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The super-bandgap laser irradiation of the in situ prepared As-S chalcogenide films was found to cause drastic structural transformations and unexpected selective diffusion processes, leading to As enrichment on the nanolayer surface. Excitation energy dependent synchrotron radiation photoelectron spectroscopy showed complete reversibility of the molecular transformations and selective laser-driven mass transport during "laser irradiation"-"thermal annealing" cycles. Molecular modeling and density functional theory calculations performed on As-rich cage-like clusters built from basic structural units indicate that the underlying microscopic mechanism of laser induced transformations is connected with the realgar-pararealgar transition in the As-S structure. The detected changes in surface composition as well as the related local and molecular structural transformations are analyzed and a model is proposed and discussed in detail. It is suggested that the formation of a concentration gradient is a result of bond cleavage and molecular reorientation during transformations and anisotropic molecular diffusion. [ABSTRACT FROM AUTHOR]

Superfluid helium nanodroplets are often considered as transparent and chemically inert nanometer-sized cryo-matrices for high-resolution or time-resolved spectroscopy of embedded molecules and clusters. On the other hand, when the helium nanodroplets are resonantly excited with XUV radiation, a multitude of ultrafast processes are initiated, such as relaxation into metastable states, formation of nanoscopic bubbles or excimers, and autoionization channels generating low-energy free electrons. Here, we discuss the full spectrum of ultrafast relaxation processes observed when helium nanodroplets are electronically excited. In particular, we perform an in-depth study of the relaxation dynamics occurring in the lowest 1s2s and 1s2p droplet bands using high resolution, time-resolved photoelectron spectroscopy. The simplified excitation scheme and improved resolution allow us to identify the relaxation into metastable triplet and excimer states even when exciting below the droplets' autoionization threshold, unobserved in previous studies. [ABSTRACT FROM AUTHOR]

A combined experimental and theoretical approach has been used to disentangle the fundamental mechanisms of the fragmentation of the three isomers of nitroimidazole induced by vacuum ultra-violet (VUV) radiation, namely, 4-, 5-, and 2-nitroimidazole. The results of mass spectrometry as well as photoelectron-photoion coincidence spectroscopy display striking differences in the radiation-induced decomposition of the different nitroimidazole radical cations. Based on density functional theory (DFT) calculations, a model is proposed which fully explains such differences, and reveals the subtle fragmentation mechanisms leading to the release of neutral species like NO, CO, and HCN. Such species have a profound impact in biological media and may play a fundamental role in radiosensitising mechanisms during radiotherapy. [ABSTRACT FROM AUTHOR]

As-deposited, annealed, and in situ As2S3 nanolayers, illuminated by blue (405 nm) and red (650 nm) laser light, were studied using synchrotron radiation photoelectron spectroscopy and DFT electronic structure calculations. Changes in composition and local atomic coordination occurring in the irradiated region of As2S3 films were monitored by analysis of As 3d and S 2p core levels. These studies show that both the thermal treatment and the red laser illumination of As2S3 nanolayers decrease the concentration of homopolar (As-As and S-S) bonds. From the other hand, an increasing concentration of As-rich structural units (s.u.) with a homopolar As-As bond was observed under in situ blue laser illumination of As2S3 nanolayers. Molecular orbital energies were calculated for different As- and S-centered s.u. and used for the interpretation of the core levels and valence band spectra. The surface local structure of the As2S3 nanolayers and its photoinduced transformation are discussed in detail. [ABSTRACT FROM AUTHOR]

The ionization dynamics of pure He nanodroplets irradiated by Extreme ultraviolet radiation is studied using Velocity-Map Imaging PhotoElectron-PhotoIon COincidence spectroscopy. We present photoelectron energy spectra and angular distributions measured in coincidence with the most abundant ions He+, He2+, and He3+. Surprisingly, below the autoionization threshold of He droplets, we find indications for multiple excitation and subsequent ionization of the droplets by a Penning-like process. At high photon energies we observe inelastic collisions of photoelectrons with the surrounding He atoms in the droplets. [ABSTRACT FROM AUTHOR]

