  Continuous spectral hole burning and spin-level population oscillations are
studied in an inhomogeneously broadened diamond-based spin ensemble composed of
substitutional nitrogen and nitrogen-vacancy centres created through neutron
irradiation and annealing. The burnt spectral features highlight a
detuning-dependent homogeneous hole linewidth that is up to three orders of
magnitude narrower than the total inhomogeneous ensemble linewidth. Continuous
population oscillations are observed to quickly decay beyond a pump and probe
detuning of 5 Hz, and are numerically modelled using a five-level system of
coupled rate equations. Fourier analysis of these oscillations highlight
discrete $^{13}$C hyperfine interactions, with energies within the
inhomogeneous ensemble linewidth, as well as suspected nuclear $3/2$-spin
coupled signatures likely related to the $^7$Li byproduct of neutron
irradiation.
