Direct comparison of molecular-beam vs liquid-phase pump-probe and two-dimensional spectroscopy on the example of...

Although azulene's anomalous fluorescence originating from S₂ rather than from S₁ is a textbook example for the violation of Kasha's rule, an understanding of the underlying processes is still a subject of investigation. Here, we use action-based coherent two-dimensional electronic spectroscopy (2DES) to measure a single Liouville-space response pathway from S₀ via S₁ to the S₂ state of azulene. We directly compare this sequential excitation in the liquid phase detecting S₂ fluorescence and in a molecular beam detecting photoionized cations, using the S₂ anomalous emission to our advantage. We complement the 2DES study with pump-probe measurements of S₁ excitation dynamics, including vibrational relaxation and passage through a conical intersection. A direct comparison of the liquid and gas phase allows us to assess the effect of the solvent and the interplay of intra- and intermolecular energy relaxation.