Transient Electronic Polarizability of β-Carotene from Ultrafast Terahertz Stark Spectroscopy

The prototypical polyene-like β-carotene molecule displays a substantial electric polarizability but a negligible electric dipole moment in the electronic ground state. Changes of such quantities in excited states of β-carotene have remained unknown in liquid and/or protein environments fluctuating at ambient temperature. Here, we study the polarizability and dipole changes of β-carotene in apolar n-hexane by ultrafast terahertz (THz) Stark spectroscopy. An external picosecond THz electric field induces a pronounced red-shift of electronic absorption on the optically allowed transition from the 1¹A_g ground state to the 1¹B_u excited state. We derive an increase in polarizability of Δα = 450 ± 80 ų in the 1¹B_u state, while the electric dipole moment changes by less than 2.7 D. Such values are in agreement with high-level electronic structure simulations that reveal pronounced non-Condon effects and a variation of polarizability with the electronic transition frequency. Transient structural inhomogeneities of the solvation shell have a minor impact on the electric behavior of β-carotene.