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Inspired by the period-four oscillation in flash-induced oxygen evolution of photosystem II discovered by Joliot in 1969, Kok performed additional experiments and proposed a five-state kinetic model for photosynthetic oxygen evolution, known as Kok’s S-state clock or cycle 1 , 2 . The model comprises four (meta)stable intermediates (S 0 , S 1 , S 2 and S 3 ) and one transient S 4 state, which precedes dioxygen formation occurring in a concerted reaction from two water-derived oxygens bound at an oxo-bridged tetra manganese calcium (Mn 4 CaO 5 ) cluster in the oxygen-evolving complex 3 – 7 . This reaction is coupled to the two-step reduction and protonation of the mobile plastoquinone Q B at the acceptor side of PSII. Here, using serial femtosecond X-ray crystallography and simultaneous X-ray emission spectroscopy with multi-flash visible laser excitation at room temperature, we visualize all (meta)stable states of Kok’s cycle as high-resolution structures (2.04–2.08 Å). In addition, we report structures of two transient states at 150 and 400 µs, revealing notable structural changes including the binding of one additional ‘water’, Ox, during the S 2 →S 3 state transition. Our results suggest that one water ligand to calcium (W3) is directly involved in substrate delivery. The binding of the additional oxygen Ox in the S 3 state between Ca and Mn1 supports O–O bond formation mechanisms involving O5 as one substrate, where Ox is either the other substrate oxygen or is perfectly positioned to refill the O5 position during O 2 release. Thus, our results exclude peroxo-bond formation in the S 3 state, and the nucleophilic attack of W3 onto W2 is unlikely. Crystallography and spectroscopy are used to solve high-resolution structures of the intermediates of Kok’s S-state clock in photosystem II.

The coherence and dephasing of vibrational motions of molecules constitute an integral part of chemical dynamics, influence material properties and underpin schemes to control chemical reactions. Considerable progress has been made in understanding vibrational coherence through spectroscopic measurements, but precise, direct measurement of the structure of a vibrating excited-state polyatomic organic molecule has remained unworkable. Here, we measure the time-evolving molecular structure of optically excited N -methylmorpholine through scattering with ultrashort X-ray pulses. The scattering signals are corrected for the differences in electron density in the excited electronic state of the molecule in comparison to the ground state. The experiment maps the evolution of the molecular geometry with femtosecond resolution, showing coherent motion that survives electronic relaxation and seems to persist for longer than previously seen using other methods. Quantum coherence and dephasing in molecular motions determine the behaviour of many chemical reactions and are the fundamental basis for the concept of coherent control. Now, ultrafast X-ray scattering combined with a detailed structural determination analysis precisely measures the coherent vibrational motions of a polyatomic organic molecule following photoexcitation.

Bacteriorhodopsin (bR) is a light-driven proton pump. The primary photochemical event upon light absorption is isomerization of the retinal chromophore. Here we used time-resolved crystallography at an X-ray free-electron laser to follow the structural changes in multiphoton-excited bR from 250 femtoseconds to 10 picoseconds. Quantum chemistry and ultrafast spectroscopy were used to identify a sequential two-photon absorption process, leading to excitation of a tryptophan residue flanking the retinal chromophore, as a first manifestation of multiphoton effects. We resolve distinct stages in the structural dynamics of the all- trans retinal in photoexcited bR to a highly twisted 13- cis conformation. Other active site sub-picosecond rearrangements include correlated vibrational motions of the electronically excited retinal chromophore, the surrounding amino acids and water molecules as well as their hydrogen bonding network. These results show that this extended photo-active network forms an electronically and vibrationally coupled system in bR, and most likely in all retinal proteins. Bacteriorhodopsin (bR) is a light-driven proton pump. Here the authors combine time-resolved crystallography at a free-electron laser, ultrafast spectroscopy and quantum chemistry to study the structural changes following multiphoton photoexcitation of bR and find that they occur within 300 fs not only in the light-absorbing chromophore but also in the surrounding protein.

Light-driven oxidation of water to molecular oxygen is catalyzed by the oxygen-evolving complex (OEC) in Photosystem II (PS II). This multi-electron, multi-proton catalysis requires the transport of two water molecules to and four protons from the OEC. A high-resolution 1.89 Å structure obtained by averaging all the S states and refining the data of various time points during the S 2 to S 3 transition has provided better visualization of the potential pathways for substrate water insertion and proton release. Our results indicate that the O1 channel is the likely water intake pathway, and the Cl1 channel is the likely proton release pathway based on the structural rearrangements of water molecules and amino acid side chains along these channels. In particular in the Cl1 channel, we suggest that residue D1-E65 serves as a gate for proton transport by minimizing the back reaction. The results show that the water oxidation reaction at the OEC is well coordinated with the amino acid side chains and the H-bonding network over the entire length of the channels, which is essential in shuttling substrate waters and protons. The oxygen-evolving complex in Photosystem II (PSII) catalyzes the light-driven oxidation of water to oxygen and it is still under debate how the water reaches the active site. Here, the authors analyse time-resolved XFEL-based crystal structures of PSII that were determined at room temperature and report the structures of the waters in the putative channels surrounding the active site at various time-points during the reaction cycle and conclude that the O1 channel is the likely water intake pathway and the Cl1 channel the likely proton release pathway.

