Differences of the radiative line width in single-impurity molecule spectroscopy

A theory of the dependence of the spontaneous emission rate and, consequently, the dependence of the radiative line width of a single-impurity molecule on the orientation of the position vector in a birefringent host crystal is developed for the case of the electric quadrupole radiation. Depending on the orientation of the electron position vector, the differences in the values of the spontaneous emission rate of the electric quadrupole radiation may be considerable and they depend on the values of the principal refractive indices of the host crystal—the larger refractive indices lead to larger differences in the values.