The negatively charged nitrogen-vacancy (NV-) center in diamond is a promising candidate for many quantum applications. Here, we examine the splitting and broadening of the centers infrared (IR) zero-phonon line (ZPL). We develop a model for these effects that accounts for the strain induced by photo-dependent microscopic distributions of defects. We apply this model to interpret observed variations of the IR ZPL shape with temperature and photoexcitation conditions. We identify an anomalous temperature dependent broadening mechanism and that defects other than the substitutional nitrogen center significantly contribute to strain broadening. The former conclusion suggests the presence of a strong Jahn-Teller effect in the centers singlet levels and the latter indicates that major sources of broadening are yet to be identified. These conclusions have important implications for the understanding of the center and the engineering of diamond quantum devices. Finally, we propose that the IR ZPL can be used as a sensitive spectroscopic tool for probing microscopic strain fields and performing defect tomography.
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