  Some aspects of lasing at vibronic transitions in impurity crystals are
theoretically studied. The threshold conditions for a vibronic laser are shown
to be dependent on the strength of interaction of optical centers with a local
vibration, which forms the vibronic spectrum, and the crystal lattice
temperature. The theory can be easily generalized to the spectrum containing a
structureless phonon sideband and well agrees with the experimental temperature
dependence of the output power of a Mg2SiO4:Cr4+ forsterite laser.
