Excitons and Plasmons in Insulators

The relation between excitons and “plasmons” in insulators is studied, and it is shown that both excitons and “plasmons” may be obtained in the singlet longitudinal states. When the energy of interband transition is nearly equal to the free electron plasmon energy \(ωₚ\), the energy and the angular dispersion of “plasmon” loss peaks are closely related to the structure of the pair band. In this case the “plasmon” solution of the dispersion relation is expected on the higher energy side of M 3 -(or M 2 -) van Hove singularity provided the mean value of imaginary part of the dielectric constant exceeds some critical value below this energy and is small above this energy. Recent experimental data of energy loss spectra on alkali-halides are analyzed making use of the optical data and the results of band structure calculations.