Rigid dipoles vibrational state

For a molecule in a given electronic and vibrational state, it is convenient to define the permanent dipole operator d = (i/r // i/r), where v/) is a product of the electronic and vibrational states. This vector operator depends on the angles that specify the orientation of the molecule with respect to the external field axis. For diatomic molecules, d is directed along the intermolecular axis. The Stark shifts of the molecule in a DC electric field can (almost always) be found by treating the molecule as a rigid rotor and diagonalizing the matrix of the operator... 

An important simplification of the VIB state is proposed, which allows consideration of the translational vibration of charges and reorientation of HB rigid dipoles as uncoupled processes. Then the VIB state is determined by two Lorentz lines. 

The contribution of the mechanism a to the static permittivity is 0r,s = ss — Ags. The symbol A in A s refers to the contribution provided by the vibrating rigid and nonrigid dipoles pertinent to the VIB state. This symbol A is employed, since the optical constant n is included in the permittivity component or,s The value A s will be estimated in Section E. 

---