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Photon Echo Spectroscopy Reveals Structure-Dynamics Relationships in Carotenoids

Dr Tõnu Pullerits

Chemical Physics, Lund University, Sweden

3:30 pm Friday, 17 July 2009, EN101 (Ground Floor, EN Building), Hawthorn.

Recent developments in laser technology have enabled novel ultrafast multi‐pulse experiments which are analogous to multidimensional nuclear magnetic resonance (NMR) spectroscopy. Femtosecond coherent multidimensional laser spectroscopy is capable of resolving the structural and electronic dynamics on truly molecular timescale. By choosing different directions of incoming laser beams, pulse sequences and wavelengths one can construct a large number of qualitatively different nonlinear spectroscopy experiments providing a rich package of complementary information. Here we report results of a set of experiments corresponding to three pulse photon echo configuration carried out on carotenoids. Based on simultaneous analysis of the frequency resolved transient grating, peak shift and echo width signals we present a model for the third order optical response of carotenoids including population dynamics and system‐bath interactions. Our frequency resolved photon echo experiments clearly show that the model needs to incorporate the excited state absorption from both the S2 and the S1 states. We apply our model to analyse the experimental results on astaxanthin and lycopene, aiming to elucidate the relation between structure and system‐bath interactions. Our analysis allows us to relate structural motifs to changes in the energy‐gap correlation functions. We find that the terminal rings of astaxanthin lead to increased coupling between slow molecular motions and the electronic transition. We also find evidence for stronger coupling to higher frequency over‐damped modes in astaxanthin, pointing to the importance of the functional groups in providing coupling to fluctuations influencing the dynamics in the passage through the conical intersection governing the S2‐S1 relaxation. Very recent results about electronic 2D spectroscopy of carotenoids will be reported.

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