Applied molecular quantum electrodynamics: geometric aspects of the efficiency of photonic interactions

This thesis presents applications of molecular quantum electrodynamics (MQED) to the analysis of resonance energy transfer (RET), molecular absorption and emission, and light scattering by molecules. An MQED framework describes such processes as a series of microscopic photonic interaction events. Multi-interaction processes entail intermediate states of the system’s evolution remaining unspecified, requiring careful interpretation. RET, as modified by coupling with the nearest molecule of the surrounding refractive medium, is investigated. Special attention is given to a system geometry where unmodified RET is impossible, so coupling with the third chromophore is essential. Two distinct treatments are given to emission by a multi-chromophore system, distinguished by different ways of framing the quantum system: Either all photons are virtual and chromophores share excitation, or real photons interact with a single unspecified chromophore. Anomalously high fluorescence-anisotropy is explainable with the latter analysis. Off-resonant light is known to modify the absorption behaviour of molecules: This weak-interaction is analysed with an MQED formulation modified by field dressing, modelling advanced media effects in the condensed phase. Within the electric-dipole approximation, hyper-Rayleigh scattering (HRS) is considered forbidden for centrosymmetric molecules: By including higher-multipole interactions, mechanisms enabling conventionally-forbidden HRS are discovered. For each process analysed, the main results are predictions for the efficiency or observable rate. The relative positions and orientations of the molecules and fields are the key variables, so the rate equations are typically complicated functions thereof. Where rate equations depend on molecular orientation, it is often appropriate to calculate the average value over all orientations, giving results applicable to the fluid phase. System geometry may exert very fine control – a process forbidden in one case may become allowed by a minor change of one chromophore’s alignment. This thesis contributes to understanding the precise requirements of molecular geometry that must inform the design of energy-transfer systems.