Chromophores absorb light in photosensitive proteins and thereby initiate fundamental biological processes such as photosynthesis, vision and biofluorescence. An important goal in their understanding is the provision of detailed structural descriptions of the ultrafast photochemical events that they undergo, in particular of the excited states that connect chemistry to biological function. Here we report on the structures of two excited states in the reversibly photoswitchable fluorescent protein rsEGFP2. We populated the states through femtosecond illumination of rsEGFP2 in its non-fluorescent off state and observed their build-up (within less than one picosecond) and decay (on the several picosecond timescale). Using an X-ray free-electron laser, we performed picosecond time-resolved crystallography and show that the hydroxybenzylidene imidazolinone chromophore in one of the excited states assumes a near-canonical twisted configuration halfway between the trans and cis isomers. This is in line with excited-state quantum mechanics/molecular mechanics and classical molecular dynamics simulations. Our new understanding of the structure around the twisted chromophore enabled the design of a mutant that displays a twofold increase in its off-to-on photoswitching quantum yield.

Intramolecular charge transfer and the associated changes in molecular structure in N,N′-dimethylpiperazine are tracked using femtosecond gas-phase X-ray scattering. The molecules are optically excited to the 3p state at 200 nm. Following rapid relaxation to the 3s state, distinct charge-localized and charge-delocalized species related by charge transfer are observed. The experiment determines the molecular structure of the two species, with the redistribution of electron density accounted for by a scattering correction factor. The initially dominant charge-localized state has a weakened carbon–carbon bond and reorients one methyl group compared with the ground state. Subsequent charge transfer to the charge-delocalized state elongates the carbon–carbon bond further, creating an extended 1.634 Å bond, and also reorients the second methyl group. At the same time, the bond lengths between the nitrogen and the ring-carbon atoms contract from an average of 1.505 to 1.465 Å. The experiment determines the overall charge transfer time constant for approaching the equilibrium between charge-localized and charge-delocalized species to 3.0 ps.

The structural versatility of light underpins an outstanding collection of optical phenomena where both geometrical and topological states of light can dictate how matter will respond or display. Light possesses multiple degrees of freedom such as amplitude, and linear, spin angular, and orbital angular momenta, but the ability to adaptively engineer the spatio-temporal distribution of all these characteristics is primarily curtailed by technologies used to impose any desired structure to light. We demonstrate a laser architecture based on coherent beam combination offering integrated spatio-temporal field control and programmability, thereby presenting unique opportunities for generating light by design to exploit its topology.

When a molecule interacts with light, its electrons can absorb energy from the electromagnetic field by rapidly rearranging their positions. This constitutes the first step of photochemical and photophysical processes that include primary events in human vision and photosynthesis. Here, we report the direct measurement of the initial redistribution of electron density when the molecule 1,3-cyclohexadiene (CHD) is optically excited. Our experiments exploit the intense, ultrashort hard x-ray pulses of the Linac Coherent Light Source (LCLS) to map the change in electron density using ultrafast x-ray scattering. The nature of the excited electronic state is identified with excellent spatial resolution and in good agreement with theoretical predictions. The excited state electron density distributions are thus amenable to direct experimental observation. Photoabsorption is a fundamental process that leads to changes in the electron density in matter. Here, the authors show a direct measurement of the distribution of electron density when a cyclohexadine molecule is excited by pulsed UV radiation and probed by a time delayed X-ray pulse generated at LCLS.

A long-sought-after goal for photocathode electron sources has been to improve performance by temporally shaping the incident excitation laser pulse. The narrow bandwidth, short wavelength, and picosecond pulse duration make it challenging to employ conventional spectral pulse shaping techniques. We present a novel and efficient intensity-envelope shaping technique achieved during nonlinear upconversion through opposite-chirp sum-frequency mixing. We also present a numerical case study of the linac coherent light source-II photoinjector where transverse electron emittance is improved by at least 25%.

Bremsstrahlung—the spontaneous emission of broadband radiation from free electrons that are deflected by atomic nuclei—contributes to the majority of X-rays emitted from X-ray tubes and used in applications ranging from medical imaging to semiconductor chip inspection. Here, we show that the bremsstrahlung intensity can be enhanced significantly—by more than three orders of magnitude—through shaping the electron wavefunction to periodically overlap with atoms in crystalline materials. Furthermore, we show how to shape the bremsstrahlung X-ray emission pattern into arbitrary angular emission profiles for purposes such as unidirectionality and multi-directionality. Importantly, we find that these enhancements and shaped emission profiles cannot be attributed solely to the spatial overlap between the electron probability distribution and the atomic centers, as predicted by the paraxial and non-recoil theory for free electron light emission. Our work highlights an unprecedented regime of free electron light emission where electron waveshaping provides multi-dimensional control over practical radiation processes like bremsstrahlung. Our results pave the way towards greater versatility in table-top X-ray sources and improved fundamental understanding of quantum electron-light interactions.Shaping the quantum free electron wavefunction leads to over thousand-fold intensity enhancements and greater directionality in X-ray bremsstrahlung.