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In the mixed-valence manganites, a near-infrared laser typically melts the orbital and spin order simultaneously, corresponding to the photoinduced $d^{1}d^{0}$ $\xrightarrow{}$ $d^{0}d^{1}$ excitations in the Mott-Hubbard bands of manganese. Here, we use ultrafast methods -- both femtosecond resonant x-ray diffraction and optical reflectivity -- to demonstrate that the orbital response in the layered manganite Nd$_{1-x}$Sr$_{1+x}$MnO$_{4}$ ($\it{x}$ = 2/3) does not follow this scheme. At the photoexcitation saturation fluence, the orbital order is only diminished by a few percent in the transient state. Instead of the typical $d^{1}d^{0}$ $\xrightarrow{}$ $d^{0}d^{1}$ transition, a near-infrared pump in this compound promotes a fundamentally distinct mechanism of charge transfer, the $d^{0}$ $ \xrightarrow{}$ $d^{1}L$, where $\it{L}$ denotes a hole in the oxygen band. This novel finding may pave a new avenue for selectively manipulating specific types of order in complex materials of this class.

Nature / Physical science 520(7545), 78 - 81(2015). doi:10.1038/nature14296, Transition-metal complexes have long attracted interest for fundamental chemical reactivity studies and possible use in solar energy conversion1, 2. Electronic excitation, ligand loss from the metal centre, or a combination of both, creates changes in charge and spin density at the metal site3, 4, 5, 6, 7, 8, 9, 10, 11 that need to be controlled to optimize complexes for photocatalytic hydrogen production8 and selective carbon–hydrogen bond activation9, 10, 11. An understanding at the molecular level of how transition-metal complexes catalyse reactions, and in particular of the role of the short-lived and reactive intermediate states involved, will be critical for such optimization. However, suitable methods for detailed characterization of electronic excited states have been lacking. Here we show, with the use of X-ray laser-based femtosecond-resolution spectroscopy and advanced quantum chemical theory to probe the reaction dynamics of the benchmark transition-metal complex Fe(CO)$_5$ in solution, that the photo-induced removal of CO generates the 16-electron Fe(CO)$_4$ species, a homogeneous catalyst12, 13 with an electron deficiency at the Fe centre14, 15, in a hitherto unreported excited singlet state that either converts to the triplet ground state or combines with a CO or solvent molecule to regenerate a penta-coordinated Fe species on a sub-picosecond timescale. This finding, which resolves the debate about the relative importance of different spin channels in the photochemistry of Fe(CO)$_5$ (refs 4, 16,17,18,19 and 20), was made possible by the ability of femtosecond X-ray spectroscopy to probe frontier-orbital interactions with atom specificity. We expect the method to be broadly applicable in the chemical sciences, and to complement approaches that probe structural dynamics in ultrafast processes., Published by Macmillan28177, London

We performed ultrafast time-resolved near-infrared pump, resonant soft X-ray diffraction probe measurements to investigate the coupling between the photoexcited electronic system and the spin cycloid magnetic order in multiferroic TbMnO3 at low temperatures. We observe melting of the long range antiferromagnetic order at low excitation fluences with a decay time constant of 22.3 +- 1.1 ps, which is much slower than the ~1 ps melting times previously observed in other systems. To explain the data we propose a simple model of the melting process where the pump laser pulse directly excites the electronic system, which then leads to an increase in the effective temperature of the spin system via a slower relaxation mechanism. Despite this apparent increase in the effective spin temperature, we do not observe changes in the wavevector q of the antiferromagnetic spin order that would typically correlate with an increase in temperature under equilibrium conditions. We suggest that this behavior results from the extremely low magnon group velocity that hinders a change in the spin-spiral wavevector on these time scales., Comment: 9 pages, 4 figures

We use femtosecond resonant soft x-ray diffraction to measure the optically stimulated ultrafast changes of charge density wave correlations in underdoped YBa2Cu3O6.6. We find that when coherent interlayer transport is enhanced by optical excitation of the apical oxygen distortions, at least 50% of the in-plane charge density wave order is melted. These results indicate that charge ordering and superconductivity may be competing up to the charge ordering transition temperature, with the latter becoming a hidden phase that is accessible only by nonlinear phonon excitation., Comment: 22 pages (including Supplement), 3 figures

We report femtosecond resonant soft X-ray diffraction measurements of the dynamics of the charge order and of the crystal lattice in non-superconducting, stripe-ordered La1.875Ba0.125CuO4. Excitation of the in-plane Cu-O stretching phonon with a mid-infrared pulse has been previously shown to induce a transient superconducting state in the closely related compound La1.675Eu0.2Sr0.125CuO4. In La1.875Ba0.125CuO4, we find that the charge stripe order melts promptly on a sub-picosecond time scale. Surprisingly, the low temperature tetragonal distortion is only weakly reduced, reacting on significantly longer time scales that do not correlate with light-induced superconductivity. This experiment suggests that charge modulations alone, and not the LTT distortion, prevent superconductivity in equilibrium., 16 pages, 3 figures

