  We study spectroscopy of negatively charged nitrogen-vacancy center in
diamond at high temperatures under high vacuum conditions. Spin resonances are
studied using optical detection of magnetic resonance (ODMR), and optical
spectroscopy is employed to study radiative transitions. Upon increasing the
temperature the intensity of radiative decay in visible and infra-red
decreased. In addition, the ODMR resonance frequencies were decreased, and the
phonon line emission shifted to higher wavelengths. Fitting the measured
intensity of photo-luminescence with the theoretical predictions of the
Mott-Seitz model yields the value of $0.24 \operatorname{eV}$ for the energy
barrier associated with nonradiative decay.
