  Hyperpolarisation at room temperature is one of the most important research
fields in order to improve liquid, gas or nanoparticle tracer for Magnetic
Resonance Imaging (MRI) in medical applications. In this paper we utilize
nuclear magnetic resonance (NMR) to investigate the hyperpolarisation effect of
negatively charged nitrogen vacancy (NV) centres on carbon-13 nuclei and their
spin diffusion in a diamond single crystal close to the excited state level
anti crossing (ESLAC) around 50 mT. Whereas the electron spins of the NV centre
can be easily polarized in its m = 0 ground state at room temperature just by
irradiation with green light , the swop of the NV electron spin polarization to
a carbon-13 nuclei is a complex task. We found that the coupling between the
polarized NV electron spin, the electron spin of a substitutional nitrogen
impurity (P1) as well as its nitrogen-14 nuclei and the carbon-13 nuclear spin
has to be considered. Here we show that through an optimization of this
procedure, in about two minutes a signal to noise ratio which corresponds to a
23 hour standard measurement without hyperpolarisation and an accumulation of
460 single scans can be obtained. Furthermore we were able to identify several
polarisation peaks of different sign at different magnetic fields in a region
of some tens of gauss. Most of the peaks can be attributed to a coupling of the
NV centres to nearby P1 centres. We present a new theoretical model in a
framework of cross polarisation of a four spin dynamic model in good agreement
with our experimental data. The results demonstrate the opportunities and power
as well as limitations of hyperpolarisation in diamond via NV centres. We
expect that the current work may have a significant impact on future
applications.
