text: For polyatomic molecules, progressions are most often observed when the change in bond lengths upon electronic excitation coincides with the change due to a ″totally symmetric″ vibration. This is the same process that occurs in resonance Raman spectroscopy. For example, in formaldehyde (methanal), HCO, the n → π* transition involves excitation of an electron from a non-bonding orbital to an antibonding pi orbital which weakens and lengthens the C-O bond. This produces a long progression in the C-O stretching vibration. Another example is furnished by benzene, CH. In both gas and liquid phase the band around 250 nm shows a progression in the symmetric ring-breathing vibration.
  As an example from inorganic chemistry the permanganate ion, , in aqueous solution has an intense purple colour due to an O → Mn ligand-to-metal charge transfer band (LMCT) in much of the visible region. This band shows a progression in the symmetric Mn-O stretching vibration.  The individual lines overlap each other extensively, giving rise to a broad overall profile with some coarse structure.
  Progressions in vibrations which are not totally symmetric may also be observed.
  d-d electronic transitions in atoms in a centrosymmetric environment are electric-dipole forbidden by the Laporte rule. This will apply to octahedral coordination compounds of the transition metals. The spectra of many of these complexes have some vibronic character. The same rule also applies to f-f transitions in centrosymmetric complexes of lanthanides and actinides. In the case of the octahedral actinide chloro-complex of uranium(IV), UCl the observed electronic spectrum is entirely vibronic. At the temperature of liquid helium, 4K, the vibronic structure was completely resolved, with zero intensity for the purely electronic transition, and three side-lines corresponding to the asymmetric U-Cl stretching vibration and two asymmetric Cl-U-Cl bending modes. Later studies on the same anion were also able to account for vibronic transitions involving low-frequency lattice vibrations.