Vibronic coupling explains the ultrafast carotenoid-to-bacteriochlorophyll energy transfer in natural and artificial light harvesters

The initial energy transfer in photosynthesis occurs between the light-harvesting pigments and on ultrafast timescales. We analyze the carotenoid to bacteriochlorophyll energy transfer in LH2 Marichromatium purpuratum as well as in an artificial light-harvesting dyad system by using transient grating and two-dimensional electronic spectroscopy with 10 fs time resolution. We find that F\"orster-type models reproduce the experimentally observed 60 fs transfer times, but overestimate coupling constants, which leads to a disagreement with both linear absorption and electronic 2D-spectra. We show that a vibronic model, which treats carotenoid vibrations on both electronic ground and excited state as part of the system's Hamiltonian, reproduces all measured quantities. Importantly, the vibronic model presented here can explain the fast energy transfer rates with only moderate coupling constants, which are in agreement with structure based calculations. Counterintuitively, the vibrational levels on the carotenoid electronic ground state play a central role in the excited state population transfer to bacteriochlorophyll as the resonance between the donor-acceptor energy gap and vibrational ground state energies is the physical basis of the ultrafast energy transfer rates in these systems

Theoretische Untersuchungen zur Spektroskopie von Molekülaggregaten

Die spektroskopischen Eigenschaften von Molekülaggregaten wurden mittels quantendynamischer Berechnungen untersucht. Hierbei wurden sowohl lineare als auch nichtlineare Spektroskopietechniken einbezogen. Zur Simulation von Absorptions- und CD-Spektroskopie wurden Kopplungseffekte sowie die relative Orientierung der Monomer-Einheiten in den Modellen berücksichtigt, um gemessene Spektren reproduzieren und so die entsprechenden Parameter zu bestimmen. Zur genaueren Beschreibung wurden auch Ergebnisse quantenchemischer Rechnungen verwendet. Darüber hinaus wurden Untersuchungen zur nichtlinearen optischen Spektroskopie an Dimeren durchgeführt.The spectroscopic properties of molecular aggregates have been investigated by means of quantum dynamical calculations. Thereby both linear and nonlinear spectroscopic techniques have been taken into account. For the simulation of absorption and CD-spectra, coupling effects were regarded as well as the relative orientation of the monomer units in order to determine the parameters by reproducing measured spectra. For a more detailled description, results from quantum chemical calculations have also been included. Furthermore, investigations on nonlinear spectroscopy of molecular dimers have been performed

Beating Signals in 2D Spectroscopy: Electronic or Nuclear Coherences? Application to a Quantum Dot Model System

In 2D electronic spectroscopy, oscillatory signals have recently received increased attention. The key issue is the origin of such beating-whether it is vibrational or electronic. In analogy to the distinction between rephasing and nonrephasing contributions to 2D spectra, we separate signal components with positive and negative coherence beating frequency via a third Fourier transformation and consider two-dimensional cuts at the respective positions. We apply this approach to a model system for the description of quantum dots (QDs) and analyze the possibility to distinguish between vibrational effects associated with the longitudinal optical (LO) phonon mode and electronic fine structure splitting

Combined treatment of relaxation and fluctuation dynamics in the calculation of two-dimensional electronic spectra.

While the theoretical description of population transfer subsequent to electronic excitation in combination with a line shape function description of vibrational dynamics in the context of 2D-spectroscopy is well-developed under the assumption of different timescales of population transfer and fluctuation dynamics, the treatment of the interplay between both kinds of processes lacks a comprehensive description. To bridge this gap, we use the cumulant expansion approach to derive response functions, which account for fluctuation dynamics and population transfer simultaneously. We compare 2D-spectra of a model system under different assumptions about correlations between fluctuations and point out under which conditions a simplified treatment is justified. Our study shows that population transfer and dissipative fluctuation dynamics cannot be described independent of each other in general. Advantages and limitations of the proposed calculation method and its compatibility with the modified Redfield description are discussed