High-intensity extreme ultraviolet (XUV) pulses from a free-electron laser can be used to create a nanoplasma in clusters. In reference Michiels et al (2020 Phys. Chem. Chem. Phys. 22 7828–34) we investigated the formation of excited states in an XUV-induced nanoplasma in ammonia clusters. In the present article we expand our previous study with a detailed analysis of the nanoplasma evolution and ion kinetics. We use a time-delayed UV laser as probe to ionize excited states of H and in the XUV-induced plasma. Employing covariance mapping techniques, we show that the correlated emission of protons plays an important role in the plasma dynamics. The time-dependent kinetic energy of the ions created by the probe laser is measured, revealing the charge neutralization of the cluster happens on a sub-picosecond timescale. Furthermore, we observe ro-vibrationally excited molecular hydrogen ions being ejected from the clusters. We rationalize our data through a qualitative model of a finite-size non-thermal plasma. [ABSTRACT FROM AUTHOR]

We report on the observation of time-resolved quantum beats in the helium fluorescence from the transition 1s3p → 1s2s, where the initial state is excited by XUV free electron laser radiation. The quantum beats originate from the Zeeman splitting of the magnetic substates due to an external magnetic field. We perform a systematic study of this effect and discuss the possibilities of studying this phenomenon in the XUV and x-ray regime. [ABSTRACT FROM AUTHOR]

Autoionizing hole states with electron configuration nsnp5mp are studied in Ar+ and Kr+. Total and partial photoionization cross sections, photoelectron spectra and photoelectron angular distributions in the region of the resonances are obtained theoretically in extensive R-matrix calculations. The states of Ar+ are observed by means of excitation by a free-electron laser operating in the vacuum- and extreme-ultraviolet wavelength regime combined with angle-resolved photoelectron spectroscopy. Fine tuning of the photon energy allows scanning of the resonances and the observation of the shape of the partial cross section ratio, as well as the asymmetry parameter of the angular distribution of the photoelectrons. The calculations are in good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

The ionization dynamics of helium droplets irradiated by intense, femtosecond extreme ultraviolet (XUV) pulses is investigated in detail by photoelectron spectroscopy. Helium droplets are resonantly excited to atomic-like 2p states with a photon energy of 21.5 eV and autoionize by interatomic Coulombic decay (ICD). A complex evolution of the electron spectra as a function of droplet size (250 to 106 He atoms per droplet) and XUV intensity (109–1012 W cm−2) is observed, ranging from narrow atomic-like peaks that are due to binary autoionization, to an unstructured feature characteristic of electron emission from a nanoplasma. The experimental results are analyzed and interpreted with the help of a numerical simulation based on rate equations taking into account all relevant processes—multi-step ionization, electronic relaxation, ICD, secondary inelastic collisions, desorption of electronically excited atoms, and collective autoionization (CAI). [ABSTRACT FROM AUTHOR]

The inner shell excitation of pyrimidine and some halogenated pyrimidines near the C and N K-edges has been investigated experimentally by near edge x-ray absorption fine structure spectroscopy and theoretically by density functional theory calculations. The selected targets, 5-Br-pyrimidine, 2-Br-pyrimidine, 2-Cl-pyrimidine, and 5-Br-2-Cl-pyrimidine, allow the effects of the functionalization of the pyrimidine ring to be studied either as a function of different halogen atoms bound to the same molecular site or as a function of the same halogen atom bound to different molecular sites. The results show that the individual characteristics of the different spectra of the substituted pyrimidines can be rationalized in terms of variations in electronic and geometrical structures of the molecule depending on the localization and the electronegativity of the substituent. [ABSTRACT FROM AUTHOR]

We have studied the adsorption of low dimensional gold on ceria, produced by evaporation onto the surface. The interaction of gold with CeO2(111) layers was investigated with x-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, and resonant photoelectron spectroscopy (RPES). Gold was deposited in steps onto a 1.5 nm thick CeO2(111) layer epitaxially grown on a Cu(111) substrate. The RPES showed a partial Ce4+→Ce3+ reduction, observed as a resonant enhancement of the 4f level of the Ce3+ species. This can be explained by possible creation of a new Au+ ionic state. The observed effects are stronger for Au deposition at room temperature than at 250 °C. The obtained results are in agreement with already published density functional theory calculations reporting weakening of bond between the oxygen and the Ce atoms in ceria caused by the presence of gold. [ABSTRACT FROM AUTHOR]

