| 1. |
- De Vico, Luca, et al.
(författare)
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Macrocycle ring deformation as the secondary design principle for light-harvesting complexes
- 2018
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424. ; 115:39, s. 9051-9057
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Tidskriftsartikel (refereegranskat)abstract
- Natural light-harvesting is performed by pigment–protein complexes, which collect and funnel the solar energy at the start of photosynthesis. The identity and arrangement of pigments largely define the absorption spectrum of the antenna complex, which is further regulated by a palette of structural factors. Small alterations are induced by pigment–protein interactions. In light-harvesting systems 2 and 3 from Rhodoblastus acidophilus, the pigments are arranged identically, yet the former has an absorption peak at 850 nm that is blue-shifted to 820 nm in the latter. While the shift has previously been attributed to the removal of hydrogen bonds, which brings changes in the acetyl moiety of the bacteriochlorophyll, recent work has shown that other mechanisms are also present. Using computational and modeling tools on the corresponding crystal structures, we reach a different conclusion: The most critical factor for the shift is the curvature of the macrocycle ring. The bending of the planar part of the pigment is identified as the second-most important design principle for the function of pigment–protein complexes—a finding that can inspire the design of novel artificial systems.
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| 2. |
- Osipov, Vladimir Al, et al.
(författare)
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Accumulation effects in modulation spectroscopy with high-repetition-rate pulses : Recursive solution of optical Bloch equations
- 2017
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Ingår i: Physical Review A. - 2469-9926. ; 96:4
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Tidskriftsartikel (refereegranskat)abstract
- Application of the phase-modulated pulsed light for advance spectroscopic measurements is the area of growing interest. The phase modulation of the light causes modulation of the signal. Separation of the spectral components of the modulations allows to distinguish the contributions of various interaction pathways. The lasers with high repetition rate used in such experiments can lead to appearance of the accumulation effects, which become especially pronounced in systems with long-living excited states. Recently it was shown that such accumulation effects can be used to evaluate parameters of the dynamical processes in the material. In this work we demonstrate that the accumulation effects are also important in the quantum characteristics measurements provided by modulation spectroscopy. In particular, we consider a model of quantum two-level system driven by a train of phase-modulated light pulses, organized in analogy with the two-dimensional spectroscopy experiments. We evaluate the harmonics' amplitudes in the fluorescent signal and calculate corrections appearing from the accumulation effects. We show that the corrections can be significant and have to be taken into account at analysis of experimental data.
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| 3. |
- Osipov, Vladimir Al., et al.
(författare)
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Multiscale wavelet decomposition of time-resolved X-ray diffraction signals in cyclohexadiene
- 2018
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 115:41, s. 10269-10274
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate how the wavelet transform, which is a powerful tool for compression, filtering, and scaling analysis of signals, may be used to separate large- and short-scale electron density features in X-ray diffraction patterns. Wavelets can isolate the electron density associated with delocalized bonds from the much stronger background of highly localized core electrons. The wavelet-processed signals clearly reveal the bond formation and breaking in the early steps of the photoinduced pericyclic ring opening reaction of 1,3-cyclohexadiene, which are not resolved in the bare signal.
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| 4. |
- Osipov, Vladimir Al, et al.
(författare)
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Nature of relaxation processes revealed by the action signals of intensity-modulated light fields
- 2016
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Ingår i: Physical Review A (Atomic, Molecular and Optical Physics). - 1050-2947. ; 94:5
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Tidskriftsartikel (refereegranskat)abstract
- We present a generalized theory and experimental results of the action signals induced by the absorption of two photons from two phase-modulated laser beams. In our experiment, the phases of the laser beams are modulated at the frequencies φ1 and φ2, respectively. Their collinear combination leads to the modulation of the total intensity at the frequency φ=φ2-φ1. The action signals, such as photoluminescence and photocurrent, which result from the absorption of two photons, are isolated at frequencies mφ, m{0,1,2,»}. We demonstrate that the ratio of the amplitudes of the secondary (m=2) and the primary (m=1) signals A2φ:Aφ is sensitive to the type of relaxation processes in the media. Such sensitivity originates from the cumulative effects of the nonequilibrated state of the matter between the pulses. When the cumulative effects are small, i.e., the relaxation time is much shorter than the laser repetition rate or the laser intensity is high enough to dominate the system behavior, the ratio achieves the reference value of 1:4. This ratio decreases monotonically as the relaxation time increases. Our experimental results from fluorescent molecules rhodamine 6G and rubrene support these theoretical findings. In the case of a second-order relaxation process, the ratio changes rapidly with the excitation intensity. When the recombination rate in the second-order process is significantly slower than the repetition rate of the laser, we observe nonmonotonic behavior of the ratio as a function of excitation population at low excitation intensity, and when the recombination rate and the excitation intensity are high, the ratio approaches the value of 1:4. We also use the model to determine the value of the recombination rate of charge carriers in a GaP photodiode.
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| 5. |
- Riser, Roman, et al.
(författare)
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Power Spectrum of Long Eigenlevel Sequences in Quantum Chaotic Systems
- 2017
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Ingår i: Physical Review Letters. - 0031-9007. ; 118:20
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Tidskriftsartikel (refereegranskat)abstract
- We present a nonperturbative analysis of the power spectrum of energy level fluctuations in fully chaotic quantum structures. Focusing on systems with broken time-reversal symmetry, we employ a finite-N random matrix theory to derive an exact multidimensional integral representation of the power spectrum. The N→ limit of the exact solution furnishes the main result of this study - a universal, parameter-free prediction for the power spectrum expressed in terms of a fifth Painlevé transcendent. Extensive numerics lends further support to our theory which, as discussed at length, invalidates a traditional assumption that the power spectrum is merely determined by the spectral form factor of a quantum system.
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| 6. |
- Täuber, Daniela, et al.
(författare)
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Fluorescence correlation spectroscopy in thin films at reflecting substrates as a means to study nanoscale structure and dynamics at soft-matter interfaces
- 2016
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Ingår i: Physical Review E: covering statistical, nonlinear, biological, and soft matter physics. - 2470-0045. ; 94:1
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Tidskriftsartikel (refereegranskat)abstract
- Structure and dynamics at soft-matter interfaces play an important role in nature and technical applications. Optical single-molecule investigations are noninvasive and capable to reveal heterogeneities at the nanoscale. In this work we develop an autocorrelation function (ACF) approach to retrieve tracer diffusion parameters obtained from fluorescence correlation spectroscopy (FCS) experiments in thin liquid films at reflecting substrates. This approach then is used to investigate structure and dynamics in 100-nm-thick 8CB liquid crystal films on silicon wafers with five different oxide thicknesses. We find a different extension of the structural reorientation of 8CB at the solid-liquid interface for thin and for thick oxide. For the thin oxides, the perylenediimide tracer diffusion dynamics in general agrees with the hydrodynamic modeling using no-slip boundary conditions with only a small deviation close to the substrate, while a considerably stronger decrease of the interfacial tracer diffusion is found for the thick oxides.
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