Optical detection of NMR J-spectra at zero magnetic field
M Ledbetter, C.W. Crawford, A Pines, D.E. Wemmer, Svenja A. Knappe, John E. Kitching, D Budker
Nuclear magnetic resonance (NMR) has emerged as one of the most powerful analytical tools for elucidating molecular structure. One of the parameters typically extracted from NMR spectra which is necessary to fully understand the structure of a molecule is the parameter J in the scalar interaction JI1 •I2, first discovered by Hahn and Maxwell and independently by Gutowsky, McCall and Slichter. Conventional detection of NMR relies on large magnetic fields to achieve high sensitivity, and hence, with the exception of field-cycling techniques, nearly all experiments involving J-coupling have been performed under conditions where the Zeeman interaction is by far the dominant term in the Hamiltonian. Here we demonstrate direct detection of hetero- and homonuclear J-coupling in a zero-field environment using an optical atomic magnetometer. We obtain linewidths as low as 0.1 Hz, measure J-coupling constants to an accuracy of 0.004 Hz and realize high signal-to-noise ratio with a sample volume of 80 υL, significantly smaller than has been used in other low-field experiments. Our measurements are in good agreement with density-matrix calculations, and show that the spectra of certain functional groups become greatly simplified at zero field compared to spectra obtained in the Earth's field.