Electric dipole interactions in molecular crystals

Abstract

The importance of dipolar interactions is highlighted in a wide range of molecular crystal properties. Molecules respond via an effective polarizability (appropriate to the crystal phase) to a local field arising from the applied field and the fields of the surrounding induced dipoles, determined self-consistently. An exact treatment is possible for perfect crystals and crystals with localized imperfections, yielding the inverse dielectric function characterizing the crystal response. From this, results follow for the crystal linear and nonlinear dielectric response, for Coulomb exciton energies, and for the intensities of lattice vibrational spectra. A direct extension yields the polarization energy associated with the induced dipoles. This provides insight into the energetics of excess charge carriers in perfect and imperfect crystals and of charge-transfer excitons, into electronic Stark spectra, and into aspects of lattice dynamics and phase transitions.