Abstract

Carotenoids are among the most investigated molecules in organic physics and chemistry [1]. They perform several vital functions in photosynthetic systems: (i) light harvesting, by absorbing energy in the blue-green region and transferring it to the chlorophylls with very high efficiency; (ii) photoprotection via quenching of the chlorophyll triplet states; (iii) singlet oxygen scavenging. In carotenoids both the ground state (S₀) and the first excited state (S₁) possess A_g character (according to the approximate C_2h symmetry), while the second excited state (S₂) possesses B_u symmetry. Therefore only the S₀→S₂ transition is optically allowed; however, after photoexcitation, an ultrafast internal conversion (IC) to S₁ takes place. The dynamics of this process is important for the understanding of photosynthesis, because energy transfer from the carotenoids to the chlorophylls can take place either from S₂ or from S₁. The recent availability of sub-10-fs laser pulses in the visible has allowed to directly study IC in carotenoids with unprecedented time resolution [2].

© 2002 Optical Society of America