3D spectroscopy of vibrational coherences in quantum dots:Theory

In semiconductor nanocrystals, called quantum dots (QD), electronic transition energies, phonon frequencies, and electron-phonon coupling strengths are all reported to depend on the size of the crystals. The size dependencies of the transition energies and the mode frequencies are well characterized and understood. At the same time, the electron-phonon coupling dependence on size is controversial-even the sign of the change is not settled. In this article, third-order response functions of a model QD resembling CdSe are calculated. The longitudinal optical (LO) mode is included as a relatively narrow Lorentzian contribution to the spectral density. A novel version of electronic 2D spectroscopy is investigated where a third Fourier transform is taken over a so-called population time, leading to 3D spectral representation. The amplitude and phase of the 2D cuts of the 3D spectral body around the LO mode frequency are analyzed. The analytical power and sensitivity of the cuts in determining the possible Huang-Rhys factor (electron-phonon coupling strength) and the LO mode frequency dependence on the QD size are investigated. Peak structures in the cuts with a tilt relative to the diagonal are identified as sensitive signatures for the size dependencies. The study elucidates the 3D representation of the electronic 2D spectroscopy as a powerful tool for obtaining insight into otherwise hardly accessible characteristics of the system

Messung der kardiorespiratorischen Fitness mit unterschiedlichen Kurzbelastungstests

Problem- und Zielstellung: Eingeschränkte kardiorespiratorische Fitness (CRF) gilt als Risikofaktor für die Entwicklung von Herz-Kreislauf-Erkrankungen. Häufig erfolgt die CRF-Ermittlung mit dem submaximalen Belastungs-EKG (Fahrradergometrie). Es mangelt an Testverfahren, die ortsunabhängig und ökonomisch als Screeninginstrument zur Früherkennung eingeschränkter CRF einsetzbar sind. Deshalb wurde untersucht, ob auch kurze Belastungstests (nachfolgend Kurztests) eine zuverlässige Aussage zur CRF ermöglichen und eine Alternative zum Belastung-EKG sein können. - Methodik: Die Untersuchungen erfolgten im Rahmen der arbeitsmedizinischen Vorsorge. Diese bestand aus einem Fragebogen (u. a. anamnestische, soziodemografische, lebensstilbezogene Angaben), körperlichen Untersuchungen (u. a. Body Mass Index, Herzfrequenz, Blutdruck) und vier Belastungstests (submaximales Belastungs-EKG, Kurztests: Kurzergometrie, Stepp-Test, Kniebeugetest). In die Analysestichprobe gingen 66 Erwerbstätige (30 Frauen, 36 Männer; Durchschnittsalter: 48 Jahre) ein. Die CRF wurde nach der Formel von Wicks et al. (2011) ermittelt. - Ergebnisse: Die CRF war in den Kurztests signifikant geringer als im Belastungs-EKG (η²partial = 0,23). Mit den Kurztests wurde der submaximale Belastungsbereich nicht erreicht. Die CRF korrelierte zwischen Belastungs-EKG und Kurztests sehr gering bis gering (r = 0,16–0,23). Zwischen den Kurztests bestanden mittlere Korrelationen (r = 0,54–0,62). Geschlecht, Alter und Sportstunden/Woche beeinflussen die CRF. - Schlussfolgerung: Die Kurztests können das Belastungs-EKG nicht ersetzen. Bei ihnen wird nicht die eigentliche CRF ermittelt, sondern eine neu definierte Kurzbelastungsfitness. Ob die Kurzbelastungsfitness Aussagen über das kardiorespiratorische Erkrankungsrisiko ermöglicht, ist weiter zu untersuchen.Background and objectives: Reduced cardiorespiratory fitness (CRF) is considered a risk factor for developing cardiovascular diseases. Very often CRF is determined with submaximal exercise ECG (bicycle ergometry). There is a lack of test procedures that can be used location-independently and economically as a screening instrument for the early detection of restricted CRF. Therefore, we investigated whether short exercise tests (hereinafter referred as short tests) also enable a reliable statement about CRF and whether they can be an alternative to exercise ECG. - Methods: The examinations were carried out as part of preventive occupational health care. They consisted of a questionnaire (including anamnestic, sociodemographic, lifestyle-related information), physical examination (including body mass index, heart rate, blood pressure) and four exercise tests (submaximal exercise ECG, short tests: short ergometry, step test, squat test). In all, 66 employees (30 women, 36 men; average age: 48 years) were included. The CRF was calculated according to the formula of Wicks et al. (2011). - Results: CRF was significantly lower in the short tests than in the exercise ECG (η²partial = 0.23); the submaximal exposure range was not reached with the short tests. The CRF correlation between exercise ECG and short tests (r = 0.16–0.23) was very low to low. Between the short tests, there were moderate correlations (r = 0.54–0.62). Gender, age and exercise hours per week all influence CRF. - Conclusions: The short tests cannot replace the exercise ECG. The short tests do not determine the actual CRF, but a newly defined short-term fitness. Whether the short-term fitness allows statements regarding risk for cardiorespiratory diseases remains to be investigated