Quantum Coherence as a Witness of Vibronically Hot Energy Transfer in Bacterial Reaction Centre

Photosynthetic proteins have evolved over billions of years so as to undergo optimal energy transfer to the sites of charge separation. Based on spectroscopically detected quantum coherences, it has been suggested that this energy transfer is partially wavelike. This conclusion critically depends on assignment of the coherences to the evolution of excitonic superpositions. Here we demonstrate for a bacterial reaction centre protein that long-lived coherent spectroscopic oscillations, which bear canonical signatures of excitonic superpositions, are essentially vibrational excited state coherences shifted to the ground state of the chromophores . We show that appearance of these coherences is brought about by release of electronic energy during the energy transfer. Our results establish how energy migrates on vibrationally hot chromophores in the reaction centre and they call for a re-examination of claims of quantum energy transfer in photosynthesis

Distinctive character of electronic and vibrational coherences in disordered molecular aggregates

Coherent dynamics of coupled molecules are effectively characterized by the two-dimensional (2D) electronic coherent spectroscopy. Depending on the coupling between electronic and vibrational states, oscillating signals of purely electronic, purely vibrational or mixed origin can be observed. Even in the "mixed" molecular systems two types of coherent beats having either electronic or vibrational character can be distinguished by analyzing oscillation Fourier maps, constructed from time-resolved 2D spectra. The amplitude of the beatings with the electronic character is heavily affected by the energetic disorder and consequently electronic coherences are quickly dephased. Beatings with the vibrational character depend weakly on the disorder, assuring their long-time survival. We show that detailed modeling of 2D spectroscopy signals of molecular aggregates providesdirect information on the origin of the coherent beatings.Comment: 7 pages, 4 figures, 1 tabl

Compressive imaging of transient absorption dynamics on the femtosecond timescale

Femtosecond spectroscopy is an important tool for tracking rapid photoinduced processes in a variety of materials. To spatially map the processes in a sample would substantially expand the capabilities of the method. This is, however, difficult to achieve due to the necessity to use low-noise detection and to maintain feasible data acquisition time. Here we demonstrate realization of an imaging pump-probe setup, featuring sub-100 fs temporal resolution, by a straightforward modification of a standard pump-probe technique, using a randomly structured probe beam. The structured beam, made by a diffuser, enabled us to computationally reconstruct the maps of transient absorption dynamics based on the concept of compressed sensing. We demonstrate the functionality of the setup in two proof-of-principle experiments, were we achieve spatial resolution of 20 \mu m. The presented concept provides a feasible route to imaging, using the pump-probe technique and ultrafast spectroscopy in general.Comment: 13 pages, 6 figure

Freestanding sample holder for ultrafast optical spectroscopy at low temperatures.

Ultrafast optical spectroscopy techniques are often employed to gain information about samples that are liquid at room temperature and frozen at cryogenic temperatures. However, the measurements suffer from the presence of unwanted, non-resonant signals originating in the sample cell walls. Most of these artifacts can be avoided in the measurements performed at room temperature by using liquid jet systems, i.e., by removing the sample cell. However, these systems cannot be used in low temperature measurements, when the sample is frozen. Herein we describe a freestanding sample holder that allows low temperature ultrafast spectroscopy measurements free of artifacts caused by the sample cell

In situ mapping of the energy flow through the entire photosynthetic apparatus

Absorption of sunlight is the first step in photosynthesis, which provides energy for the vast majority of organisms on Earth. The primary processes of photosynthesis have been studied extensively in isolated light-harvesting complexes and reaction centres, however, to understand fully the way in which organisms capture light it is crucial to also reveal the functional relationships between the individual complexes. Here we report the use of two-dimensional electronic spectroscopy to track directly the excitation-energy flow through the entire photosynthetic system of green sulfur bacteria. We unravel the functional organization of individual complexes in the photosynthetic unit and show that, whereas energy is transferred within subunits on a timescale of subpicoseconds to a few picoseconds, across the complexes the energy flows at a timescale of tens of picoseconds. Thus, we demonstrate that the bottleneck of energy transfer is between the constituents

