EXCITED-STATE RELAXATION OF beta-CAROTENE IN ORGANIC SOLVENT STUDIED BY FEMTOSECOND MID-INFRARED PUMP-PROBE SPECTROSCOPY

摘要

Carotenoids play an important role in photosynthetic antenna complexes, where they function as light harvesting pigments and photoprotectors. To date, the fundamental obstacle in understanding of their function is the unclear picture of the electronicenergy levels, and recently several 'new' electronic states have been proposed additionally to the well-established S_0, S_1 and S_2 states (Polivka & Sundstrom 2004). Ultrafast midinfrared (mid-IR) time resolved spectroscopy is one of the tools available to shed light on structural dynamics, by recording the time variations of the vibrational absorption spectrum. Starting from the typical mid-IR absorption of the ground state, and following the changes in the absorption of the system under investigation upon ultrafast photo-excitation, it is possible to relate these to breaking or formation, rocking, strengthening or weakening of C—H, C—CH3, C=O, C—C, C=C bonds, etc. Time resolved vibrational spectroscopy might be a way to identify and classify each electronic level by its vibrational structure and, using that as a finger print, track it during the photoexcitation and following relaxation process. Here, we present our first preliminary results from femtosecond mid-infrared spectroscopy on beta-carotene.