1,720,984 research outputs found
Ultrafast relaxation dynamics of multichromophoric systems through advanced optical spectroscopies
Full text linkThe light-matter interaction is one of the most interesting phenomena occurring in Nature. The interaction with light provides energy to the molecules, which is then released in the process of relaxation. This thesis deals with the investigation of relaxation mechanisms of molecules from a semi-quantum point of view. The use of quantum mechanics models is necessary to understand the operations and the results of advanced optical spectroscopies, such as 2D electronic spectroscopy, used precisely for the study of excitation and relaxation of molecules. The study and characterization of relaxation processes is of crucial importance to devise strategies to exploit and gain energy from them. Relaxation, as widely known, is strongly influenced by the surroundings and by the vibrations of the molecules. Therefore, we compared the relaxation dynamics of two systems with very different couplings with the environment. The first are aggregates of porphyrins that have weak coupling with the environment. However, it will be shown that the excitation is coupled with low frequency vibrations. The second is a quadrupolar system studied in its monomeric and dimeric form in different solvents. Surprisingly, a strong coupling between the dye transitions and a vibration of the solvent molecules was found. If confirmed, this effect could lead to new strategies for engineering light harvesting systems
Unifying Nonlinear Response and Incoherent Mixing in Action-2D Electronic Spectroscopy
Full text linkAction-detection has expanded the scope and applicability of 2D electronic spectroscopy, while posing new challenges for the unambiguous interpretation of spectral features. In this context, identifying the origin of cross-peaks at early waiting times is not trivial, and incoherent mixing is often invoked as an unwanted contribution masking the nonlinear signal. In this work, we elaborate on the relation between the nonlinear response and the incoherent mixing contribution by analyzing the action signal in terms of one- and two-particle observables. Considering a weakly interacting molecular dimer, we show how cross-peaks at early waiting times, reflecting exciton-exciton annihilation dynamics, can be equivalently interpreted as arising from incoherent mixing. This equivalence, on the one hand, highlights the information content of spectral features related to incoherent mixing and, on the other hand, provides an efficient numerical scheme to simulate the action response of weakly interacting systems
Global analysis of coherence and population dynamics in 2D electronic spectroscopy
Full text link2D electronic spectroscopy is a widely exploited tool to study excited state dynamics. A high density of information is enclosed in 2D spectra. A crucial challenge is to objectively disentangle all the features of the third order optical signal. We propose a global analysis method based on the variable projection algorithm, which is able to reproduce simultaneously coherence and population dynamics of rephasing and non-rephasing contributions. Test measures at room temperature on a standard dye are used to validate the procedure and to discuss the advantages of the proposed methodology with respect to the currently employed analysis procedures
Correlated Fluctuations and Intraband Dynamics of J-Aggregates Revealed by Combination of 2DES Schemes
Full text linkThe intraband exciton dynamics of molecular aggregates is a crucial initial step to determine the possibly coherent nature of energy transfer and its implications for the ensuing interband relaxation pathways in strongly coupled excitonic systems. In this work, we fully characterize the intraband dynamics in linear J-aggregates of porphyrins, good model systems for multichromophoric assemblies in biological antenna complexes. Using different 2D electronic spectroscopy schemes together with Raman spectroscopy and theoretical modeling, we provide a full characterization of the inner structure of the main one-exciton band of the porphyrin aggregates. We find that the redistribution of population within the band occurs with a characteristic time of 280 fs and dominates the modulation of an electronic coherence. While we do not find that the coupling to vibrations significantly affects the dynamics of excitonic coherence, our results suggest that exciton fluctuations are nevertheless highly correlated
Versatile setup for high-quality rephasing, non-rephasing, and double quantum 2D electronic spectroscopy
Full text linkDelocalized triplet state in porphyrin J-aggregates revealed by EPR spectroscopy
No full textIn this work, the electronic structure of the triplet state of self-assembled J-aggregates of tetrakis(4-sulfonatophenyl)porphyrin (TPPS) has been characterized by means of time-resolved electron paramagnetic resonance spectroscopy. Several insights into the triplet properties of the aggregate have been gained through comparison with the corresponding monomeric unit in both free base and acidified forms. Molecular distortions in the monomeric acidified TPPS cause variation in its zero-field splitting parameters and a redirection of triplet spin sublevel activity. The aggregation process does not alter the mechanism of triplet state population compared to the acidified monomer but it is accompanied by a further reduction in the zero-field splitting parameter D, which is possibly indicative of the formation of a delocalized triplet state species. The detection of a long-lived spin-polarized radical species also proves polaron generation and movement to a trap site in the J-aggregates
Mechanistic insight into internal conversion process within Q-bands of chlorophyll a
Full text linkAbstractThe non-radiative relaxation of the excitation energy from higher energy states to the lowest energy state in chlorophylls is a crucial preliminary step for the process of photosynthesis. Despite the continuous theoretical and experimental efforts to clarify the ultrafast dynamics of this process, it still represents the object of an intense investigation because the ultrafast timescale and the congestion of the involved states makes its characterization particularly challenging. Here we exploit 2D electronic spectroscopy and recently developed data analysis tools to provide more detailed insights into the mechanism of internal conversion within the Q-bands of chlorophyll a. The measurements confirmed the timescale of the overall internal conversion rate (170 fs) and captured the presence of a previously unidentified ultrafast (40 fs) intermediate step, involving vibronic levels of the lowest excited state.</jats:p
Lifetime Shortening and Fast Energy-Tansfer Processes upon Dimerization of a A-π-D-π-A Molecule
No full textTime-resolved fluorescence and transient absorption experiments uncover a distinct change in the relaxation dynamics of the homo-dimer formed by two 2,5-bis[1-(4-N-methylpyridinium)ethen-2-yl)]-N-methylpyrrole ditriflate (M) units linked by a short alkyl chain when compared to that of the monomer M. Fluorescence decay traces reveal characteristic decay times of 1.1 ns and 210 ps for M and the dimer, respectively. Transient absorption spectra in the spectral range of 425-1050 nm display similar spectral features for both systems, but strongly differ in the characteristic relaxation times gathered from a global fit of the experimental data. To rationalize the data we propose that after excitation of the dimer the energy localizes on one M branch and then decays to a dark state, peculiar only of the dimer. This dark state relaxes to the ground state within 210 ps through non-radiative relaxation. The nature of the dark state is discussed in relation to different possible photophysical processes such as excimer formation and charge transfer between the two M units. Anisotropy decay traces of the probe-beam differential transmittance of M and the dimer fall on complete different time scales as well. The anisotropy decay for M is satisfactorily ascribed to rotational diffusion in DMSO, whereas for the dimer it occurs on a faster time scale and is likely caused by energy-transfer processes between the two monomer M units
Silicon-Based Dual Linear Polarizer Exploiting Quasi-Bound States In The Continuum
No full textWe present a theoretical and experimental demonstration of a single layer silicon-based metasurface able to convert any arbitrarily polarized incoming light into linearly polarized light. Our metasurface leverages the topological features of symmetry protected quasi-bound states in the continuum and shows an experimental extinction ratio of ~40dB for two linear cross-polarization excitations, paving the way for a novel class of ultra-compact multi-frequency linear polarizers
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