Beyond the absorption peak: An all-experimental analysis of resonance conditions in doubly resonant sum frequency generation via intensity and phase-resolved spectroscopy

Doubly resonant infrared-visible sum frequency generation spectra of diverse origins are analyzed using a spectral overlap function determined from in situ measurements. New and previously published datasets are compared through both homodyne (intensity) and internal heterodyne (phase-resolved) spectra across varied visible and infrared excitation conditions. We demonstrate that the conventional damping of excitation spectra for vibrational modes-a long-standing practice in the literature-is not required to reproduce the experimental observations when the spectral overlap function is obtained experimentally. Moreover, we show that the doubly resonant character cannot be inferred from the absorption spectrum alone, as resonance conditions extend well beyond the energy range implied by the absorption peak. Finally, this all-experimental method for data analysis accurately predicts the vibrational-electronic phase extracted from phase-resolved spectra, a critical parameter for robust spectral interpretation and an independent validation of the approach.