Search

We present the first lasing results of SwissFEL, a hard X-ray free-electron laser (FEL) that recently came into operation at the Paul Scherrer Institute in Switzerland. SwissFEL is a very stable, compact and cost-effective X-ray FEL facility driven by a low-energy and ultra-low-emittance electron beam travelling through short-period undulators. It delivers stable hard X-ray FEL radiation at 1-A wavelength with pulse energies of more than 500 μJ, pulse durations of ~30 fs (root mean square) and spectral bandwidth below the per-mil level. Using special configurations, we have produced pulses shorter than 1 fs and, in a different set-up, broadband radiation with an unprecedented bandwidth of ~2%. The extremely small emittance demonstrated at SwissFEL paves the way for even more compact and affordable hard X-ray FELs, potentially boosting the number of facilities worldwide and thereby expanding the population of the scientific community that has access to X-ray FEL radiation. The first lasing results at SwissFEL, an X-ray free-electron laser, are presented, highlighting the facility’s unique capabilities. A general comparison to other major facilities is also provided.

The dissociation energy in the X1Σg+ electronic ground state of the carbon dimer C2 is determined spectroscopically from a combined treatment involving resonant four-wave mixing experiments and hig...

Excited xylyl (methyl-benzyl) radical isomers have been studied by femtosecond time-resolved photoelectron spectroscopy and mass spectrometry. Depending on the substitution we find different deactivation channels after excitation into the D3(2A'') state (310 nm, 4 eV). While the ortho and para isomer exhibit deactivation rates similar to the benzyl radical, meta-xylyl sticks out and depletes twice as fast into the vibrationally hot ground state. We found that a ring deformation mode rather than the methyl pseudorotation enables access to a conical intersection, which is responsible for the faster deactivation. Transitions in the photoelectron spectrum can be assigned to several Rydberg series with mostly d angular momentum components. Absorption of two 4 eV photons triggers hydrogen loss reactions on a femtosecond timescale.

Optical-domain Transient Grating (TG) spectroscopy is a versatile background-free four-wave-mixing technique used to probe vibrational, magnetic and electronic degrees of freedom in the time domain. The newly developed coherent X-ray Free Electron Laser sources allow its extension to the X-ray regime. Xrays offer multiple advantages for TG: their large penetration depth allows probing the bulk properties of materials, their element-specificity can address core-excited states, and their short wavelengths create excitation gratings with unprecedented momentum transfer and spatial resolution. We demonstrate for the first time TG excitation in the hard X-ray range at 7.1 keV. In Bismuth Germanate (BGO), the nonresonant TG excitation generates coherent optical phonons detected as a function of time by diffraction of an optical probe pulse. This experiment demonstrates the ability to probe bulk properties of materials and paves the way for ultrafast coherent four-wave-mixing techniques using X-ray probes and involving nanoscale TG spatial periods., Comment: 11 pages, 4 figures

Wastewater treatment plants are major point sources of (micro)pollutant emissions and advanced wastewater treatment technologies can improve their removal capacity. While abundant data on individual advanced treatment technologies is available, there is limited knowledge regarding the removal performance of ozonation combined with multiple post-treatments and stand-alone membrane bioreactors. This is especially true for the removal of in vitro and in vivo toxicity. Therefore, we investigated the removal of 40 micropollutants and toxicity by a pilot-scale ozonation with four post-treatments: non-aerated and aerated granular activated carbon and biological filtration. In addition, two stand-alone membrane bioreactors fed with untreated wastewater and one MBR operating with ozonated partial flow recirculation were analysed. Aqueous and extracted samples were analysed in vitro for (anti)estrogenic, (anti)androgenic and mutagenic effects. To assess in vivo effects, the mudsnail Potamopyrgus antipodarum was exposed in an on-site flow-through system. Multiple in vitro effects were detected in conventionally treated wastewater including estrogenic and anti-androgenic activity. Ozonation largely removed these effects, while anti-estrogenic and mutagenic effects increased suggesting the formation of toxic transformation products. These effects were significantly reduced by granular activated carbon being more effective than biological filtration. The membrane bioreactor performed similarly to the conventional treatment while the membrane bioreactor with ozonation had a comparable removal performance like ozonation. Conventionally treated wastewater increased the growth of P. antipodarum. Ozonation reduced the reproduction indicating a potential formation of toxic transformation products. In the post-treatments, these effects were compensated or remained unaffected. The effluents of the membrane bioreactors induced reproductive toxicity. Our results show that ozonation is effective in further reducing toxicity and micropollutant concentrations. However, the formation of toxicity requires a post-treatment. Here, ozonation coupled to granular activated carbon filtration seemed the most promising treatment process.

OLED technology beyond small or expensive devices requires light-emitters, luminophores, based on earth-abundant elements. Understanding and experimental verification of charge transfer in luminophores are needed for this development. An organometallic multicore Cu complex comprising Cu–C and Cu–P bonds represents an underexplored type of luminophore. To investigate the charge transfer and structural rearrangements in this material, we apply complementary pump-probe X-ray techniques: absorption, emission, and scattering including pump-probe measurements at the X-ray free-electron laser SwissFEL. We find that the excitation leads to charge movement from C- and P- coordinated Cu sites and from the phosphorus atoms to phenyl rings; the Cu core slightly rearranges with 0.05 Å increase of the shortest Cu–Cu distance. The use of a Cu cluster bonded to the ligands through C and P atoms is an efficient way to keep structural rigidity of luminophores. Obtained data can be used to verify computational methods for the development of luminophores., Nature Communications, 11 (1), ISSN:2041-1723

