Letteratura scientifica selezionata sul tema "Vaccum Ultraviolet absorption spectroscopy with synchrotron radiation"

A high-precision XYZ translator was developed for the microanalysis of electronic structures and chemical compositions on material surfaces by electron spectroscopy techniques, such as photoelectron spectroscopy and absorption spectroscopy, utilizing the vacuum ultraviolet and soft X-ray synchrotron radiation at an undulator beamline BL-13B at the Photon Factory. Using the high-precision translator, the profile and size of the undulator beam were estimated. They were found to strongly depend on the photon energy but were less affected by the polarization direction. To demonstrate the microscopic measurement capability of an experimental apparatus incorporating a high-precision XYZ translator, the homogeneities of an SnO film and a naturally grown anatase TiO2 single crystal were investigated using X-ray absorption and photoemission spectroscopies. The upgraded system can be used for elemental analyses and electronic structure studies at a spatial resolution in the order of the beam size.

Since spring 2019 an experimental setup consisting of an electron spectrometer and an ion time-of-flight mass spectrometer for diluted samples has been available for users at the FinEstBeAMS beamline of the MAX IV Laboratory in Lund, Sweden. The setup enables users to study the interaction of atoms, molecules, (molecular) microclusters and nanoparticles with short-wavelength (vacuum ultraviolet and X-ray) synchrotron radiation and to follow the electron and nuclear dynamics induced by this interaction. Test measurements of N2 and thiophene (C4H4S) molecules have demonstrated that the setup can be used for many-particle coincidence spectroscopy. The measurements of the Ar 3p photoelectron spectra by linear horizontal and vertical polarization show that angle-resolved experiments can also be performed. The possibility to compare the electron spectroscopic results of diluted samples with solid targets in the case of Co2O3 and Fe2O3 at the Co and Fe L 2,3-absorption edges in the same experimental session is also demonstrated. Because the photon energy range of the FinEstBeAMS beamline extends from 4.4 eV up to 1000 eV, electron, ion and coincidence spectroscopy studies can be executed in a very broad photon energy range.

We report absolute photoabsorption cross sections for gas-phase 2- and 5-bromopyrimidine in the 3.7–10.8 eV energy range, in a joint theoretical and experimental study. The measurements were carried out using high-resolution vacuum ultraviolet synchrotron radiation, with quantum chemical calculations performed through the nuclear ensemble approach in combination with time-dependent density functional theory, along with additional Franck–Condon Herzberg–Teller calculations for the first absorption band (3.7–4.6 eV). The cross sections of both bromopyrimidines are very similar below 7.3 eV, deviating more substantially from each other at higher energies. In the 7.3–9.0 eV range where the maximum cross-section is found, a single and broad band is observed for 5-bromopyrimidine, while more discernible features appear in the case of 2-bromopyrimidine. Several π* ← π transitions account for the most intense bands, while weaker ones are assigned to transitions involving the nitrogen and bromine lone pairs, the antibonding σ*Br orbital, and the lower-lying Rydberg states. A detailed comparison with the available photo-absorption data of bromobenzene is also reported. We have found significant differences regarding the main absorption band, which is more peaked in bromobenzene, becoming broader and shifting to higher energies in both bromopyrimidines. In addition, there is a significant suppression of vibrational structures and of Rydberg states in the pair of isomers, most noticeably for 2-bromopyrimidine.

Context. Absorption lines of H2 and HD molecules observed at high redshift in the line of sight towards quasars are a test ground to search for variation of the proton-to-electron mass ratio μ. For this purpose, results from astronomical observations are compared with a compilation of molecular data of the highest accuracy, obtained in laboratory studies as well as in first-principles calculations. Aims. A comprehensive line list is compiled for H2 and HD absorption lines in the Lyman (B1Σu+ − X1Σg+) and Werner (C1Πu − X1Σg+) band systems up to the Lyman cutoff at 912 Å. Molecular parameters listed for each line i are the transition wavelength λi, the line oscillator strength fi, the radiative damping parameter of the excited state Γi, and the sensitivity coefficient Ki for a variation of the proton-to-electron mass ratio. Methods. The transition wavelengths λi for the H2 and HD molecules are determined by a variety of advanced high-precision spectroscopic experiments involving narrowband vacuum ultraviolet lasers, Fourier-transform spectrometers, and synchrotron radiation sources. Results for the line oscillator strengths fi, damping parameters Γi, and sensitivity coefficients Ki are obtained in theoretical quantum chemical calculations. Results. A new list of molecular data is compiled for future analyses of cold clouds of hydrogen absorbers, specifically for studies of μ-variation from quasar data. The list is applied in a refit of quasar absorption spectra of B0642–5038 and J1237+0647 yielding constraints on a variation of the proton-to-electron mass ratio Δμ/μ consistent with previous analyses.

