Knowledge Bank

The splitting of spectral lines by an electric field, in the accepted sense, is dependent on the interaction of the field with degenerate states. However, molecular crystals having no degenerate states can be subject to a linear Stark mixing due to the interaction of the field with the orientational degeneracy of the inequivalent molecules in the unit cell. The resultant splitting of spectral lines, $\Delta S=EG\Delta \mu$, is directly proportional to the field strength, E, a geometrical factor, G, and the change in dipole moment, $\Delta \mu$, between the ground and excited states. The dipole moments of benzophenone in its first excited triplet and singlet states, azulene in its first and second states, and phenanthrene have been measured and will be discussed.