Multidimensional spectroscopy of β-carotene: Vibrational cooling in the excited state

Pump-degenerate four wave mixing (Pump-DFWM) is used for investigating the vibrational dynamics in the excited state of β-carotene in solution. In this 2D technique, an initial pump pulse promotes the system to the excited state, which is then probed by the succeeding DFWM sequence. We focus particularly on the internal conversion between the S₂ and S₁ state with high temporal and spectral resolution. The frequency shift of the excited state vibrations is measured and is explained as mode-specific vibrational cooling. Our results suggest an internal conversion in a time range between 260 and 500 fs without any intermediate states.