A comparative study has been made for the non-catalyst based few layer graphene (FLG) and Fe-catalyst based multiwall carbon nanotubes (MWCNTs). Magnetic and electronic properties of FLG and MWCNTs were studied using magnetic M-H hysteresis loops and synchrotron radiation based X-ray absorption fine structure spectroscopy measurements. Structural defects and electronic and bonding properties of FLG/MWCNTs have been studied using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). The work functions of FLG and MWCNTs are 4.01 eV and 3.79 eV, respectively, obtained from UPS (He-I) spectra. UPS (He-II) results suggest that the density of states (DOS) of MWCNTs is higher than FLG and is consistent with Raman spectroscopy result that shows the defect of MWCNTs is higher than FLG. The magnetic coercivity (Hc) of the MWCNTs (~750 Oe) is higher than FLG (~85 Oe) which could be used for various technological magnetic applications.

Experimental characterization of the structural, electronic and dynamic properties of dilute systems in aqueous solvents, such as nanoparticles, molecules and proteins, are nowadays an open challenge. X-ray absorption spectroscopy (XAS) is probably one of the most established approaches to this aim as it is element-specific. However, typical dilute systems of interest are often composed of light elements that require extreme-ultraviolet to soft X-ray photons. In this spectral regime, water and other solvents are rather opaque, thus demanding radical reduction of the solvent volume and removal of the liquid to minimize background absorption. Here, we present an experimental endstation designed to operate a liquid flat jet of sub-micrometre thickness in a vacuum environment compatible with extreme ultraviolet/soft XAS measurements in transmission geometry. The apparatus developed can be easily connected to synchrotron and free-electron-laser user-facility beamlines dedicated to XAS experiments. The conditions for stable generation and control of the liquid flat jet are analyzed and discussed. Preliminary soft XAS measurements on some test solutions are shown.

AbstractThe electronic state spectroscopy of 2(5H)-thiophenone, C4H4OS, has been investigated by high-resolution vacuum ultraviolet photoabsorption in the 3.76–10.69 eV energy range using synchrotron radiation, together with novel quantum chemical calculations performed at the equation of motion coupled cluster singles and doubles (EOM-CCSD) level of theory. The major electronic transitions have been assigned to valence and Rydberg character, with relevant C=O, C=C and C–C stretching vibrations across the entire absorption spectrum. Photolysis lifetimes in the Earth’s atmosphere (0–50 km altitude) have been estimated from the absolute photoabsorption cross-sections, indicating that solar photolysis can be expected to be a strong sink mechanism. Graphical abstract

Illumination stress (IS) and negative bias under illumination stress (NBIS) cause considerable device instability in thin-film transistors based on amorphous In–Ga–Zn–O (a-IGZO). Models using in-gap states are suggested to explain device instability. Therefore, to provide reliably their density of states (DOS), this study investigated the valence band, conduction band, and in-gap states of an a-IGZO thin film. The DOS of in-gap states was directly determined in a dynamic range of six orders of magnitude through constant final state yield spectroscopy (CFS-YS) using low-energy and low-flux photons. Furthermore, light irradiation irreversibly induced extra in-gap states near the Fermi level and shifted the Fermi level to the vacuum level side, which should be related to the device instability due to IS and NBIS. Hard x-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy using synchrotron radiation observed the large DOS of in-gap states near the Fermi level as in previous works. Here, we reveal that they are not intrinsic electronic states of undamaged a-IGZO, but induced by the intense measurement light of synchrotron radiation. This study demonstrates that CFS-YS is useful for determining the reliable DOS of the in-gap states for samples that are sensitive to light irradiation. The absorption spectrum measured through photothermal deflection spectroscopy is interpreted based on DOS directly determined via photoemission spectroscopies. This indicates that the line shape in the energy region below the region assigned to the Urbach tail in previous works actually roughly reflects the DOS of occupied in-gap states.

