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QUINTELA RODRIGUEZ, FRANK ERNESTO, (2023)  - Approccio perturbativo alla simulazione della spettroscopia coerente multidimensionale: effetti dell'interazione tra gradi di libertą elettronici e vibrazionali.  - , Tesi di dottorato - (, , Universitą degli studi di Modena e Reggio Emilia ) - pagg. -

Abstract: Over the past few decades, multidimensional coherent spectroscopy has been widely used to improve our understanding of ultrafast processes in the femtosecond regime. By distributing the complicated optical response of a quantum system into a multi-dimensional spectrum, it provides insight into complicated processes, resulting from the interplay of coherent and incoherent dynamics. The multi-dimensional spectrum of a typical sample system is usually congested and difficult to analyze, as a result of the integration of many physical phenomena. Effective models are thus employed to describe the system in terms of few electronic levels coupled to a set of vibrational modes. The dominant contribution in the electronic-vibrational coupling is often represented in terms of the displaced- oscillator model, where one or more harmonic oscillators undergo an electronic-state specific displacement in the nuclear coordinates. Within such adiabatic approach, the vibrational wave packets modulate the electronic spectra by evolving on a given potential energy surface. This model accounts for many complex spectral features, which show up as the number of electronic levels and of exciting laser pulses increase. The present thesis work includes both theoretical developments and simulations of observed spectra, carried out within a collaboration with experimental partners. In the theoretical work, we analytically calculate the vibrational component of the M-th order response functions, for a system with N electronic levels, with arbitrary M and N (so far, analytical solutions were only known for N=2 and M≤3). This is done by exploiting the fact that the time evolution of the vibrational state preserves the coherent-state character of the vibrational wave packet. This allows to compute the response functions within a coherent state representation, for a generic initialization of the vibrational state (coherent and thermal state). Besides, we include the effects of vibrational relaxation through an effective non-hermitian Hamiltonian [1]. In the joint experimental-theoretical work, we numerically simulate the observed 2D spectra, with the inclusion of non-adiabatic terms. These have been shown to underlie several important photo-chemical reactions, where the specific vibronic mechanism can dramatically affect the overall system dynamics. In particular, we study the internal conversion dynamics of a functionalised porphyrin monomer. Porphyrins are tetrapyrrole macrocycles that are ubiquitous pigments in nature, performing different functions in living systems. Our simulations, along with ab initio (DFT) calculations performed by our collaborators, allow us to introduce and validate an effective model that, within a density-matrix approach, reproduces the main observed features and unveils the fundamental mechanisms at the heart of these processes. More specifically, the simulations allow us to unveil the role played in the internal conversion process by specific vibrational modes, to disentangle the intricate interplay between adiabatic and non-adiabatic couplings on the one hand, and the coherent and incoherent excitation evolution on the other [2]. [1] FEQ Rodriguez, F Troiani, "Vibrational response functions for multidimensional electronic spectroscopy in the adiabatic regime: A coherent-state approach”, J. Chem. Phys. 157, 034107 (2022) [2] Vasilis Petropoulos, Pavel Rukin, Frank Quintela, Mattia Russo, Luca Moretti, Ana Moore, Thomas Moore, Devens Gust, Deborah Prezzi, Gregory Scholes, Elisa Molinari, Giulio Cerullo, Filippo Troiani, Carlo Rozzi, Margherita Maiuri "Real-time evolution of vibrational wavepackets in a free base porphyrin”, (Manuscript in preparation)