The capabilities of a double-pulse (DP) femtosecond (fs) laser ablation of solid materials as an ion source for application in a miniature Laser ionisation time-of-flight Mass Spectrometer (LMS) system designed for space research are investigated. The studies are conducted by irradiating a high-purity Mg sample with sequences of two femtosecond laser pulses. The positively charged fraction of the Mg plasma is analysed as a function of the inter-pulse delay in the range from 0 to 300 ps and for pulse energies in the range from 0.2 to 1 μJ. The DP ablation studies with both pulses of similar energy show a Mg+ ion yield enhancement within the inter-pulse delay range from 1 to 35 ps near the ablation energy threshold (AET) and from 1 to ∼300 ps if larger pulse energies are applied for the first ablation pulse. For the same total energies but different individual pulse energies, Mg+ ions are produced more efficiently if the weaker pulse is applied first. The analyses of Mg+, multiple-charged Mg and Mg cluster ion yields as a function of the inter-pulse delay and the pulse energies improve the understanding of the ablation mechanism and add some insights into the dynamics of the Mg-surface melting and cooling phases, thermal characteristics of the expanding plasma plume and atomic/cluster ion production mechanisms. By applying sufficiently high DP energies > (0.3 + 0.3) μJ the clusters which are likely produced in the initial ablation phase at the surface can be decomposed effectively in the heated plasma plume. Moreover, by tuning the inter-pulse delay one can also efficiently suppress the neutral-ion reactions in post-plasma chemistry. Our studies show that the DP femtosecond laser ablation ion source can improve the mass spectrometric analysis of solid samples by increasing the ion yield of atomic ions and reducing the abundance of cluster ions. This can improve the quantitative analyses of elements and their isotopes by increasing signal-to-noise ratios and reducing isobaric interferents arising from cluster ions. The current DP studies are conducted successfully with pulse energies lower than 1 μJ, which is sufficiently low to be realised in a compact, lightweight system using fibre technology based lasers, suitable for in situ space applications.

The available knowledge of the electronically excited states of the copper dimer is limited. This is common for transition metals, as the high density of states hinders both experimental assignment and computation. In this work, two-color resonant four-wave mixing spectroscopy was applied to neutral Cu2 in the gas phase. The method yielded accurate positions of individual rovibrational lines in the I-X and J-X electronic systems. This revealed the term symbols for the I and J states as 1Πu (1u) and 1Σu+ (0u+), respectively. For the 63Cu2 isotopologue, accurate molecular constants were obtained. The characterization of the J state finally allowed decisive determination of its electron configuration. The J state is obtained from the ground state by promotion of a 3dπg electron into the weakly bonding 4pπu molecular orbital. From the data analysis, lifetimes of the I state (between 10 ps and 5 ns) and J state (66 ns) were inferred.

For reducing organic micropollutants (MP) in municipal wastewater effluents, granular activated carbon (GAC) has been tested in various studies. We did systematic literature research and found 44 studies dealing with the adsorption of MPs (carbamazepine, diclofenac, sulfamethoxazole) from municipal wastewater on GAC in pilot- and large-scale plants. Within our meta-analysis we plot the bed volumes (BV [m3water/m3GAC]) until the breakthrough criterion of MP-BV20% was reached, dependent on potential relevant parameters (empty bed contact time EBCT, influent DOC DOC0 and manufacturing method). Moreover, we performed statistical tests (ANOVAs) to check the results for significance. Single adsorbers operating time differs i.e. by 2500% until breakthrough of diclofenac-BV20% was reached (800-20,000 BV). There was still elimination of the "very well/well" adsorbable MPs such as carbamazepine and diclofenac even when the equilibrium of DOC had already been reached. No strong statistical significance of EBCT and DOC0 on MP-BV20% could be found due to lack of data and the high heterogeneity of the studies using GAC of different qualities. In further studies, adsorbers should be operated ≫20,000 BV for exact calculation of breakthrough curves, and the following parameters should be recorded: selected MPs; DOC0; UVA254; EBCT; product name, manufacturing method and raw material of GAC; suspended solids (TSS); backwash interval; backwash program and pressure drop within adsorber. Based on our investigations we generally recommend using reactivated GAC to reduce the environmental impact and to carry out tests on pilot scale to collect reliable data for process design.

Advanced wastewater treatment technologies are generally known to be an effective tool for reducing micropollutant discharge into the aquatic environment. Nevertheless, some processes such as ozonation result in stable transformation products with often unknown toxicity. In the present study, whole effluents originating from nine different steps of advanced treatment combinations were compared for their aquatic toxicity. Assessed endpoints were survival, growth and reproduction of Lumbriculus variegatus, Daphnia magna and Lemna minor chronically exposed in on-site flow-through tests based on standard guidelines. The treatment combinations were activated sludge treatment followed by ozonation with subsequent filtration by granular activated carbon or biofilters and membrane bioreactor treatment of raw wastewater followed by ozonation. Additionally, the impact of treated wastewater on the immune response of invertebrates was investigated by challenging D. magna with a bacterial endoparasite. Conventionally treated wastewater reduced reproduction of L. variegatus by up to 46%, but did not affect D. magna and L. minor with regard to survival, growth, reproduction and parasite resistance. Instead, parasite susceptibility was significantly reduced in D. magna exposed to conventionally treated as well as ozonated wastewater in comparison to D. magna exposed to the medium control. None of the three test organisms provided clear evidence that wastewater ozonation leads to increased aquatic toxicity. Rather than to the presence of toxic transformation products, the affected performance of L. variegatus could be linked to elevated concentrations of ammonium and nitrite that likely resulted from treatment failures.