AbstractCerium doped Gd3Ga3Al2O12 (GGAG) single crystals as well as GGAG:Ce single crystals co-doped by divalent (Mg2+, Ca2+) and tetravalent (Zr4+, Ti4+) ions have been studied by means of time-resolved luminescence as well as the excitation luminescence spectroscopy in vacuum ultraviolet (VUV) and soft X-ray (XUV) spectral range. Tunable laser excitation was applied for time-resolved experiments in order to obtain luminescence decay curves under excitations in Ce3+, Gd3+ and excitonic absorption bands. The influence of the co-dopant ions on the Ce3+ luminescence decay kinetics is elucidated. The fastest luminescence decay was observed for the Mg2+ co-doped crystals under any excitation below bandgap energy indicating the perturbation of the 5d states of Ce3+ by Mg2+ ions. Synchrotron radiation was utilized for the luminescence excitation in the energy range from 4.5 to 800 eV. Special attention was paid to the analysis of Ce3+ excitation spectra in VUV and XUV spectral range where multiplication of electronic excitation (MEE) processes occur. Our results demonstrated that GGAG:Ce single crystals co-doped by Mg2+ ions as well as the GGAG:Ce crystal annealed in vacuum reveal the most efficient excitation of Ce3+ emission in VUV-XUV excitation range. The role of intrinsic defects in MEE processes in the co-doped as well as in the annealed GGAG:Ce single crystals is discussed.

AbstractSince the early 1980s, the Antarctic environment has served as a natural field laboratory for researchers to investigate the effects of stratospheric ozone depletion, which has resulted in increased surface ultraviolet radiation levels. However, its effective threats still present gaps. We report new pieces of evidence of increased ultraviolet radiation impacting West Antarctica sea salt aerosols. Salt aerosols, particularly in the Southern Ocean Sea, play an important role in the radiative earth balance. To disclose the molecular details of sea salt aerosols, we used a synchrotron-based multi-element microscopic speciation of individual microparticles (Scanning Transmission X-ray Microscopy with Near-Edge X-ray Absorption Fine Structure Spectroscopy combined with Computer-Controlled Scanning Electron Microscopy). Here we identified substantial abundances of chlorine-enriched aerosols in sea salt generated by photolytic products, whereas ice core records revealed increased chlorine depletion from the onset of ozone depletion. Our findings reveal that modern sea salt modification has no Holocene precedent.

