Electronic Coherence Provides a Direct Proof for Energy-Level Crossing in Photoexcited Lutein and β-Carotene

We investigate femtosecond transient absorption dynamics of lutein and beta-carotene. Strong oscillations up to about 400 fs are observed, depending on excitation or detection wavelength and solvent. We propose electronic quantum beats as the origin of these oscillations. They provide direct proof for strong coupling of the 1B(u)+ with another electronic dark state predicted by quantum chemical calculations to be the 1B(u)- state resulting in a crossing within a dynamic relaxation model. The overall dynamics can be described well by an optical Bloch equation approach.