Simulation of Ab Initio Optical Absorption Spectrum of β-Carotene with Fully Resolved S0 and S2 Vibrational Normal Modes

The electronic absorption spectrum of beta-carotene (beta-Car) is studied using quantum chemistry and quantum dynamics simulations. Vibrational normal modes were computed in optimized geometries of the electronic ground state S-0 and the optically bright excited S-2 state using the time-dependent density functional theory. By expressing the S-2-state normal modes in terms of the ground-state modes, we find that no one-to-one correspondence between the ground- and excited-state vibrational modes exists. Using the ab initio results, we simulated the beta-Car absorption spectrum with all 282 vibrational modes in a model solvent at 300 K using the time-dependent Dirac-Frenkel variational principle and are able to qualitatively reproduce the full absorption line shape. By comparing the 282-mode model with the prominent 2-mode model, widely used to interpret carotenoid experiments, we find that the full 282-mode model better describes the high-frequency progression of carotenoid absorption spectra; hence, vibrational modes become highly mixed during the S-0 -> S-2 optical excitation. The obtained results suggest that electronic energy dissipation is mediated by numerous vibrational modes.

Automated summary

The electronic absorption spectrum of beta-carotene was studied using quantum chemistry and quantum dynamics simulations. The results showed that there is no one-to-one correspondence between the ground- and excited-state vibrational modes, and electronic energy dissipation is mediated by numerous vibrational modes.