Ultrafast relaxation dynamics of n-propylbenzene (n-PB) cation were studied by delayed photofragmentation and photoelectron spectroscopy. Multiphoton ionization (MPI) by the fundamental pulse at ∼800 nm was used to prepare the n-PB cations. The photoelectron image for MPI was captured and characterized by the photoelectron imaging technique, and the observed photoelectron bands were assigned to the corresponding cationic states. The ultrafast relaxation dynamics of these cation states were tracked by monitoring the time-dependent parent-ion (C6H5C3H7+) depletion and the fragment-ion (C6H5CH2+) appearance. An ultrafast relaxation time of 1.1 (±0.1) ps for the cations was obtained, which was ascribed to a dissipative intramolecular vibrational redistribution process within the ground D0 state.

A multidisciplinary concept has been developed to compare advanced wastewater treatment processes for their efficacy of eliminating micropollutants and pathogens. The concept is based on (i) the removal/formation of selected indicator substances and their transformation products (TPs), (ii) the assessment of ecotoxicity via in vitro tests, and (iii) the removal of pathogens and antibiotic resistant bacteria. It includes substances passing biological wastewater treatment plants regulated or proposed to be regulated in the European Water Framework Directive, TPs formed in biological processes or during ozonation, agonistic/antagonistic endocrine activities, mutagenic/genotoxic activities, cytotoxic activities, further activities like neurotoxicity as well as antibiotics resistance genes, and taxonomic gene markers for pathogens. At a pilot plant, ozonation of conventionally treated wastewater resulted in the removal of micropollutants and pathogens and the reduction of estrogenic effects, whereas the in vitro mutagenicity increased. Subsequent post-treatment of the ozonated water by granular activated carbon (GAC) significantly reduced the mutagenic effects as well as the concentrations of remaining micropollutants, whereas this was not the case for biofiltration. The results demonstrate the suitability of the evaluation concept to assess processes of advanced wastewater treatment including ozonation and GAC by considering chemical, ecotoxicological, and microbiological parameters.

Stochastic processes are highly relevant in research fields as different as neuroscience, economy, ecology, chemistry, and fundamental physics. However, due to their intrinsic unpredictability, stochastic mechanisms are very challenging for any kind of investigations and practical applications. Here we report the deliberate use of stochastic X-ray pulses in two-dimensional spectroscopy to the simultaneous mapping of unoccupied and occupied electronic states of atoms in a regime where the opacity and transparency properties of matter are subject to the incident intensity and photon energy. A readily transferable matrix formalism is presented to extract the electronic states from a dataset measured with the monitored input from a stochastic excitation source. The presented formalism enables investigations of the response of the electronic structure to irradiation with intense X-ray pulses while the time structure of the incident pulses is preserved., Free electron X-ray laser pulses, generated by self-amplified spontaneous emission, are stochastic in nature. Here the authors present a reconstruction method for 2D spectroscopy while preserving the intrinsic properties of the incident pulses and apply it to a study towards X-ray intensity induced effects.

The extension of transient grating spectroscopy to the x-ray regime will create numerous opportunities, ranging from the study of thermal transport in the ballistic regime to charge, spin, and energy transfer processes with atomic spatial and femtosecond temporal resolution. Studies involving complicated split-and-delay lines have not yet been successful in achieving this goal. Here we propose a novel, simple method based on the Talbot effect for converging beams, which can easily be implemented at current x-ray free electron lasers. We validate our proposal by analyzing printed interference patterns on polymethyl methacrylate and gold samples using similar to 3 keV X-ray pulses. (C) 2019 Optical Society of America

Time-resolved transient grating (TG) spectroscopy facilitates detailed studies of electron dynamics and transport phenomena by means of a periodic excitation of matter with coherent ultrashort light pulses. Several current and next generation free-electron laser (FEL) facilities provide fully coherent pulses with few femtosecond pulse durations and extreme ultraviolet (XUV) photon energies. Thus, they allow for transient grating experiments with periodicities as small as tens of nanometers and with element specific photon energies. Here, we demonstrate the element specificity of XUV TG (X-TG) experiments by tuning the photon energy across the Si L2,3-edge of Si3N4. We observe a shortening of the signal decay when increasing the XUV photon energy above the absorption edge. The analysis of the wavelength dependent signal shows that the faster decay is driven by the increase in the charge carrier density. From the decay constants the interband Auger coefficient at elevated temperatures and high electron densities has been determined.Time-resolved transient grating (TG) spectroscopy facilitates detailed studies of electron dynamics and transport phenomena by means of a periodic excitation of matter with coherent ultrashort light pulses. Several current and next generation free-electron laser (FEL) facilities provide fully coherent pulses with few femtosecond pulse durations and extreme ultraviolet (XUV) photon energies. Thus, they allow for transient grating experiments with periodicities as small as tens of nanometers and with element specific photon energies. Here, we demonstrate the element specificity of XUV TG (X-TG) experiments by tuning the photon energy across the Si L2,3-edge of Si3N4. We observe a shortening of the signal decay when increasing the XUV photon energy above the absorption edge. The analysis of the wavelength dependent signal shows that the faster decay is driven by the increase in the charge carrier density. From the decay constants the interband Auger coefficient at elevated temperatures and high electron dens...

