Two-Dimensional Electronic Spectroscopy Reveals Excited State Displacements

Nuclear-electronic couplings, dictated by the shape of multidimensional potential energy surfaces (PESs), drive photochemical and photophysical processes. Information on the absolute displacements between PESs are encoded in vibrational coherences, as their amplitude scales to the Franck-Condon integrals. We demonstrate here how insights into the potential energy landscape of excited states are accessed by “half-broadband” two-dimensional electronic spectroscopy (2DES). We performed 2DES on the oxazine dye cresyl violet (CV) in ethanol. Impulsive excitation centred at 16400 cm-1 prepares multiple ground and excited state nuclear wavepackets, which are probed by a delayed, recompressed, self-heterodyned white-light continuum. The 2D beatmaps of the 585 cm-1 and 350 cm-1 Raman active modes show strikingly different amplitude distributions. Whilst 585 cm-1 coherences modulate the whole 2D spectrum, the 350 cm-1 beats are localised on the excited state absorption region (i.e. for ω_3⁄(2πc ≥) 18500 cm-. Such amplitude distributions are consistent with S1←S0 being strongly displaced along the 585 cm-1 coordinate, but not along the 350 cm-1 coordinate, in agreement with literature on CV. Conversely, Sn←S1 is displaced along both vibrational coordinates. Our analysis includes spectral filtering considerations and is validated against simulations and TD-DFT calculations.