  The emission intensity of diamond samples containing nitrogen-vacancy centres
are measured as a function of magnetic field along a <111> direction for
various temperatures. At low temperatures the responses are sample and stress
dependent and can be modeled in terms of the previous understanding of the 3E
excited state fine structure which is strain dependent. At room temperature the
responses are largely sample and stress independent, and modeling involves
invoking a strain independent excited state with a single zero field splitting
of 1.42 GHz. The change in behaviour is attributed to a temperature dependent
averaging process over the components of the excited state orbital doublet. It
decouples orbit and spin and at high temperature the spin levels become
independent of any orbit splitting. Thus the models can be reconciled and the
parameters for low and high temperatures are shown to be consistent.
