All-experimental analysis of doubly resonant sum-frequency generation spectra for Franck-Condon and Herzberg-Teller vibronic modes.

The transform technique applied to the analysis of doubly resonant sum-frequency generation (DR-SFG) spectra is extended to include Herzberg-Teller (HT) vibronic modes. The experimentally measured overlap spectral function generates all the energy resonant amplitudes of the DR-SFG excitation function for both Franck-Condon (FC) and HT modes. When FC modes dominate the DR-SFG spectra, a methodology is provided to perform efficient curve fitting and orientation analysis in order to extract FC activities of the various vibration modes from experimental spectra with the help of a molecular model. Determination of the FC or HT natures of the vibration modes from DR-SFG data is also shown to be possible through their visible line shapes with an appropriate choice of polarizations. As an example, experimental DR-SFG data suggest that a known HT-active mode in the vibronic structure of Rhodamine 6G monomers exhibits a FC behavior in molecular aggregates.