  The intersystem crossing (ISC) is an important process in many solid-state
atomlike impurities. For example, it allows the electronic spin state of the
nitrogen-vacancy (NV) center in diamond to be initialized and read out using
optical fields at ambient temperatures. This capability has enabled a wide
array of applications in metrology and quantum information science. Here, we
develop a microscopic model of the state-selective ISC from the optical excited
state manifold of the NV center. By correlating the electron-phonon
interactions that mediate the ISC with those that induce population dynamics
within the NV center's excited state manifold and those that produce the phonon
sidebands of its optical transitions, we quantitatively demonstrate that our
model is consistent with recent ISC measurements. Furthermore, our model
constrains the unknown energy spacings between the center's spin-singlet and
spin-triplet levels. Finally, we discuss prospects to engineer the ISC in order
to improve the spin initialization and readout fidelities of NV centers.