Perturbation facilitated double-resonant four-wave mixing is applied to access high-lying vibrational levels of the X 1Σg+ (0g+) ground state of Cu2. Rotationally resolved transitions up to v″ = 102 are measured. The highest observed level is at 98% of the dissociation energy. The range and accuracy of previous measurements are significantly extended. By applying the near dissociation equation developed by Le Roy [R. J. Le Roy, J. Quant. Spectrosc. Radiat. Transfer 186, 197 (2017)], a dissociation energy of De = 16 270(7) hc cm−1 is determined, and an accurate potential energy function for the X 1Σg+ (0g+) ground state is obtained. Molecular constants are determined from the measured transitions and by solving the radial Schrodinger equation using this function and are compared with results from earlier measurements. In addition, benchmark multi-reference configuration interaction computations are performed using the Douglas–Kroll–Hess Hamiltonian and the appropriate basis of augmented valence quadruple ζ type. Coupled-cluster single, double, and perturbative triple calculations were performed for comparison.

The potential of two-color resonant and degenerate four-wave mixing spectroscopy for investigations of the complex spectra of transition metal dimers is explored. Two-color resonant and degenerate four-wave mixing spectroscopy scans of the well-known A-X and B-X transitions in Cu2 are reported and compared with previous experimental data obtained from standard single-resonance techniques. The selectivity of the method is shown to enable the measurement of isotopologue pure spectra without the need for isotopically enriched metal targets. Specific subsets of the rovibronic structure are separated in a congested spectral region of overlapping transitions. The sensitivity of the method compares satisfactorily with linear spectroscopic methods such as laser-induced fluorescence and cavity ring-down. A new laser vaporization source for the production of transition metal dimers and clusters has been constructed. The new design aims for a high number density and maximum possible shot-to-shot stability. The possibilities of further applications of non-linear four-wave mixing spectroscopy to Cu2 and other transition metal dimers are discussed. Copyright © 2015 John Wiley & Sons, Ltd.

In order to obtain fundamental insight into the bonding properties of transition metals, the ( v ′ ,0) B-X transitions ( v ′ = 0 - 2 ) of the heteronuclear CuAg dimer are re-investigated by applying non-linear four-wave mixing and laser-induced fluorescence spectroscopy. The CuAg molecules in a molecular beam are produced by laser ablation of a Cu/Ag alloy target. Rotationally resolved spectra of the B-X transition are recorded. The measurements yield accurate molecular constants for four isotopologues and an equilibrium distance r e = 2.4577 ± 0.0003 A for the B state. In particular, the B state is identified as an Ω = 0 + ion-pair state. The results corroborate the important contribution of the charge-transfer state to the metal–metal bond. The results are in good agreement with the recent calculations at the MRCI level of theory (Alizadeh et al., 2014).

Many of the scientific applications for X-ray free-electron lasers seek to exploit the ultrashort pulse durations of intense X-rays to obtain femtosecond time resolution of various processes in a “pump-probe” scheme. One of the limiting factors for such experiments is the timing jitter between the X-rays and ultrashort pulses from more conventional lasers operating at near-optical wavelengths. In this work, we investigate the potential of using X-ray-induced changes in the optical second harmonic generation efficiency of a nonlinear crystal to retrieve single-shot arrival times of X-ray pulses with respect to optical laser pulses. Our experimental results and simulations show changes to the efficiency of the second harmonic generation of 12%, approximately three times larger than the measured changes in the transmission of the 800 nm center-wavelength fundamental pulse. Further experiments showing even larger changes in the transmission of 400 nm center-wavelength pulses show that the mechanism of the second harmonic generation efficiency modulation is mainly the result of X-ray-induced changes in the linear absorption coefficients near 400 nm. We demonstrate and characterize a cross-correlation tool based on this effect in reference to a previously demonstrated method of X-ray/optical cross-correlation.

Anthropogenic micropollutants and transformation products (TPs) negatively affect aquatic ecosystems and water resources. Wastewater treatment plants (WWTP) represent major point sources for (micro)pollutants and TPs in urban water cycles. The aim of the current study was to assess the removal of micropollutants and toxicity during conventional and advanced wastewater treatment. Using wild-type and transgenic Caenorhabditis elegans, the endpoint reproduction, growth, and cytochrome P450 (CYP) 35A3 induction (via cyp-35A3::GFP) were assessed. Samples were collected at four WWTPs and a receiving surface water. One WWTP included the advanced treatments: ozonation followed by granular activated carbon (GAC) or biological filtration (BF), respectively. Relevant micropollutants and WWTP parameters (n = 111) were included. Significant reproductive toxicity was detected for one WWTP effluent (31–83% reduced brood size). Three of four effluents significantly promoted the growth of C. elegans larvae (49–55% increased lengths). This effect was also observed for the GAC (34–41%) and BF (30%) post-treatments. Markedly, significant cyp-35A3::GFP induction was detected for one effluent before and after ozonation, being more pronounced for the ozonated samples (5- and 7.4-fold above controls). While the advanced treatments decreased the concentrations of most micropollutants, the observed effects may be attributed to effects of residual target compounds and/or compounds not included in the target chemical analysis. This highlights the need for an integrated assessment of (advanced) wastewater treatment covering both biological and chemical parameters. This is a post-peer-review, pre-copyedit version of an article published in Environmental Science and Pollution Research. Locked until 6 March 2019 due to copyright restrictions. The final authenticated version is available online at: http://dx.doi.org/10.1007/s11356-018-1605-2.

