Electronic Double-Quantum Coherences and Their Impact on Ultrafast Spectroscopy: The Example of β-Carotene

The energy level structure and dynamics of biomolecules are important for understanding their photoinduced function. In particular, the role of carotenoids in light-harvesting is heavily studied, yet not fully understood. The conventional approach to investigate these processes involves analysis of the third-order optical polarization in one spectral dimension. Here, we record two-dimensional correlation spectra for different time-orderings to characterize all components of the transient molecular polarization and the optical signal. Single- and double-quantum two-dimensional experiments provide insight into the energy level structure as well as the ultrafast dynamics of solvated β-carotene. By analysis of the lineshapes, we obtain the transition energy and characterize the potential energy surfaces of the involved states. We obtain direct experimental proof for an excited state absorption transition in the visible (S₂→S_n2). The signatures of this transition in pump-probe transients are shown to lead to strongly damped oscillations with characteristic pump and probe frequency dependence.