The spontaneous emission rate (SER) of a single impurity molecule for the electric dipole transition has been found in
the case where the birefringence of a host crystal is taken into account. SER depends on the orientation of the dipole
moment with respect to the principal axes of the dielectric (permittivity) tensor of the host crystal and on the principal
refractive indices of the crystal. We use the model where the average electromagnetic field in the crystal is classical and
effective electric field vectors at the site of the impurity molecule are found for the interstitial impurity which is located
in the center of the crystal cell. The results of calculations for seven biaxial host crystals (anthracene, chrysene, diphenyl,
fluorine, naphthalene, phenanthrene, terphenyl) are presented. It is shown that values of SER form in the space a three-
axial ellipsoid. Differences in the values of SER for the electric dipole transition in the same host crystal are up to 34%.
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