We investigate the excitation of coherent acoustic and optical phonons by ultrashort extreme ultraviolet (EUV) pulses produced by a free electron laser. Two crossed femtosecond EUV (wavelength 12.7 nm) pulses are used to excite coherent phonons at a wavelength of 280 nm, which are detected via diffraction of an optical probe beam. Longitudinal and surface acoustic waves are measured in BK-7 glass, diamond, and Bi4Ge3O12; in the latter material, the excitation of a coherent optical phonon mode is also observed. We discuss probing different acoustic modes in reflection and transmission geometries and frequency mixing of surface and bulk acoustic waves in the signal. The use of extreme ultraviolet radiation will allow the creation of tunable GHz to THz acoustic sources in any material without the need to fabricate transducer structures. The contribution by the MIT group was supported by the U.S. Department of Energy, Office of Basic Energy Sciences under Award No. DE-FG02-00ER15087. The JILA group graciously acknowledges support from the Department of Energy BES Award No. DE-FG02-99ER14982 and the Gordon and Betty Moore Foundation's EPiQS Initiative through Grant No. GBMF: 4538, and J.K. acknowledges support from an SRC Fellowship. The Elettra group acknowledges the European Research Council Grant No. 202804-TIMER. A.C. and T.F. acknowledge the Swiss NSF through the NCCR-MUST.

The low energy electronic structure of the copper dimer has been re-investigated using non-linear four-wave mixing spectroscopy and high level ab initio calculations. In addition to the measurement of the previously reported A, B, and C electronic states, a new state denoted A′ is identified with T0 = 20 100.4090(16) cm−1 (63Cu2). Rotational analysis of the A′–X (0,0) and (1,0) transitions leads to the assignment of A′ 1u. Ab initio calculations present the first theoretical description of the low energy states of the copper dimer in Hund’s case (c) and confirm the experimental assignment. The discovery of this new low energy excited state emphasizes that spin-orbit coupling is significant in states with d-hole electronic configurations and resolves a decades-long mystery in the initial assignment of the A state.

We report on the application of a short working distance von Hamos geometry spectrometer to measure the inelastic X-ray scattering (IXS) signals from solids and liquids. In contrast to typical IXS instruments where the spectrometer geometry is fixed and the incoming beam energy is scanned, the von Hamos geometry allows measurements to be made using a fixed optical arrangement with no moving parts. Thanks to the shot-to-shot capability of the spectrometer setup, we anticipate its application for the IXS technique at X-ray free electron lasers (XFELs). We discuss the capability of the spectrometer setup for IXS studies in terms of efficiency and required total incident photon flux for a given signal-to-noise ratio. The ultimate energy resolution of the spectrometer, which is a key parameter for IXS studies, was measured to the level of 150 meV at short crystal radius thanks to the application of segmented crystals for X-ray diffraction. The short working distance is a key parameter for spectrometer efficiency that is necessary to measure weak IXS signals.

The SwissFEL X-ray Free Electron Laser (XFEL) facility started construction at the Paul Scherrer Institute (Villigen, Switzerland) in 2013 and will be ready to accept its first users in 2018 on the Aramis hard X-ray branch. In the following sections we will summarize the various aspects of the project, including the design of the soft and hard X-ray branches of the accelerator, the results of SwissFEL performance simulations, details of the photon beamlines and experimental stations, and our first commissioning results.

The development of free electron laser (FEL) sources has provided an unprecedented bridge between the scientific communities working with ultrafast lasers and extreme ultraviolet (XUV) and X-ray radiation. Indeed, in recent years an increasing number of FEL-based applications have exploited methods and concepts typical of advanced optical approaches. In this context, we recently used a seeded FEL to demonstrate a four-wave-mixing (FWM) process stimulated by coherent XUV radiation, namely the XUV transient grating (X-TG). We hereby report on X-TG measurements carried out on a sample of silicon nitride (Si3N4). The recorded data bears evidence for two distinct signal decay mechanisms: one occurring on a sub-ps timescale and one following slower dynamics extending throughout and beyond the probed timescale range (100 ps). The latter is compatible with a slower relaxation (time decay > ns), that may be interpreted as the signature of thermal diffusion modes. From the peak intensity of the X-TG signal we could estimate a value of the effective third-order susceptibility which is substantially larger than that found in SiO2, so far the only sample with available X-TG data. Furthermore, the intensity of the time-coincidence peak shows a linear dependence on the intensity of the three input beams, indicating that the measurements were performed in the weak field regime. However, the timescale of the ultrafast relaxation exhibits a dependence on the intensity of the XUV radiation. We interpreted the observed behaviour as the generation of a population grating of free-electrons and holes that, on the sub-ps timescale, relaxes to generate lattice excitations. The background free detection inherent to the X-TG approach allowed the determination of FEL-induced electron dynamics with a sensitivity largely exceeding that of transient reflectivity and transmissivity measurements, usually employed for this purpose. This journal is © The Royal Society of Chemistry.

X-ray techniques have evolved over decades to become highly refined tools for a broad range of investigations. Importantly, these approaches rely on X-ray measurements that depend linearly on the number of incident X-ray photons. The advent of X-ray free electron lasers (XFELs) is opening the ability to reach extremely high photon numbers within ultrashort X-ray pulse durations and is leading to a paradigm shift in our ability to explore nonlinear X-ray signals. However, the enormous increase in X-ray peak power is a double-edged sword with new and exciting methods being developed but at the same time well-established techniques proving unreliable. Consequently, accurate knowledge about the threshold for nonlinear X-ray signals is essential. Herein we report an X-ray spectroscopic study that reveals important details on the thresholds for nonlinear X-ray interactions. By varying both the incident X-ray intensity and photon energy, we establish the regimes at which the simplest nonlinear process, two-photon X-ray absorption (TPA), can be observed. From these measurements we can extract the probability of this process as a function of photon energy and confirm both the nature and sub-femtosecond lifetime of the virtual intermediate electronic state., Scientific Reports, 6, ISSN:2045-2322

