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

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.