Impurities and excitons in solids

The Rydberg series of another member of the sequence, C2H5I, have also been studied [100]. They are more diffuse due the opening of new ionisation and dissociation channels and, as a result, cannot be followed to such high members. 

There are many other examples of long Rydberg series in still heavier molecules for instance, very high Rydberg states of CeH6 have been reported [101]. 

The previous three sections have demonstrated that Rydberg series do not merely arise for atoms. We now take the matter further, and explore 

At first sight, it might seem, from the example outlined briefly in section 2.14, that Rydberg excitations would arise mainly from impurity atoms in a solid, and would only contain a few low members, small enough to fit within the interstices of the lattice. Indeed, it might seem impossible to associate any other form of Rydberg excitation with condensed matter. 

There are, however, Rydberg excitations in solids closely related to 

---