An ozone treatment system was investigated to analyze its impact on clinically relevant antibiotic resistant bacteria (ARB) and antibiotic resistant genes (ARGs). A concentration of 0.9±0.1g ozone per 1g DOC was used to treat conventional clarified wastewater. PCR, qPCR analyses, Illumina 16S Amplicon Sequencing, and PCR-DGGE revealed diverse patterns of resistances and susceptibilities of opportunistic bacteria and accumulations of some ARGs after ozone treatment. Molecular marker genes for enterococci indicated a high susceptibility to ozone. Although they were reduced by almost 99%, they were still present in the bacterial population after ozone treatment. In contrast to this, Pseudomonas aeruginosa displayed only minor changes in abundance after ozone treatment. This indicated different mechanisms of microorganisms to cope with the bactericidal effects of ozone. The investigated ARGs demonstrated an even more diverse pattern. After ozone treatment, the erythromycin resistance gene (ermB) was reduced by 2 orders of magnitude, but simultaneously, the abundance of two other clinically relevant ARGs increased within the surviving wastewater population (vanA, blaVIM). PCR-DGGE analysis and 16S-Amplicon-Sequencing confirmed a selection-like process in combination with a substantial diversity loss within the vital wastewater population after ozone treatment. Especially the PCR-DGGE results demonstrated the survival of GC-rich bacteria after ozone treatment.

Conventional wastewater treatment plants are ineffective in removing a broad range of micropollutants, resulting in the release of these compounds into the aquatic environment, including natural drinking water resources. Ozonation is a suitable treatment process for micropollutant removal, although, currently, little is known about the formation, behavior, and removal of transformation products (TP) formed during ozonation. We investigated the elimination of 30 selected micropollutants (pharmaceuticals, X-ray contrast media, industrial chemicals, and TP) by biological treatment coupled with ozonation and, subsequently, in parallel with two biological filters (BF) or granular activated carbon (GAC) filters. The selected micropollutants were removed to very different extents during the conventional biological wastewater treatment process. Ozonation (specific ozone consumption: 0.87 ± 0.29 gO3 gDOC(-1), hydraulic retention time: 17 ± 3 min) eliminated a large number of the investigated micropollutants. Although 11 micropollutants could still be detected after ozonation, most of these were eliminated in subsequent GAC filtration at bed volumes (BV) of approximately 25,000 m(3) m(-3). In contrast, no additional removal of micropollutants was achieved in the BF. Ozonation of the analgesic tramadol led to the formation of tramadol-N-oxide that is effectively eliminated by GAC filters, but not by BF. For the antiviral drug acyclovir, the formation of carboxy-acyclovir was observed during activated sludge treatment, with an average concentration of 3.4 ± 1.4 μg L(-1) detected in effluent samples. Subsequent ozonation resulted in the complete elimination of carboxy-acyclovir and led to the formation of N-(4-carbamoyl-2-imino-5-oxo imidazolidin)-formamido-N-methoxyacetetic acid (COFA; average concentration: 2.6 ± 1.0 μg L(-1)). Neither the BF nor the GAC filters were able to remove COFA. These results highlight the importance of considering TP in the evaluation of advanced wastewater treatment processes. The results further indicate that post-treatment of ozonated wastewater with GAC filtration seems to be more suitable than BF, due to the sorption of formed TP to the activated carbon.

The potential of four-wave mixing spectroscopy in combination with a discharge slit-jet source is explored by performing an analysis of the perturbations occurring between vibronic levels of the d 3 Π g , v ′ = 4 and b 3 Σ g - , v ′ = 16 states of C2. Unambiguous assignments of perturbed transitions are achieved by intermediate level labeling in the a 3 Π u state of the Swan system and results in the re-assignment of several line positions found in the literature. Furthermore, extra transitions are observed and assigned in a straightforward manner that originate from the perturbing b 3 Σ g - , v ′ = 16 electronic state. Significantly perturbed rotational transitions in the (4, 3) band of the Swan system are reported for the first time. A least-squares fit to 35 perturbed and perturbing transitions yields the vibronic origin, rotational and spin-spin coupling constant of the b 3 Σ g - , v ′ = 16 state as well as the spin-orbit and L-uncoupling parameters for the interaction between the d 3 Π g , v ′ = 4 and b 3 Σ g - , v ′ = 16 states.

In this paper, we demonstrate that degenerate and two-color resonant four-wave mixing spectroscopy is applicable for the sensitive and selective characterization of negative ions in a molecular beam environment. Results are shown for C, which is produced by discharging a mixture of acetylene and argon prior to supersonic expansion. Substantial signal-to-noise ratios of ≈ 150 show that the method is generally applicable for high-resolution optical double-resonance spectroscopy of negative ions. A detection sensitivity for C of ≈ 107 ions/cm3 is estimated. Copyright © 2010 John Wiley & Sons, Ltd.

Ultrafast electronic relaxation dynamics in cyclohexene are studied using femtosecond photoelectron imaging spectroscopy. Transient photoelectron kinetic energy distributions and photoelectron angular distributions are obtained and analyzed. Photoelectron bands are discussed and assigned to the ionization of the related electronic states. The formation process of cyclopentylcarbene, together with the ultrafast relaxation of the involved electronic states, is evidenced by the transient photoelectron images. The lifetime for the observed carbene is very short and determined to be 395 (±67) fs. The ionization dynamics of the produced cyclopentylcarbene are also discussed via an analysis of the corresponding photoelectron band.