We investigate the order parameter dynamics of the stripe-ordered nickelate, La$_{1.75}$Sr$_{0.25}$NiO$_4$, using time-resolved resonant X-ray diffraction. In spite of distinct spin and charge energy scales, the two order parameters' amplitude dynamics are found to be linked together due to strong coupling. Additionally, the vector nature of the spin sector introduces a longer re-orientation time scale which is absent in the charge sector. These findings demonstrate that the correlation linking the symmetry-broken states does not unbind during the non-equilibrium process, and the time scales are not necessarily associated with the characteristic energy scales of individual degrees of freedom., Comment: 4 figures. Accepted by Physical Review Letter

We utilized femtosecond time-resolved resonant inelastic X-ray scattering and ab initio theory to study the transient electronic structure and the photoinduced molecular dynamics of a model metal carbonyl photocatalyst Fe(CO)(5) in ethanol solution. We propose mechanistic explanation for the parallel ultrafast intra-molecular spin crossover and ligation of the Fe(CO)(4) which are observed following a charge transfer photoexcitation of Fe(CO)(5) as reported in our previous study [ Wernet et al., Nature 520, 78 (2015)]. We find that branching of the reaction pathway likely happens in the (1)A(1) state of Fe(CO)(4). A sub-picosecond time constant of the spin crossover from B-1(2) to B-3(2) is rationalized by the proposed B-1(2) -> (1)A(1) -> B-3(2) mechanism. Ultrafast ligation of the B-1(2) Fe(CO)(4) state is significantly faster than the spin-forbidden and diffusion limited ligation process occurring from the B-3(2) Fe(CO)(4) ground state that has been observed in the previous studies. We propose that the ultrafast ligation occurs via B-1(2) -> (1)A(1) -> (1)A'Fe(CO)(4)EtOH pathway and the time scale of the (1)A(1) Fe(CO)(4) state ligation is governed by the solute-solvent collision frequency. Our study emphasizes the importance of understanding the interaction of molecular excited states with the surrounding environment to explain the relaxation pathways of photoexcited metal carbonyls in solution. (C) 2016 Author(s).

Using time-resolved resonant soft x-ray diffraction, we measure the evolution of the full three-dimensional scattering volume of the antiferromagnetic superlattice reflection in the single-layer manganite La0.5Sr1.5MnO4on femtosecond time scales following photoexcitation. We find that the in-plane correlations are unchanged as a metastable state is entered, however there are subtle changes in the c-axis correlations. We observe a transient shift of the scattering ellipsoid along (00L) at very short times, and at longer time scales the short-range c-axis correlations are more robust than they are in equilibrium. Such results are not obtainable with any other techniques and hint at previously unresolved processes in the dynamics of photomelting in strongly correlated systems.

The coupling of multiple degrees of freedom - charge, spin, and lattice - in manganites has mostly been considered at the microscopic level. However, on larger length scales, these correlations may be affected by strain and disorder, which can lead to short range order in these phases and affect the coupling between them. To better understand these effects, we explore the dynamics of orbitally ordered domains in a half-doped manganite near the orbital ordering phase transition. Our results suggest that the domains are largely static, and exhibit only slow fluctuations near domain boundaries., 10 pages, 4 figures

