Coherence as an analytic tool of 2D electronic photon-echo spectroscopy: application to vibronic coupling

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We explore computationally a new analytic tool of two-dimensional (2D) photon-echo (PE) spectroscopy - so called beating maps - in order to characterize and classify vibronic coupling in terms of these experimental observables. The analysis by means of beating maps is possible whenever the laser pulses employed in a 2D PE experiment lead to excitation and detection of coherent oscillations in a molecular system under study. The oscillations carry information on the nature of the excited states, and the beating maps enable an efficient and reliable analysis of this information. The main advantage of the beating maps as compared to the conventional 2D PE signal is their selectivity (each map selects one particular oscillation) and sensitivity even to very weak optical transitions.As the result, broadening and peak overlaps present in conventional 2D PE signals are significantly reduced, and more precise and detailed information can be obtained.In this project, we employ the beating maps to probe the origin of vibronic coupling. Vibronic coupling, i.e. the coupling between electronic and nuclear degrees of freedom, may have quite distinct physical nature and manifestations. The most well known case is related to crossings of potential-energy surfaces and breakdown of Born-Oppenheimer approximation. The non-separability of electron and nuclear motion may lead to changes in electron and nuclear configurations at the same timescale and is often responsible for reaction dynamics and such fundamental processes as electron transfer and photoprotection. There is a number of well established models describing the state crossings. Here we will address the model of avoided crossing as well as the vibronic-Hamiltonian model commonly employed to describe dynamics at conical intersections. Further, we will also consider the simplest model of vibronic coupling in oligomers: a vibronic dimer. Vibronic effects in oligomers play an important role in the exciton transport efficiency. In these systems, vibrational motion may be coupled to delocalized electronic excitations.The main idea of the project is to express the properties of the considered model systems in terms of dipole transition strengths between the system eigenstates pertaining to optically coupled electronic states. These dipoles contribute directly to the formation of the beating maps. Therefore, we expect to observe specific patterns typical for each model of the vibronic coupling considered.The project provides a basis for classification of vibronic interactions by means of the beating maps. All the models considered can be further extended and parametrized for specific applications. Currently, only very few experimental groups have realized the potential of the beating-maps analysis.We envision that the research will encourage further experimental realizations.