Transfer of Vibrational Coherence Through Incoherent Energy Transfer Process in F\"{o}rster Limi

We study transfer of coherent nuclear oscillations between an excitation energy donor and an acceptor in a simple dimeric electronic system coupled to an unstructured thermodynamic bath and some pronounced vibrational intramolecular mode. Our focus is on the non-linear optical response of such a system, i.e. we study both excited state energy transfer and the compensation of the so-called ground state bleach signal. The response function formalism enables us to investigate a heterodimer with monomers coupled strongly to the bath and by a weak resonance coupling to each other (F\"{o}rster rate limit). Our work is motivated by recent observation of various vibrational signatures in 2D coherent spectra of energy transferring systems including large structures with a fast energy diffusion. We find that the vibrational coherence can be transferred from donor to acceptor molecules provided the transfer rate is sufficiently fast. The ground state bleach signal of the acceptor molecules does not show any oscillatory signatures, and oscillations in ground state bleaching signal of the donor prevail with the amplitude which is not decreasing with the relaxation rate.Comment: 11 pages, 9 figure

Unraveling the nature of coherent beatings in chlorosomes.

Coherent two-dimensional (2D) spectroscopy at 80 K was used to study chlorosomes isolated from green sulfur bacterium Chlorobaculum tepidum. Two distinct processes in the evolution of the 2D spectrum are observed. The first being exciton diffusion, seen in the change of the spectral shape occurring on a 100-fs timescale, and the second being vibrational coherences, realized through coherent beatings with frequencies of 91 and 145 cm(-1) that are dephased during the first 1.2 ps. The distribution of the oscillation amplitude in the 2D spectra is independent of the evolution of the 2D spectral shape. This implies that the diffusion energy transfer process does not transfer coherences within the chlorosome. Remarkably, the oscillatory pattern observed in the negative regions of the 2D spectrum (dominated by the excited state absorption) is a mirror image of the oscillations found in the positive part (originating from the stimulated emission and ground state bleach). This observation is surprising since it is expected that coherences in the electronic ground and excited states are generated with the same probability and the latter dephase faster in the presence of fast diffusion. Moreover, the relative amplitude of coherent beatings is rather high compared to non-oscillatory signal despite the reported low values of the Huang-Rhys factors. The origin of these effects is discussed in terms of the vibronic and Herzberg-Teller couplings

High-order harmonic generation using a high-repetition-rate turnkey laser

We generate high-order harmonics at high pulse repetition rates using a turnkey laser. High-order harmonics at 400 kHz are observed when argon is used as target gas. In neon we achieve generation of photons with energies exceeding 90 eV ($\sim$13 nm) at 20 kHz. We measure a photon flux of 4.4$\cdot10^{10}$ photons per second per harmonic in argon at 100 kHz. Many experiments employing high-order harmonics would benefit from higher repetition rates, and the user-friendly operation opens up for applications of coherent extreme ultra-violet pulses in new research areas

Revealing vibronic coupling in chlorophyll c1 by polarization-controlled 2D electronic spectroscopy

Vibronic coupling between molecules has been recently discussed to play an important role in photosynthetic functions. Furthermore, this type of coupling between electronic states has been suggested to define photophysical properties of chlorophylls, a family of photosynthetic molecules. However, experimental investigation of vibronic coupling presents a major challenge. One subtle way to study vibronic coupling is by excitation and observation of superpositions of vibrational states via transitions to vibronically mixed states. Such superpositions, called coherences, are then observed as quantum beats in non-linear spectroscopy experiments. Here we present polarization-controlled two-dimensional electronic spectroscopy study of the chlorophyll c1 molecule at cryogenic (77 K) temperature. By applying complex analysis to the oscillatory signals we are able to unravel vibronic coupling in this molecule. The vibronic mixing picture that we see is much more complex than was thought before