Studies of the Jahn - Teller effect III. The rotational and vibrational spectra of symmetric-top molecules in electronically degenerate states

We have developed the physical theory required for interpreting the infra-red, Raman and microwave spectra of molecules in orbitally degenerate electronic states in which spin-orbit coupling is unimportant. Formulae are obtained which relate the oscillator strengths and scattering cross-sections to the electrical and mechanical properties of the molecule, and group theoretical selection rules are derived and applied to the common point groups. Among other results we find: (a)that in an electronically degenerate state a molecule may exhibit a dipole moment which would be symmetry-forbidden if the electronic state were non-degenerate, (b) that certain lines in the Raman spectrum may have depolarization ratio greater than f, because of the possibility of pseudo-vector scattering, and (c) that in both the infra-red and Raman spectra a Jahn-Teller active vibration may give rise to overtones of extremely high intensity but irregular spacing.