Generalized Huang-Rhys factors for molecular aggregates

Stimulated Raman spectra of molecular aggregates are often remarkably dissimilar to other vibrational spectra e.g. resonance Raman and hole-burning. Particularly in photosynthetic proteins, extremely sparse and predominantly low-frequency spectra are common, unlike the rich structure typical in spectra obtained by frequency-domain techniques. Thus, a mechanism which selectively enhances the intensity of only a small subset of vibronic transitions under stimulated Raman conditions in molecular complexes seems to be required. In this work, we explore how pigment-localized vibrations couple to the excitonic states of molecular aggregates – and the consequences for spectral observables. To aid in the analysis we introduce the concept of a generalized Huang-Rhys factor for molecular aggregates, we derive an analytical expression for this quantity, and establish two conditions required for strong vibronic coupling. We illustrate the effect of such coupling by simulation of absorption and resonance Raman spectra of a two-mode model dimer.