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 cycle1,2. The model comprises four (meta)stable intermediates (S0, S1, S2 and S3) and one transient S4 state, which precedes dioxygen formation occurring in a concerted reaction from two water-derived oxygens bound at an oxo-bridged tetra manganese calcium (Mn4CaO5) cluster in the oxygen-evolving complex3–7. This reaction is coupled to the two-step reduction and protonation of the mobile plastoquinone QB 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 S2→S3 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 S3 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 O2 release. Thus, our results exclude peroxo-bond formation in the S3 state, and the nucleophilic attack of W3 onto W2 is unlikely.

Light-induced oxidation of water by photosystem II (PS II) in plants, algae and cyanobacteria has generated most of the dioxygen in the atmosphere. PS II, a membrane-bound multi-subunit pigment-protein complex, couples the one-electron photochemistry at the reaction center with the four-electron redox chemistry of water oxidation at the Mn4CaO5 cluster in the oxygen-evolving complex (OEC) (Fig. 1a, Extended Data Fig. 1). Under illumination, the OEC cycles through five intermediate S-states (S0 to S4)1, where S1 is the dark stable state and S3 is the last semi-stable state before O-O bond formation and O2 evolution2,3. A detailed understanding of the O-O bond formation mechanism remains a challenge, and elucidating the structures of the OEC in the different S-states, as well as the binding of the two substrate waters to the catalytic site4-6, is a prerequisite for this purpose. Here we report the use of femtosecond pulses from an X-ray free electron laser (XFEL) to obtain damage free, room temperature (RT) structures of dark-adapted (S1), two-flash illuminated (2F; S3-enriched), and ammonia-bound two-flash illuminated (2F-NH3; S3-enriched) PS II. Although the recent 1.95 Å structure of PS II7 at cryogenic temperature using an XFEL provided a damage-free view of the S1 state, RT measurements are required to study the structural landscape of proteins under functional conditions8,9, and also for in situ advancement of the S-states. To investigate the water-binding site(s), ammonia, a water analog, has been used as a marker, as it binds to the Mn4CaO5 cluster in the S2 and S3 states10. Since the ammonia-bound OEC is active, the ammonia-binding Mn site is not a substrate water site10-13. Thus, this approach, together with a comparison of the native dark and 2F states, is used to discriminate between proposed O-O bond formation mechanisms.

X-ray crystallography at X-ray free-electron laser (XFEL) sources is a powerful method for studying macromolecules at biologically relevant temperatures. Moreover, when combined with complementary techniques like X-ray emission spectroscopy (XES), both global structures and chemical properties of metalloenzymes can be obtained concurrently, providing new insights into the interplay between the protein structure/dynamics and chemistry at an active site. Implementing such a multimodal approach can be compromised by conflicting requirements to optimize each individual method. In particular, the method used for sample delivery greatly impacts the data quality. We present here a new, robust way of delivering controlled sample amounts on demand using acoustic droplet ejection coupled with a conveyor belt drive that is optimized for crystallography and spectroscopy measurements of photochemical and chemical reactions over a wide range of time scales. Studies with photosystem II, the phytochrome photoreceptor, and ribonucleotide reductase R2 illustrate the power and versatility of this method.

We demonstrate strong stimulated inelastic x-ray scattering by resonantly exciting a dense gas target of neon with femtosecond, high-intensity x-ray pulses from an x-ray free-electron laser (XFEL). A small number of lower energy XFEL seed photons drive an avalanche of stimulated resonant inelastic x-ray scattering processes that amplify the Raman scattering signal by several orders of magnitude until it reaches saturation. Despite the large overall spectral width, the internal spiky structure of the XFEL spectrum determines the energy resolution of the scattering process in a statistical sense. This is demonstrated by observing a stochastic line shift of the inelastically scattered x-ray radiation. In conjunction with statistical methods, XFELs can be used for stimulated resonant inelastic x-ray scattering, with spectral resolution smaller than the natural width of the core-excited, intermediate state. [5][6][7] with pulse durations in the fs range and unprecedented high intensities, hold the potential for transferring these techniques to the x-ray domain, to ultimately study the coherent interplay of electronic and vibrational degrees of freedom with high temporal and spatial resolution [8][9][10]. The high penetration depth of x rays, combined with the element and chemical sensitivity of inelastic x-ray scattering [11][12][13] could open pathways to temporally resolve complex dynamical processes such as energy transfer in light harvesting complexes or reaction dynamics of catalytic processes [14,15]. However, the cross section for x-ray Raman scattering is small compared to that in the visible spectral domain. Therefore, even at XFELs, single shot measurements are challenging, especially in dilute samples in the gas or liquid phase.This difficulty could be overcome by stimulating the Raman scattering process to produce a strong coherent amplification of the signal. We present the first demonstration of stimulated resonant electronic x-ray Raman scattering (SRXRS) in an atomic gas. Resonant x-ray Raman scattering, also referred to as resonant inelastic x-ray scattering [12], was discovered in 1974 [11] and since then has developed into a standard tool to study electronic and vibrational excitations in solids [16][17][18], liquids [19], gases [20], and on surfaces and interfaces [21,22]. Although several theoretical feasibility studies of SRXRS have been presented [23][24][25][26], no experiment has yet demonstrated this effect.SRXRS would ideally be realized by a narrow-band, tuneable two-color x-ray source: one frequency to excite an electronic inner-shell transition and the other, lower frequency to dump an electron into the short-lived core hole. Although the production of narrow-band x rays has been realized [27,28], and schemes for two-color self seeded sources have been proposed [29], there are currently no tunable, two-color, coherent sources available to perform such an experiment. Instead of this ideal, presently unrealizable approach, we report the demonstration of SRXRS in neon with a self-amplified...