The electronic structure of cluster assembled nanostructured TiO2 thin films has been investigated by resonant photoemission experiments with photon energies across the Ti L2,3 edge. The samples were produced by supersonic cluster beam deposition with a pulsed microplasma cluster source. The valence band shows resonance enhancements in the binding energy region between 4 and 8 eV, populated by O 2p and hybridized Ti 3d states, and in the region about 1 eV below the Fermi level associated with defects related Ti 3d states. The data show that in as-deposited films Ti atoms are mainly fully (sixfolds) coordinated to oxygen atoms in octahedral symmetry and only a small fraction is in a broken symmetry environment. Since resonant photoemission is closely linked to the local electronic and structural configurations around the Ti atom, it is possible to correlate the resonant photoemission intensity and lineshape with the presence of defects of the films and with the degree of hybridization between the titanium and oxygen atoms. [ABSTRACT FROM AUTHOR]

The electronic structure of the single molecule magnet system {M[Fe(L1)2]3}·4CHCl3 [M=Fe,Cr;L1=CH3N(CH2CH2O)22-] has been studied using x-ray photoelectron spectroscopy, x-ray-absorption spectroscopy, soft-x-ray emission spectroscopy, as well as theoretical density-functional-based methods. There is a good agreement between theoretical calculations and experimental data. The valence band mainly consists of three bands between 2 and 30 eV. Both theory and experiments show that the top of the valence band is dominated by the hybridization between Fe 3d and O 2p bands. From the shape of the Fe 2p spectra it is argued that Fe in the molecule is most likely in the 2+ charge state. Its neighboring atoms (O,N) exhibit a magnetic polarization yielding effective spin S=5/2 per iron atom, giving a high-spin state molecule with a total S=5 effective spin for the case of M=Fe. [ABSTRACT FROM AUTHOR]

The CO and CO2 carbon and oxygen Auger spectra have been measured by electron impact and compared with accurate theoretical calculations accounting for the effects of the dynamics of the nuclei on the energy and linewidth of the Auger bands. The calculations for CO were previously published [L. S. Cederbaum et al., J. Chem. Phys. 95, 6634 (1991)], while for CO2 they are new and presented here for the first time. For both molecules, particular attention has been paid to the low-kinetic-energy region of the spectra, which corresponds to doubly charged ion states with the two holes mainly localized in the inner valence region. New bands have been observed. It is shown that a proper consideration of the vibrational broadening and shift of the bands due to the dynamics of the nuclei is needed to assign these features. For CO, very large energy shifts between corresponding features in the C 1s and O 1s spectra have been observed, confirming the theoretical predictions of 1991. The new computed spectra of CO2 allow a very accurate analysis of the experiments over the whole energy range. [ABSTRACT FROM AUTHOR]

The behavior of liquid Au-Si alloys on Si surfaces covered by a monolayer of gold has been investigated by ultrahigh-vacuum scanning electron microscopy. On the (111) surface, the alloy displays a constant contact angle with the surface from the eutectic temperature up to a temperature of 650°C and thereafter the contact angle increases linearly with temperature. As observed in previous work, the shape of the liquid droplets changes from circular at lower temperature to hexagonal at higher temperature. In contrast, on the (100) surface, the contact angle increases linearly from the eutectic temperature to high temperature. The behavior of the shape of the droplets is, however, reversed: it is polygonal (octagonal) at lower temperature and becomes round at higher temperature. This behavior is explained in terms of the relative surface energy of the two surfaces and changing line tension of the liquid-solid-vapor phase line. In addition, the behavior of Au-Si droplets on vicinal and patterned surfaces of Si has been examined. The droplets cause step bunching and modify the local surface structure. Solidification of the droplets on all surfaces leads to phase separation. [ABSTRACT FROM AUTHOR]

As40Se60 nanolayers, as-deposited, annealed and in situ illuminated by green (532 nm) laser light, were studied using synchrotron radiation photoelectron spectroscopy. Changes in composition and local atomic coordination occurring in the irradiated region of As40Se60 films were monitored by analysis of As 3d and Se 3d core levels. It was found that the thermal treatment causes a decrease of the concentration of homopolar (As–As and Se–Se) bonds. On the other hand, an increasing concentration of both As-rich and Se-rich structural units (s.u.) with homopolar As–As and Se–Se bonds was observed under in situ green laser illumination of As40Se60 nanolayers. This process appeared to be reversible for a few sequences of annealing and illuminating of the sample. After a few cycles storing at ambient conditions the As40Se60 film composition was gradually changing, i.e. the aging effect was detected due to a drastic loss of As under ambient conditions. The surface local structure of the As40Se60 nanolayers and their photoinduced transformation are discussed in detail. [ABSTRACT FROM AUTHOR]

