  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 center's 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 center's 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.
