Unified theory of internal conversion and fluorescence under macroscopic quantum electrodynamics framework

Abstract

We develop a unified first-principles formulation of fluorescence and internal conversion (IC) within the framework of macroscopic quantum electrodynamics (QED). For molecules with negligible spin–orbit coupling, the approach accounts for both radiative and non-radiative processes in complex photonic environments. Our theory reveals that the IC, fluorescence, and two quantum electrodynamic non-adiabatic emission (QED-NAE) channels can all be regarded as non-adiabatic processes. First-principles simulations not only recover the established Chance–Prock–Silbey description of emitter–surface interactions but also quantify the contribution of vibrational overlap. We further find that cavity loss governs the competition between fluorescence and QED-NAE: reducing fluorescence in low-loss cavities results in the prominence of QED-NAE. By integrating radiative and non-radiative processes within a single macroscopic QED framework, this work provides a general foundation for photonic non-adiabatic phenomena and opens avenues for investigating electron–nucleus–photon and electron–nucleus–polariton interactions in photochemistry.