The L-lysine biosynthetic pathway provides a unique and exciting target for the design of novel antibacterial agents through inhibition of the synthesis of meso—DAP or L—lysine, crucial components of the bacterial peptidoglycan cell wall. The first two enzymes in this pathway—dihydrodipicolinate synthase (DHDPS) and dihydrodipicolinate reductase (DHDPR)—were targeted for inhibition. DHDPS catalyses the first committed step to L—lysine biosynthesis, the condensation of pyruvate and (S)-aspartate B-semialdehyde to give DHDP. DHDPR, the next enzyme in the pathway, then reduces DHDP in an NAD(P)H dependent reaction to give THDP. A suite of inhibitors were designed and synthesised to act as product- or substrate-based analogues of DHDPS and DHDPR, respectively. Heterocyclic compounds based on chelidamic acid or thiazane-3,5-dicarboxylates were synthesised as potential inhibitors of DHDPS. Stereoselective procedures for the oxidation of thiazane-3,5-dicarboxylates were developed. It was found that when direct oxidants were used (such as sodium periodate, peroxides and peracids) the axial sulfur lone-pair reacts preferentially, providing the axial S—oxide. Oxidation via a two step mechanism using bromine/water gives the epimeric equatorial S—oxide. Acyclic analogues of the heterocyclic compounds were also synthesised as potential inhibitors of the intermediate in the DHDPS reaction pathway. Two methods of synthesising the DHDPS substrate aspartate B-semialdehyde (ASA) were investigated. The first four step procedure beginning from racemic allylglycine, installed the aldehyde moiety through an osmium tetroxide/sodium periodate reaction. The ASA produced by this procedure was of variable yield and purity. The second method of synthesising ASA was achieved by reduction of a Weinreb amide derivative of aspartic acid. This procedure gave ASA in excellent yield and purity and was the method of choice for preparing ASA for use in kinetic studies. Additionally the required enzymes, DHDPS and DHDPR were purified to homogeneity as judged by SDS-PAGE visualised by Coomassie brilliant blue staining and specific activity tables. The inhibitors synthesised were tested for inhibition of DHDPS and DHDPR. The heterocyclic compounds were not found to be potent inhibitors of DHDPS. Furthermore, chelidamic acid and its dimethyl ester were shown NOT to be competitive inhibitors of DHDPS. Unfortunately, none of the analogous acyclic compounds proved to be potent DHDPS inhibitors either. Two potent acyclic irreversible inhibitors synthesised en route to other target inhibitors of DHDPS were discovered, possessing micromolar—millimolar activity. These compounds provide a new lead in the design of more potent DHDPS inhibitors. From the DHDPS inhibitory results obtained in this study the mechanism of DHDPS was revised. It was postulated that the DHDPS-catalysed reaction proceeds via a protonated aldehyde intermediate that is consistent with the inhibitory data obtained herein. None of the synthesised inhibitors evaluated against DHDPR were found to be more potent then the known inhibitor dipicolinic acid. Consistent with the literature results dipicolinic acid was found to be a competitive inhibitor of DHDPR, however dimethyl chelidamate was found to be an uncompetitive inhibitor of DHDPR.

We performed ultrafast time-resolved near-infrared pump, resonant soft x-ray diffraction probe measurements to investigate the coupling between the photoexcited electronic system and the spin cycloid magnetic order in multiferroic TbMnO3 at low temperatures. We observe melting of the long range antiferromagnetic order at low excitation fluences with a decay time constant of 22.3 +/- 1.1 ps, which is much slower than the similar to 1 ps melting times previously observed in other systems. To explain the data, we propose a simple model of the melting process where the pump laser pulse directly excites the electronic system, which then leads to an increase in the effective temperature of the spin system via a slower relaxation mechanism. Despite this apparent increase in the effective spin temperature, we do not observe changes in the wave vector q of the antiferromagnetic spin order that would typically correlate with an increase in temperature under equilibrium conditions. We suggest that this behavior results from the extremely low magnon group velocity that hinders a change in the spin-spiral wave vector on these time scales.

Vacuum space charge induced kinetic energy shifts of O 1s and Ru 3d core levels in femtosecond soft X-ray photoemission spectra (PES) have been studied at a free electron laser (FEL) for an oxygen layer on Ru(0001). We fully reproduced the measurements by simulating the in-vacuum expansion of the photoelectrons and demonstrate the space charge contribution of the high-order harmonics in the FEL beam. Employing the same analysis for 400 nm pump-X-ray probe PES, we can disentangle the delay dependent Ru 3d energy shifts into effects induced by space charge and by lattice heating from the femtosecond pump pulse., LCLS, Stanford University through the Stanford Institute for Materials Energy Sciences (SIMES); Lawrence Berkeley National Laboratory (LBNL); University of Hamburg through the BMBF priority program; Center for Free Electron Laser Science (CFEL); BMBF priority program; VolkswagenStiftung

We show that coadsorbed oxygen atoms have a dramatic influence on the CO desorption dynamics from Ru(0001). In contrast to the precursor-mediated desorption mechanism on Ru(0001), the presence of surface oxygen modifies the electronic structure of Ru atoms such that CO desorption occurs predominantly via the direct pathway. This phenomenon is directly observed in an ultrafast pump-probe experiment using a soft x-ray free-electron laser to monitor the dynamic evolution of the valence electronic structure of the surface species. This is supported with the potential of mean force along the CO desorption path obtained from density-functional theory calculations. Charge density distribution and frozen-orbital analysis suggest that the oxygen-induced reduction of the Pauli repulsion, and consequent increase of the dative interaction between the CO 5 sigma and the charged Ru atom, is the electronic origin of the distinct desorption dynamics. Ab initio molecular dynamics simulations of CO desorption from Ru(0001) and oxygen-coadsorbed Ru(0001) provide further insights into the surface bond-breaking process., US Department of Energy, Basic Energy Science; Swedish Research Council (VR); Department of Energy, Laboratory Directed Research and Development; VolkswagenStiftung; LCLS, Stanford University; Lawrence Berkeley National Laboratory (LBNL); University of Hamburg; Center for Free Electron Laser Science (CFEL)