9. Spectra of polyatomic molecules

The spectra of molecules made up of more than two atoms are, in general, more complicated than those of diatomic molecules, because their rotational and vibrational motions, as well as their electronic states, are more numerous. To interpret polyatomic spectra, it is important to consider the shape of the molecule, i.e. the internal arrangement of the atoms, as certain characteristics are associated with certain shapes. The study of symmetry properties is important for understanding polyatomic molecules. Symmetry is described in group theory in terms of point symmetry groups.