Ultrafast relaxation kinetics of the dark S₁ state in all-trans-β-carotene explored by one- and two-photon pump-probe spectroscopy

Femtosecond one- and two-photon pump-probe dispersive spectroscopic measurements have been applied to the investigation of the vibrational relaxation kinetics of the dark S₁ (2¹Ag-) state in β-carotene, combining a higher sensitive detection system with tunable visible and infrared excitation pulses. The two-photon excitation measurements enable the preferential detection of the dark S₁ state. The tunable infrared excitation pulses allowed selective excitation to a different vibrational level of S₁. The S₁ dynamics at early delay times depend strongly on excitation energy. A dependence of the initial S ₁ dynamics on excitation energy is discussed in term of the vibrational relaxation of S₁.