Abstract
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 and high-resolution spectroscopic detection, 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/sqrt{{rm{H}}}{rm{z}} sensitivity of an OPM between 10 Hz and 5.5 kHz 1H Larmor frequency suffices to detect magnetic resonance signals from ~ 0.1 mL liquid volumes imbibed in simple mesoporous materials, or inside metal tubing, following nuclear spin prepolarization adjacent to the OPM. High-resolution spectroscopic detection can resolve inter-nucleus spin-spin couplings, further widening the scope of application to chemical systems. Expected limits of the technique regarding measurement of relaxation rates above 100 s−1 are discussed.
Highlights
Pumped magnetometers (OPMs) based on alkali-atom vapors are ultra-sensitive devices for dc and low-frequency ac magnetic measurements
nuclear magnetic relaxation dispersion (NMRD) such as surface fractal dimension and roughness provide models for industrial catalysis and petrology, where liquids are confined inside porous solids and molecular diffusion is restricted by surface geometry[6] as well as adsorption[7], and in medicine assist the design of molecular agents for relaxation-contrast magnetic resonance imaging (MRI)[8]
When it covers the appropriate range of fields and time scales, the relaxation measured in NMRD can relate model parameters of interest to molecular motion, surface structure, and molecule–surface interactions[35,36,37]
Summary
Pumped magnetometers (OPMs) based on alkali-atom vapors are ultra-sensitive devices for dc and low-frequency ac magnetic measurements. NMRD is performed by transporting samples between persistent high- and ultralow-field locations[16,17,18,19,20], but relatively slow transport times limit the observable τc at the high end The limits of these existing techniques are illustrated by the magenta- and blue-shaded regions, respectively, of Fig. 1. The speed of the FFC approach is combined with the lowfrequency sensitivity of a spin-exchange-relaxation-free (SERF)[21,22,23,24,25,26,27,28] optically pumped magnetometer (OPM) to perform NMRD at 1H Larmor frequencies from 1 Hz to 10 kHz, corresponding to the region of Fig. 1 shadedpinffiffiffiffigffiffireen. The high sensitivity of SERF OPMs of order 1 fT/ Hz29,30 at signal
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