  We report direct measurement of population dynamics in the excited state
manifold of a nitrogen-vacancy (NV) center in diamond. We quantify the
phonon-induced mixing rate and demonstrate that it can be completely suppressed
at low temperatures. Further, we measure the intersystem crossing (ISC) rate
for different excited states and develop a theoretical model that unifies the
phonon-induced mixing and ISC mechanisms. We find that our model is in
excellent agreement with experiment and that it can be used to predict unknown
elements of the NV center's electronic structure. We discuss the model's
implications for enhancing the NV center's performance as a room-temperature
sensor.
