Effect of convection and B inhomogeneity on singlet relaxation experiments.

Nuclear spin singlet lifetimes can often exceed the T length scales by a large factor. This property makes them suitable for polarization storage. The measurement of such long lifetimes itself can become challenging due to the influence of even very weak relaxation mechanisms. Here we show that a judicious choice of the singlet-to-triplet conversion method is highly important in order to achieve reliable singlet relaxation measurements. In particular, we identify thermal convection, in connection with B field gradients, asa significant apparent decay mechanism, which limits the ability to measure the true singlet state lifetimes. Highly B-compensated broadband singlet excitation/detection sequences are shown to minimize the influence of macroscopic molecular motion and B inhomogeneity.