COLLECTIVE EXCITATIONS IN TIGHTLY BOUND INSULATORS

The longitudinal collective excitations of the electrons in a tightly bound insulator are studied using the dielectric matrix technique. It is found that the spectrum of excited electron-hole states changes radically as the electrons are increasingly tightly bound. A Frenkel exciton and a succession of charge transfer states are found, with the possibility of a plasmon. It is predicted that in electron energy-loss measurements, in practice, only the Frenkel exciton will be seen, and this seems to agree with present experimental evidence on molecular crystals.