Molecular Selectivity of Damping-Induced Spectral Amplification

Environmental damping in molecular spectroscopy is commonly treated as a phenomenological source of decoherence and spectral broadening, leading to a monotonic reduction of spectral intensity. Here we show that this assumption is not generally valid. Using a vibronically resolved response-function framework, we demonstrate that for certain molecular vibronic structures, intermediate damping can enhance observable spectral intensity through redistribution of vibronic interference, without any increase in total oscillator strength. The peak intensity exhibits a non-monotonic dependence on damping strength, characterized by a finite optimal value. This behavior is not universal. Weakly coupled systems lack sufficient spectral overlap to support redistribution, whereas strongly coupled systems exhibit destructive interference due to excessive vibronic density, leading to monotonic suppression. These results indicate that damping can act as a structural selector of spectral accessibility rather than as a purely degradative loss mechanism and clarify the molecular conditions under which environmental coupling reshapes spectroscopic observables.