β‐Carotene Revisited by Transient Absorption and Stimulated Raman Spectroscopy

Abstract

β-Carotene in n-hexane was examined by femtosecond transient absorption and stimulated Raman spectroscopy. Electronic change is separated from vibrational relaxation with the help of band integrals. Overlaid on the decay of S1 excited-state absorption, a picosecond process is found that is absent when the C9 -methyl group is replaced by ethyl or isopropyl. It is attributed to reorganization on the S1 potential energy surface, involving dihedral angles between C6 and C9 . In Raman studies, electronic states S2 or S1 were selected through resonance conditions. We observe a broad vibrational band at 1770 cm(-1) in S2 already. With 200 fs it decays and transforms into the well-known S1 Raman line for an asymmetric C=C stretching mode. Low-frequency activity (<800 cm(-1) ) in S2 and S1 is also seen. A dependence of solvent lines on solute dynamics implies intermolecular coupling between β-carotene and nearby n-hexane molecules.

Femtosecond Stimulated Raman Spectroscopy

“…Since the first report by Yoshizawa et al . in 2002, an enormous amount of work has contributed to our understanding of carotene's unique excited‐state dynamics . Recent reviews on carotenes and biopolyenes have been given by Hashimoto et al .…”

Ultrafast Intramolecular Proton Transfer of Alizarin Investigated by Femtosecond Stimulated Raman Spectroscopy

“…Transient stimulated Raman spectra of the sample are obtained by scanning the time delay between the stimulated Raman probe pair and the actinic pump pulses, where all the kinetic and structural information on the sample in the excited states is contained. The FSRS has been widely used to investigate many chemical and biological systems including reaction dynamics, vision process, and photosynthesis. ,− …”

A Unified Picture of S* in Carotenoids

“…This was later drawn into question, based on experiments showing different temperature dependence of S* and S 1 , which is unexpected for photoinitiated excited state isomerization . The finding that S* can be “frozen out” is central to the so-called inhomogeneous ground state model, in which S 1 and S* are both first electronic excited singlet states, but of different molecular species, represented by isomers with altered end group rotation. − An alternative interpretation of pump–probe − and pump-degenerate four wave mixing data , explains S*-related features by a vibrationally hot ground state, hot S 0 . Within this hypothesis, it is important to differentiate two distinct mechanisms of addressing hot S 0 .…”