The Role of Initial and Final States in Molecular Spectroscopies
The interpretation of molecular spectra is challenging, and understanding the relationship between experimental data obtained by different spectroscopic techniques requires a careful consideration of the initial and final states for each process. We develop a simple model for the expected energy level positions, as obtained by common spectroscopic techniques, and relate them to the energies of molecular states. Molecular charging energies in photoionization processes, as well as adsorption energies and the screening of molecular charges due to environmental polarization, are taken into account as the main causes for shifts of the measured spectroscopic features. We explain the relationship between these quantities, as well as with the transport gap, the optical gap and the exciton binding energy. Our considerations serve as a model for weakly interacting systems, e.g., various organic molecular crystals, where wave function hybridizations between adjacent molecules are negligible.