Knowledge Bank

Accurate expectation values of the electric dipole operator for molecules with a symmetric internal rotor have been calculated using a semirigid model. Whereas the contributions from dipole components along the principal axes are completely separated for a rigid rotor, they are mixed appreciably for molecules with resolved internal rotation splittings. Combinations with different relative signs of the dipole components lead to different expectation values. This is demonstrated for acetaldehyde $(CH3CHO)$ through the effect on the Stark splittings. Several transitions including excited torsional states have been analyzed and the relative signs of the dipole components have been determined.