Coherent Multidimensional Spectroscopy as a Probe of System-Bath Interactions

Sammanfattning: The interaction of molecules with their environment has a profound effect on dynamics in molecular systems. System-bath interactions, or the interaction of electronic and nuclear degrees of freedom, are important in order to understand chemical reactions and transport of energy and electrons in the condensed phase. In order to investigate system-bath interactions one needs to probe the underlying nuclear motion in real time. Ultrafast optical spectroscopy has this capability, and is a useful tool for such investigations. The work presented in this thesis utilizes coherent multidimensional spectroscopy to probe system-bath interactions in the time domain. We implement the three-pulse photon echo technique with various means of detecting the radiated signal field. One particular version of this experiment, the peak shift, directly gives the timescales of the system-bath interaction. To aid interpretation of the experimental results, analysis of the experiments is coupled to numerical simulations. In the first part of this thesis we explore how to obtain quantitative information about system-bath interactions. In this section we show that the chirp of the pulses needs to be considered in order to obtain quantitative information from experiments. We also show that the width of the three-pulse photon echo signal gives a direct and simple measure of the strength of the system-bath interaction. The second part of the thesis deals with coherent multidimensional spectroscopy of carotenoids. Here we show the presence of an excited state at roughly twice the S2 energy in many carotenoids. Another debated state in carotenoids is the so-called S' state. Our analysis of the experiments show that this state is an excited state, resolving a long-standing discussion about the position of this state. As the first to carry out coherent multidimensional spectroscopy on carotenoids, we have been able to address the system-bath interaction in these systems. Our results show that the structure of the carotenoid has a clear relation to the system-bath interaction, influencing it on multiple timescales.