A QUANTUM ELECTRODYNAMIC STUDY OF INTERMOLECULAR LINE BROADENING AND LINE SHIFT

In this paper quantum electrodynamics is used to investigate the effects of radiation field-induced energy transfer as a mechanism for the manifestation of frequency shift and both homogeneous and inhomogeneous line broadening. Using resolvent operator methods and the Power-Zienau-Woolley multipolar Hamiltonian to derive an effective Hamiltonian, it is illustrated how all coherent and incoherent field-induced line shift and line broadening including Förster energy transfer, dispersion, and cooperative Rayleigh scattering can be described solely in terms of a single multipolar interaction. The detailed analysis reveals that a description of intermolecular interactions in terms of virtual photon coupling leads a detailed range dependence that is not only different to nonretarded semiclassical theory, but also complex. This paper therefore illustrates for the first time that all intermolecular processes contribute to both line broadening and energy shift, and that the imaginary contributions to virtual photon coupling lead to significant retardation effects in condensed media. Finally by applying the results within a density matrix framework important aspects regarding the statistical averaging of these additional terms in the Hamiltonian are discussed, and it is illustrated how intermolecular coupling leads to non-Lorentzian, redshifted, asymmetric line profiles. © 1990 American Institute of Physics.