Singlet levels of the NV^- centre in diamond

The characteristic transition of the NV^- centre at 637 nmis between ³A₂ and ³E triplet states. There are also intermediate ¹A₁ and ¹E singlet states, and the infrared transition at 1042 nmbetween these singlets is studied here using uniaxial stress. The stress shift and splitting parameters are determined, and the physical interaction giving rise to the parameters is considered within the accepted electronic model of the centre. It is established that this interaction for the infrared transition is due to a modification of electronelectron Coulomb repulsion interaction. This is in contrast to the visible 637 nm transition where shifts and splittings arise from modification to the one-electron Coulomb interaction. It is also established that a dynamic JahnTeller interaction is associated with the singlet ¹E state, which gives rise to a vibronic level 115 cm^-1 above the ¹E electronic state. Arguments associated with this level are used to provide experimental confirmation that the ¹A₁ is the upper singlet level and ¹E is the lower singlet level.