Spin-orbit coupling, vibronic relaxation

The intensities of d-d bands are generally low as being forbidden by the parity and sometimes also by the multiphcity selection rules. Although forbidden, the observed intensities can be explained by relaxation of the selection rules by distortion, i.e., deviation from perfect centrosymmetry, vibronic interaction, configuration interaction or spin-orbit coupling, the details of which are beyond the scope of this article. 

We shall see in Chapter 7 that well-resolved sharp lines spectra of molecules can be obtained by incorporating them in suitable crystals. From such spectra, information about the interactions (vibronic coupling and spin-orbit coupling) and geometries (distortion) in the excited electronic state may be obtained. The results of these studies apply generally to static properties of the molecules, though the spectra also yield some information about relaxation phenomena. 

---