Vibrational levels of the recently observed high-spin transition (1 Πu5−1 Πg5) of dicarbon [P. Bornhauser et al., J. Chem. Phys. 142, 094313 (2015)] are explored by applying non-linear double-resonant four-wave mixing and laser-induced fluorescence methods. The deperturbation of the d Πg3, υ = 8 and 1 Πg5, υ = 3 states results in accurate molecular constants for the υ = 3 “dark” quintet state. In addition, the spin-orbit interaction constant is determined and parameters for the upper Swan level d Πg3, υ = 8 are improved. The first excited vibrational state of 1 Πu5 is observed by performing perturbation-assisted intersystem crossing via “gateway” states in the d Πg3, υ=6∼1 Πg5,υ= 0 system. The rotationally resolved spectra yield 11 transitions to 1 Πu5, υ = 1 that include four spin-substates. Data reduction results in accurate molecular constants of this vibrational level in the shallow potential energy surface of this state. Finally, υ = 1 and 2 of the lower quintet state (1 Πg5) are measured by performi...

A complementary measurement method based on a home-built double-sided velocity map imaging setup is introduced. This method can simultaneously obtain time-resolved photoelectron imaging and fragment ion imaging. It has been successfully applied to investigate the ultrafast dynamics of the second singlet electronically excited state (S2) in m-xylene. Time-resolved photoelectron and ion signals derived from the initial populated S2 state are tracked following two-photon absorption of a pump pulse. Time-of-flight mass spectra (TOFMS) show that there are dominant parent ions and one fragment ions with methyl loss during such a process. According to the measured photoelectron images and fragment ions images, transient kinetic energy distributions and angular distributions of the generated photoelectrons and fragments are obtained and analyzed. Compared to stand-alone photoelectron imaging, the obtained fragment ion imaging is powerful for further understanding the mechanisms especially when the dissociation occurs during the pump-probe ionization. Two competing channels intersystem crossing T3←S2 and internal conversion S1←S2 are attributed to the deactivation of the S2 state. A lifetime of ∼50 fs for the initially excited S2 state, of ∼276 fs for the secondary populated S1 state, and of 5.76 ps for the T3 state is inferred.

The ozone layer which absorbs harmful solar UV radiation is an essential umbrella for human beings. However, a large number of exhausts of chlorine compounds including freon released by people in the atmosphere pose a great threat to the ozone layer. Freon dissociates into the product of chlorine radicals induced by UV sunlight radiation, which are found to be the main culprit for the destruction of atmospheric ozone. In this paper, time-of-flight mass spectrometry and velocity map imaging technique are coupled for investigating the multiphoton dissociation dynamics of Freon 1110 (C2Cl4, Tetrachloroethylene) induced by ultrafast short laser pulse on a femtosecond time scale at 800 nm. Fragments mass spectra of C2Cl4 are measured by time-of-flight mass spectrometry. Together with the parent ion C2Cl4+, two dominant fragment ions C2Cl3+ and C2Cl2+ are discovered in the multi-photon ionization and dissociation process in the experiment. By analyzing the above mass spectra, two corresponding photodissociation mechanisms are discussed and listed as follows: 1) C2Cl4+C2Cl3+ +Cl with single CCl bond breaking and direct production of Cl radical; 2) C2Cl4+C2Cl2+ +2Cl with double CCl bonds breaking and production of two Cl radicals. Also, ion images of these two observed fragment ions C2Cl3+ and C2Cl2+ are measured by velocity map imaging apparatus. The kinetic energy distributions of these two fragment ions are determined from the measured velocity map images. The kinetic energy distributions of both C2Cl3+ and C2Cl2+ can be well fitted by two Gaussion distributions. It indicates that both fragments C2Cl3+ and C2Cl2+ are from two production channels. The peak energies for each channel are fitted. More detailed photodissociation dynamics is obtained by analyzing the angular distribution of the generated fragment ions. The anisotropy parameter values are measured to be 0.46 (low energy channel) and 0.52 (high energy channel) for the fragment C2Cl3+, and 0.41 (low energy channel) and 0.66 (high energy channel) for the fragment C2Cl2+, respectively. The ratios between parallel transition and perpendicular transition are determined for all the observed channels for producing fragments C2Cl3+ and C2Cl2+. In addition, density functional theory calculations at a high-precision level are also performed on photodissociation dynamics for further analysis and discussion. The optimized geometries of ground state and ionic state of C2Cl4 are obtained and compared with density functional theory calculation at the level of B3LYP/6-311G++(d,p). The different structures of the ground and ionic states are given and discussed. The calculated information about ionic states of C2Cl4, including energy level and oscillator strength for the ionic excited states, is also given for analyzing the photodissociation dynamics of the C2Cl4 ions.

The first high-spin transition in C2 (1 (5)Πu - 1 (5)Πg) is observed by perturbation-facilitated optical-optical double resonance spectroscopy. The experiment is performed by applying unfolded two-color resonant four-wave mixing. C2 radicals in the initial a (3)Πu, v = 5 state are produced by using a discharge source in a molecular beam environment. The final quintet state is excited via intermediate "gateway" states exhibiting both substantial triplet and quintet character due to a perturbation between the 1 (5)Πg, v = 0 and the d (3)Πg, v = 6 states. Fifty seven rotational transitions in the P, Q, and R branches of all spin sub-states are measured and yield accurate molecular constants of the newly found upper level 1 (5)Πu. In addition, satellite transitions (ΔJ ≠ ΔN) are observed and allow an accurate determination of the spin-orbit constant. The results are compared with high-level ab initio computations at the multi-reference configuration interaction level of theory. The high-lying quintet state is found to be predissociative and displays a shallow potential that accommodates three vibrational levels only.

