  Scalar couplings of the form J I_1 \cdot I_2 between nuclei impart valuable
information about molecular structure to nuclear magnetic-resonance spectra.
Here we demonstrate direct detection of J-spectra due to both heteronuclear and
homonuclear J-coupling in a zero-field environment where the Zeeman interaction
is completely absent. We show that characteristic functional groups exhibit
distinct spectra with straightforward interpretation for chemical
identification. Detection is performed with a microfabricated optical atomic
magnetometer, providing high sensitivity to samples of microliter volumes. We
obtain 0.1 Hz linewidths and measure scalar-coupling parameters with 4-mHz
statistical uncertainty. We anticipate that the technique described here will
provide a new modality for high-precision "J spectroscopy" using small samples
on microchip devices for multiplexed screening, assaying, and sample
identification in chemistry and biomedicine.
