  The application of magnetic resonance (MR) spectroscopy at progressively
smaller length scales may eventually permit "chemical imaging" of spins at the
surfaces of materials and biological complexes. In particular, the negatively
charged nitrogen-vacancy (NV-) centre in diamond has been exploited as an
optical transducer for nanoscale nuclear magnetic resonance. However, the
spectra of detected spins are generally broadened by their interaction with
proximate paramagnetic NV- centres through coherent and incoherent mechanisms.
Here we demonstrate a detection technique that can resolve the spectra of
electron spins coupled to NV- centres, namely substitutional nitrogen (NS) and
neutral nitrogen-vacancy (NV0) centres in diamond, through optically detected
cross-relaxation. The hyperfine spectra of these spins are a unique chemical
identifier, suggesting the possibility, in combination with recent results in
diamonds harbouring shallow NV- implants, that the spectra of spins external to
the diamond can be similarly detected.
