measuring the relaxation rates of the nuclear spin polarization of Xe-129 in the presence of alkali-metal vapor at different densities, we have extracted the spin-exchange rates between Xe-129 and the three alkali metals K, Rb, and Cs. By studying the alkali-metal-Xe-129 spin-exchange rates as functions of the cell number density from 0.2 to 0.7 amagat, the binary collision and van der Waals molecular terms are separated, and constants governing both mechanisms are determined. The results from our work can be used to optimize the parameter space for polarizing Xe-129, a promising agent for magnetic resonance imaging and other applications

Stability of two-dimensional steady Euler flows with concentrated vorticity

In this paper, we study the stability two-dimensional (2D) steady Euler flows with sharply concentrated vorticity in a simply-connected bounded domain. These flows are obtained as maximizers of the kinetic energy subject to the constraint that the vorticity is compactly supported in a finite number of disjoint regions of small diameter. We prove the nonlinear stability of these flows when the vorticity is concentrated in one small region, or in two small regions with opposite signs. The proof is achieved by showing that these flows constitute a compact and isolated set of local maximizers of the kinetic energy on an isovortical surface. The separation property of the stream function plays a crucial role in validating the isolatedness.Comment: 41 pages, proof of existence for multiple vortices was adde