Investigation of molecular dynamics in β-carotene using femtosecond pump-FWM spectroscopy

We have carried out two different pump four-wave mixing experiments, combining an initial pump excitation and a subsequent four-wave mixing probe process, on the photosynthetic pigment β-carotene to reveal different aspects of its molecular dynamics after photoexcitation. Firstly, the pump degenerate four-wave mixing (pump -DFWM) technique, in which the DFWM is resonant with the S₁- to S _n -transition of β-carotene, is used to monitor the events following excitation of the system. The transient shows a peculiar shape and is seen to depend on the energy of the initial pump pulse as well as on the concentration of the solute in the solvent. Secondly, pump coherent anti-Stokes Raman scattering (pump-CARS) is used to elucidate the excited state vibrational dynamics of β-carotene. This technique gives access to the dynamics of both ground and excited electronic states with vibrational selectivity.