Several studies of the magnetic properties of Sr2FeMoO6, a half metallic double perovskite showing large magneto resistance effects at room temperature, by means of site specific x-ray absorption spectroscopy (XAS) have led to very different results concerning the Fe valence state. We present a detailed study of a Sr2FeMoO6 sample, which has been probed by means of XAS and x-ray magnetic circular dichroism (XMCD) over several years. We find a mixed valent Fe2+, Fe3+ state, which shifts toward Fe3+ with time. An understanding of such a chemical change is of importance for potential applications of Sr2FeMoO6 and related transition metal oxides. [ABSTRACT FROM AUTHOR]

The first steps in photochemical processes, such as photosynthesis or animal vision, involve changes in electronic and geometric structure on extremely short time scales. Time-resolved photoelectron spectroscopy is a natural way to measure such changes, but has been hindered hitherto by limitations of available pulsed light sources in the vacuum-ultraviolet and soft Xray spectral region, which have insufficient resolution in time and energy simultaneously. The unique combination of intensity, energy resolution, and femtosecond pulse duration of the FERMI-seeded free-electron laser can now provide exceptionally detailed information on photoexcitation–deexcitation and fragmentation in pump-probe experiments on the 50-femtosecond time scale. For the prototypical system acetylacetone we report here electron spectra measured as a function of time delay with enough spectral and time resolution to follow several photoexcited species through well-characterized individual steps, interpreted using state-of-the-art static and dynamics calculations. These results open the way for investigations of photochemical processes in unprecedented detail. [ABSTRACT FROM AUTHOR]

We have performed a high-resolution synchrotron radiation photoelectron spectroscopy study of the initial growth stages of the ZnPd near-surface alloy on Pd(111), complemented by scanning tunnelling microscopy data. We show that the chemical environment for surfaces containing less than half of one monolayer of Zn is chemically distinct from subsequent layers. Surfaces where the deposition is performed at room temperature contain ZnPd islands surrounded by a substrate with dilute Zn substitutions. Annealing these surfaces drives the Zn towards the substrate top-layer, and favours the completion of the first 1 : 1 monolayer before the onset of growth in the next layer. [ABSTRACT FROM AUTHOR]

Free electron lasers (FELs) offer the unprecedented capability to study reaction dynamics and image the structure of complex systems. When multiple photons are absorbed in complex systems, a plasma-like state is formed where many atoms are ionized on a femtosecond timescale. If multiphoton absorption is resonantly-enhanced, the system becomes electronically-excited prior to plasma formation, with subsequent decay paths which have been scarcely investigated to date. Here, we show using helium nanodroplets as an example that these systems can decay by a new type of process, named collective autoionization. In addition, we show that this process is surprisingly efficient, leading to ion abundances much greater than that of direct single-photon ionization. This novel collective ionization process is expected to be important in many other complex systems, e.g. macromolecules and nanoparticles, exposed to high intensity radiation fields. [ABSTRACT FROM AUTHOR]

The photoelectron angular distributions (PADs) have been investigated for infrared (IR) ionization of He atoms excited to Rydberg states by extreme ultraviolet free-electron laser pulses. The experiment was carried out with two pulses which do not overlap in time. Depending on the intensity of the IR pulses, one IR photon ionization or additionally two-photon above-threshold ionization is observed. For low IR intensity, the PAD is well described by a contribution of s and d partial waves in accordance with early experiments. At high IR intensity, the PAD for two IR photon ionization clearly shows the contribution of higher partial waves. The experimental data are compared with the results of theoretical calculations based on solving the time-dependent Schrödinger equation. [ABSTRACT FROM AUTHOR]

We have investigated nonlinear processes in small molecules by x-ray photoelectron spectroscopy using the Linac Coherent Light Source free electron laser, and by simulations. The main focus of the experiments was the formation of the two-site double core-hole (tsDCH) states in the molecules CO2, N2O and N2. These experiments are described in detail and the results are compared with simulations of the photoelectron spectra. The double core-hole states, and in particular the tsDCH states, have been predicted to be highly sensitive to the chemical environment. The theory behind this chemical sensitivity is validated by the experiments. Furthermore, our simulations of the relative integrated intensities of the peaks associated with the nonlinear processes show that this type of simulation, in combination with experimental data, provides a useful tool for estimating the duration of ultra-short x-ray pulses. [ABSTRACT FROM AUTHOR]

The article reports on the commissioning of the ALBA synchrotron light facility near Barcelona, Spain after its operating license was awarded by the Spanish State Secretary for Energy on May 8, 2011. The facility has successfully stored 100 milliAmpere (mA) of current and three giga electron volt (GeV) of energy on April 1, 2011. Furthermore, the photons and electron beam in the facility will be used for diagnostic purposes and that it was co-financed by the Spanish and Calatan governments.

