Two-Dimensional Electronic Spectroscopy Resolves Relative Excited State Displacements

Abstract

Knowledge of relative displacements between potential energy surfaces (PES) is critical in spectroscopy and photochemistry. Information on displacements is encoded in vibrational coherences. Here we apply ultrafast two-dimensional electronic spectroscopy in a pump-probe half-broadband (HB2DES) geometry to probe the ground and excited state potential landscapes of cresyl violet. 2D coherence maps reveal that while the coherence amplitude of the dominant 585 cm-1 Raman active mode is mainly localised in ground state bleach and stimulated emission regions, a 338 cm-1 mode is enhanced in excited state absorption. Modelling these data with a three-level displaced harmonic oscillator model using the hierarchical equation of motion-phase matching approach (HEOM-PMA) shows that the S1←S0 PES displacement is greater along the 585 cm-1 than the 338 cm-1 coordinate, while Sn←S1 displacements are similar along both coordinates. HB2DES is thus a powerful tool for exploiting nuclear wavepackets to extract quantitative multidimensional, vibrational coordinate information across multiple PESs.