Calculation of Absolute Resonance Raman Intensities: Vibronic Theory vs Short-Time Approximation

Abstract

We present the absolute resonance Raman scattering (RRS) intensities of uracil, rhodamine 6G (R6G), and iron(II) porphyrin with imidazole and CO ligands (FePImCO) calculated using density functional theory (DFT). The spectra are calculated using both the vibronic theory and the short-time approximation. We find that the absolute RRS intensities calculated using the short-time approximation are severely overestimated, as compared with results obtained using the vibronic theory. This issue is attributed to the sensitivity of the absolute RRS intensities to the adjustable damping factor within the short-time approximation. This is illustrated for uracil, for which the relative intensities were predicted accurately using the short-time approximation, but the absolute intensities were still overestimated. Although intensities comparable to that obtained with the vibronic theory could be obtained using the short-time approximation, it requires a large damping factor, roughly twice that estimated from the absorption spectrum, to be used in the simulations. Furthermore, we find that DFT underestimates the absolute RRS intensities for R6G as compared to experiments, which is most likely due to the neglect of solvent effects in the calculations. For R6G and FePImCO, vibronic effects are shown to enhance the low-frequency modes relatively more, improving the agreement with experiments.