Time-resolved femtosecond x-ray diffraction patterns from laser-excited molecular iodine are used to create a movie of intramolecular motion with a temporal and spatial resolution of 30 fs and 0.3 A. This high fidelity is due to interference between the non-stationary excitation and the stationary initial charge distribution. The initial state is used as the local oscillator for heterodyne amplification of the excited charge distribution to retrieve real-space movies of atomic motion onÅngstrom and femtosecond scales. This x-ray interference has not been employed to image internal motion in molecules before. Coherent vibrational motion and dispersion, dissociation, and rotational dephasing are all clearly visible in the data, thereby demonstrating the stunning sensitivity of heterodyne methods.High brightness ultrafast hard x-ray free electron lasers (FELs) can perform time-resolved x-ray diffractive imaging. Recent demonstrations of time-resolved crystal diffraction or time-resolved non-periodic imaging illustrate the power of these sources to trackÅngstrom-scale motion [1,2]. These have spurred new insights in broad areas of science, but have not fully realized the potential of x-ray FELs to image molecules with simultaneous sub-Ångstrom and few-femtosecond resolution. Previous x-ray or electron scattering experiments have used correlations between simulations and data to extract femtosecond molecular dynamics information [3][4][5][6][7].Here we propose and demonstrate an imaging method that employs a universal but unappreciated feature of time-resolved hard x-ray scattering that dramatically improves reconstructed images of charge motion, and enables femtosecond and sub-Ångstrom x-ray movies. The method relies on the "pump-probe" protocol, where motion is initiated by a short "start" pulse, and then interrogated at a later time by a "probe" pulse. The pumped fraction is small, and the unexcited fraction is our heterodyne reference [8].

Photo-induced non-radiative energy dissipation is a potential pathway to induce structural-phase transitions in two-dimensional materials. For advancing this field, a quantitative understanding of real-time atomic motion and lattice temperature is required. However, this understanding has been incomplete due to a lack of suitable experimental techniques. Here, we use ultrafast electron diffraction to directly probe the subpicosecond conversion of photoenergy to lattice vibrations in a model bilayered semiconductor, molybdenum diselenide. We find that when creating a high charge carrier density, the energy is efficiently transferred to the lattice within one picosecond. First-principles nonadiabatic quantum molecular dynamics simulations reproduce the observed ultrafast increase in lattice temperature and the corresponding conversion of photoenergy to lattice vibrations. Nonadiabatic quantum simulations further suggest that a softening of vibrational modes in the excited state is involved in efficient and rapid energy transfer between the electronic system and the lattice.

We report the observation and analysis of the gain curve of amplified Kα x-ray emission from solutions of Mn(II) and Mn(VII) complexes using an x-ray free electron laser to create the 1s core-hole population inversion. We find spectra at amplification levels extending over 4 orders of magnitude until saturation. We observe bandwidths below the Mn 1s core-hole lifetime broadening in the onset of the stimulated emission. In the exponential amplification regime the resolution corrected spectral width of ~1.7 eV FWHM is constant over 3 orders of magnitude, pointing to the buildup of transform limited pulses of ~1 fs duration. Driving the amplification into saturation leads to broadening and a shift of the line. Importantly, the chemical sensitivity of the stimulated x-ray emission to the Mn oxidation state is preserved at power densities of ~1020 W/cm2 for the incoming x-ray pulses. Differences in signal sensitivity and spectral information compared to conventional (spontaneous) x-ray emission spectroscopy are discussed. Our findings build a baseline for nonlinear x-ray spectroscopy for a wide range of transition metal complexes in inorganic chemistry, catalysis, and materials science.

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