  Optically pumped magnetometers (OPMs) based on alkali-atom vapors are
ultra-sensitive devices for dc and low-frequency ac magnetic measurements.
Here, in combination with fast-field-cycling hardware, we demonstrate
applicability of OPMs in quantifying nuclear magnetic relaxation phenomena.
Relaxation rate dispersion across the nT to mT field range enables quantitative
investigation of extremely slow molecular motion correlations in the liquid
state, with time constants $>$1 ms, and insight into the corresponding
relaxation mechanisms. The 10-20 fT/Hz$^{1/2}$ sensitivity of an OPM between 10
Hz and 5.5 kHz $^1$H Larmor frequency suffices to detect NMR signals from
$\sim$2 mL bulk liquid, or 0.5 mL liquid imbibed in simple mesoporous
materials, following nuclear spin prepolarization adjacent to the OPM. The
expected limits of the OPM technique are discussed regarding measurement of
relaxation rates above 100 s$^{-1}$.
