  Intense "blue-green" phosphorescence is commonly observed in near colourless
lab-grown high-pressure high-temperature (HPHT) diamonds following optical
excitation at or above the indirect bandgap. We have employed a holistic
combination of optically-excited time-resolved techniques (in addition to
standard spectroscopic characterisation techniques) to study the physics of
this long-lived phosphorescence and understand luminescence-related charge
transfer processes.
  It is shown that the properties of the broad "blue-green" luminescence and
phosphorescence band can be fully explained by emission from neutral
substitutional nitrogen-boron donor-acceptor pairs
($\text{N}_\text{S}^0$...$\text{B}_\text{S}^0$) , once the configurational
change between charge states is considered, and both tunneling between defects
and thermal ionization of donors and acceptors is considered. Significant
concentrations of metastable $\text{N}_\text{S}^-$, are identified after
optical excitation at or above the indirect bandgap. $\text{N}_\text{S}^-$ is
much shallower ($\sim$0.2 eV) than previously thought and plays a key role in
resetting the $\text{N}_\text{S}^0$...$\text{B}_\text{S}^0$ donor-acceptor
pairs.