Perturbation-facilitated two-color resonant four-wave-mixing spectroscopy is realized to access the (dark) triplet manifold of the C3 molecule from the singlet X̃(1)Σg (+) ground state. The inherent nonlinear signal dependence and coherence of the technique result in a favorable detection of the excited triplet states of interest. The observation of a newly found (3)Δu electronic state is achieved by a two-step excitation via "gate-way" levels (i.e., singlet-triplet mixed levels). Additionally, by fixing the probe laser on a transition exhibiting mainly triplet-triplet character and scanning the pump laser, we demonstrate an effective spin-filtering in a four-wave mixing measurement where only transitions to the perturber (3)Σu(-) state appear exclusively in an otherwise congested spectral range of the Comet band. Ab initio calculations of excited triplet states complement our analysis with the electronic assignment of the observed resonances.

In this work we present a deperturbation study of the d (3)Π(g), v=6 state of C(2) by double-resonant four-wave mixing spectroscopy. Accurate line positions of perturbed transitions are unambiguously assigned by intermediate level labeling. In addition, extra lines are accessible by taking advantage of the sensitivity and high dynamic range of the technique. These weak spectral features originate from nearby-lying dark states that gain transition strength through the perturbation process. The deperturbation analysis of the complex spectral region in the (6,5) and (6,4) bands of the Swan system (d(3)Π(g)-a (3)Π(u)) unveils the presence of the energetically lowest high-spin state of C(2) in the vicinity of the d (3)Π(g), v=6 state. The term energy curves of the three spin components of the d state cross the five terms of the 1 (5)Π(g) state at rotational quantum numbers N ≤ 11. The spectral complexity for transitions to the v = 6 level of d (3)Π(g) state is further enhanced by an additional perturbation at N = 19 and 21 owing to the b (3)Σ(g)(-), v=19 state. The spectroscopic characterization of both dark states is accessible by the measurement of 122 "window" levels. A global fit of the positions to a conventional Hamiltonian for a linear diatomic molecule yields accurate molecular constants for the quintet and triplet perturber states for the first time. In addition, parameters for the spin-orbit and L-uncoupling interaction between the electronic levels are determined. The detailed deperturbation study unravels major issues of the so-called high-pressure bands of C(2). The anomalous nonthermal emission initially observed by Fowler in 1910 [Mon. Not. R. Astron. Soc. 70, 484 (1910)] and later observed in numerous experimental environments are rationalized by taking into account "gateway" states, i.e., rotational levels of the d (3)Π(g), v=6 state that exhibit significant (5)Π(g) character through which all population flows from one electronic state to the other.

We discuss our recently reported femtosecond (fs) X-ray emission spectroscopy results on the ligand dissociation and recombination in nitrosylmyoglobin (MbNO) in the context of previous studies on ferrous haem proteins. We also present a preliminary account of femtosecond X-ray absorption studies on MbNO, pointing to the presence of more than one species formed upon photolysis.

Light-driven sodium pumps actively transport small cations across cellular membranes1. These pumps are used by microorganisms to convert light into membrane potential and have become useful optogenetic tools with applications in neuroscience. Although the resting state structures of the prototypical sodium pump Krokinobacter eikastus rhodopsin 2 (KR2) have been solved2,3, it is unclear how structural alterations over time allow sodium to be translocated against a concentration gradient. Here, using the Swiss X-ray Free Electron Laser4, we have collected serial crystallographic data at ten pump–probe delays from femtoseconds to milliseconds. High-resolution structural snapshots throughout the KR2 photocycle show how retinal isomerization is completed on the femtosecond timescale and changes the local structure of the binding pocket in the early nanoseconds. Subsequent rearrangements and deprotonation of the retinal Schiff base open an electrostatic gate in microseconds. Structural and spectroscopic data, in combination with quantum chemical calculations, indicate that a sodium ion binds transiently close to the retinal within one millisecond. In the last structural intermediate, at 20 milliseconds after activation, we identified a potential second sodium-binding site close to the extracellular exit. These results provide direct molecular insight into the dynamics of active cation transport across biological membranes. ISSN:0028-0836 ISSN:1476-4687

We present an approach to determine the absolute thickness profile of flat liquid jets, which takes advantage of the information of thin film interference combined with light absorption, both captured in a single microscopic image. The feasibility of the proposed method is demonstrated on our compact experimental setup used to generate micrometer thin, free-flowing liquid jet sheets upon collision of two identical laminar cylindrical jets. Stable operation was achieved over several hours of the flat jet in vacuum (10-4 mbar), making the system ideally suitable for soft x-ray photon spectroscopy of liquid solutions. We characterize the flat jet size and thickness generated with two solvents, water and ethanol, employing different flow rates and nozzles of variable sizes. Our results show that a gradient of thickness ranging from a minimal thickness of 2 µm to over 10 µm can be found within the jet surface area. This enables the tunability of the sample thickness in situ, allowing the optimization of the transmitted photon flux for the chosen photon energy and sample. We demonstrate the feasibility of x-ray absorption spectroscopy experiments in transmission mode by measuring at the oxygen K-edge of ethanol. Our characterization method and the description of the experimental setup and its reported performance are expected to expand the range of applications and facilitate the use of flat liquid jets for spectroscopy experiments.