We present a combined experimental and theoretical investigation of the valence electronic structure of porphyrin-derived molecules. The valence photoemission spectra of the free-base tetraphenylporphyrin and of the octaethylporphyrin molecule were measured using synchrotron radiation and compared with theoretical spectra calculated using the GW method and the density-functional method within the generalized gradient approximation. Only the GW results could reproduce the experimental data. We found that the contribution to the orbital energies due to electronic correlations has the same linear behavior in both molecules, with larger deviations in the vicinity of the HOMO level. This shows the importance of adequate treatment of electronic correlations in these organic systems. [ABSTRACT FROM AUTHOR]

Elettra has remained open over the last year, as science is an essential service, and the staff and users have continued to carry out research, some of which is closely related to the SARS-CoV-2 coronavirus. The COVID-19 pandemic changed the world of work, and it will probably never be the same again. Like all large-scale facilities, Elettra has adopted many measures and innovations to deal with the pandemic, covering safety precautions, management systems, changes to working practices and modifications to research priorities. [Extracted from the article]

We report the Near Edge X-ray Absorption Fine Structure Spectra (NEXAFS) of a series of oxygencontaining organic molecules, namely formaldehyde, acetaldehyde, acetone, formic acid, methanol and dimethyl ether (DME), measured with high resolution at the carbon and oxygen edges. A vibrational progression has been observed at the oxygen 1s → π[sup *] resonance of formaldehyde, indicating that this state is bound with an excited state C=O stretching frequency of 136 meV. The spectra are compared with previous measurements and the applicability of the chromophore concept is tested for the functional groups present in these molecules. [ABSTRACT FROM AUTHOR]

At present the only surface electron microscope which allows true characteristic XPEEM (photoemission electron microscopy using synchrotron radiation) and structural characterization is the spectroscopic LEEM developed at the Technical University Clausthal in the early nineties. This instrument has in the past been used mainly for LEEM studies of various surface and thin film phenomena, because it had very limited access to synchrotron radiation. Now the microscope is connected quasipermanently to the undulator beamline 6.2 at the storage ring ELETTRA, operating successfully since the end of 1996 under the name SPELEEM (Spectroscopic PhotoEmission and Low Energy Electron Microscope). The high brightness of the ELETTRA light source, together with an optimized instrument, results in a spatial resolution better than 25 nm and an energy resolution better than 0.5 eV in the XPEEM mode. The instrument can be used alternately for XPEEM, LEEM, LEED (low energy electron diffraction), MEM (mirror electron microscopy) and other imaging modes, depending upon the particular problem studied. The combination of these imaging modes allows a comprehensive characterization of the specimen. This is of particular importance when the chemical identification of structurar features is necessary for the understanding of a surface or thin film process. In addition, PED (photoelectron diffraction) and VPEAD (valence photoelectron angular distribution) of small selected areas give local atomic configuration and band structure information, respectively. [ABSTRACT FROM AUTHOR]

The third generation sources of soft X-rays, ALS and ELETTRA, have been producing data in surface and interface science for over two years. The impact of their ultrahigh brightness is quite evident, particularly as far as photoemission techniques are concerned. We illustrate this point with a short review of very recent examples from ELETTRA, in the specific domains of time resolution, high lateral resolution and high energy resolution. [ABSTRACT FROM AUTHOR]

The electronic structure of croconic acid in the gas phase has been investigated by means of core level and valence band photoemission spectroscopy and compared with hybrid Heyd-Scuseria-Ernzerhof density functional theory calculations. The results have been compared with the corresponding ones of the condensed phase. In the gas phase, due to the absence of hydrogen bond intermolecular interactions, the O 1 s core level spectrum shows a shift of binding energy between the hydroxyl (O-H) and the carbonyl group (C=O) of 2.1 eV, which is larger than the condensed phase value of 1.6 eV. Interestingly, such a shift decreases exponentially with the increase of the O-H distance calculated from theory. The significant differences between the gas and condensed phase valence band spectra highlight the important role played by the hydrogen bonding in shaping the electronic structure of the condensed phase. [ABSTRACT FROM AUTHOR]