Les sources d'ions négatifs d'hydrogène font partie intégrante des accélérateurs modernes et des systèmes d'injection de neutres (NBI) dans les futurs réacteurs de fusion. Cette dernière application nécessite le développement de sources très puissantes et l'extension de leur fonctionnement au deutérium. De nombreuses activités de recherche sur des expériences à l'échelle du laboratoire ont été dédiées à ce sujet dans le but de comprendre les processus fondamentaux qui régissent la production d'ions négatifs. Les connaissances acquises ont essentiellement contribué au développement des sources prototypes qui seront employées dans le système NBI d’ITER, le plus grand réacteur Tokamak en construction destiné à démontrer le potentiel d’exploitation de la fusion en tant que source d’énergie alternative. Toutefois, d’importantes contraintes technologiques liées à la physique fondamentale gouvernant le fonctionnement de ces sources doivent être surmontées pour qu’elles puissent délivrer des faisceaux de neutres dont les caractéristiques satisfassent les exigences initiales.Dans cette thèse, la production d’ions négatifs d’hydrogène (H-) et de deutérium (D-) est étudiée dans des plasmas entretenus à la résonance cyclotron électronique. En particulier, des études expérimentales ont été réalisées dans deux réacteurs présentant des caractéristiques similaires, “Prometheus I” et “SCHEME-II+”, en utilisant des diagnostics conventionnels et avancés adaptés à l’étude des propriétés macroscopiques et atomiques des plasmas de ces deux isotopes. Dans ces réacteurs, la production d’ions négatifs est basée sur le mécanisme dit de production en volume. Dans ce cas, la réaction d’attachement dissociative des électrons (ADE) est identifiée comme la réaction prédominante. La compréhension des facteurs influençant cette réaction peut donc conduire à un meilleur contrôle de la production d’ions négatifs.Dans le réacteur “Prometheus I”, des études paramétriques dans des plasmas d’hydrogène et de deutérium, en fonction de la puissance micro-onde et de la pression du gaz, révèlent des optima pour la production d’ions négatifs et permettent de mettre en évidence des particularités isotopiques. Le rendement en ions négatifs atteint 0.57×10^16 m^(-3 ) dans les deux plasmas, mais le rapport entre les ions négatifs et la densité du plasma est systématiquement plus élevée pour H2 que pour D2. Par exemple, un rapport de 0.225 pour H2 contre 0.125 pour D2 est obtenu dans des conditions représentatives de fonctionnement. En outre, la mesure de l’énergie des ions négatifs révèle l’existence de deux populations ioniques d’énergies différentes. Ceci a été attribué principalement à deux mécanismes responsables de l’excitation des molécules à des niveaux vibrationnels/rotationnels élevés participant à la formation d’ions négatifs via la réaction ADE.En revanche, le réacteur "SCHEME-II+" est destiné à l'étude de l'influence de divers matériaux exposés au plasma pour la production de molécules ro-vibrationnellement excitées. Un diagnostic spectroscopique complexe, la spectroscopie d'absorption dans l'ultraviolet du vide à transformée de Fourier (VUV-FT) utilisant le rayonnement synchrotron, est utilisée pour les sonder directement différentes conditions de fonctionnement du plasma. L’effet significatif des surfaces métalliques sur la création de ces espèces ro-vibrationnellement excitées a pu être démontré. Une augmentation de près d’un facteur quatre pour les molécules D2 dans les états vibrationnels élevés (v"= 4-8) est observée lorsqu’une surface de tantale remplace une surface de quartz face au plasma.Enfin, des plasmas de deutérium fonctionnant en mode pulsé sont examinés. Des mesures résolues en temps des paramètres plasma, effectuées dans des impulsions de plasma de l'ordre du kHz, révèlent d'importants effets post-plasma. Notamment, un rendement en ions négatifs plus élevé que celui mesuré dans un plasma fonctionnant en mode continu a été observé
Hydrogen negative ion sources are integral components in modern accelerator facilities and in the Neutral Beam Injection (NBI) systems of future fusion reactors. The latter application necessitates the development of such very powerful sources and the extension of their operation to deuterium. Numerous research activities on laboratory-scale experiments have been devoted to this subject aiming at understanding the fundamental processes that govern negative ion production. The gained knowledge has contributed essentially to the development of the prototype sources that will be employed in the NBI system of ITER, the largest Tokamak reactor under construction which is foreseen to demonstrate the potential of exploitation of thermonuclear fusion as an alternative source of energy. Nonetheless, there are important technological issues arising from the fundamental physics underlying these sources which need to be overcome in order for them to be able to deliver neutral beams with characteristics satisfying the baseline requirements.In the present thesis, the production of hydrogen (H-) and deuterium (D-) negative ions is studied in Electron Cyclotron Resonance (ECR) driven plasmas. In particular, experimental studies have been carried out in two reactors of similar characteristics, namely “Prometheus I” and “SCHEME-II+”, by means of conventional and state-of-the-art diagnostic techniques tailored to investigate the macroscopic and atomic properties of the plasmas of the two isotopes. In these reactors, negative ion production is based on the so-called volume production mechanism. In this case, the Dissociative Electron Attachment (DEA) reaction is identified as the predominant one. Understanding those factors which influence this reaction may lead to its control and thus an even better control of the negative ion production.In the “Prometheus I” reactor, parametric studies in hydrogen and deuterium plasmas as a function of the supplied microwave power and working gas pressure reveal the existence of optima for negative ion production and allow the identification of isotopic differences. The negative ion yield reaches a value of 0.57×10^16 m^(-3 ) in both plasmas, although in the hydrogen case the ratio of the negative ions to the plasma density is consistently higher than in the case of deuterium. Indicatively, a ratio of 0.225 in H2 versus 0.125 in D2 is observed in representative operating conditions. Measurements, moreover, of the negative ion energies disclose the existence of two ionic populations of different energies. This has been attributed to the two main mechanisms leading to the excitation of molecules in high vibrational/rotational states, which in turn participate in the formation of negative ions via the DEA reaction.On the other hand, the “SCHEME-II+” reactor is intended for studies of the influence of various materials exposed to the produced plasma on the production of the highly ro-vibrationally excited molecules. An advanced spectroscopic diagnostic technique, Vacuum Ultraviolet Fourier Transform (VUV-FT) absorption spectroscopy using synchrotron radiation, is utilized in order to directly probe these species under different plasma operating conditions. The significant positive effect of metallic surfaces on the creation of these species is demonstrated, since a nearly fourfold increase of deuterium molecules in high vibrational states (v"= 4-8) is observed when the plasma faces a tantalum surface as opposed to a Quartz surface.Finally, deuterium plasmas sustained in the pulsed mode of operation are investigated. Time-resolved measurements of the basic plasma parameters, performed in plasma pulses lying in the kHz range, reveal important post-plasma effects. In particular, a higher negative ion yield with respect to that measured in a plasma sustained in the continuous mode of operation has been observed

Atoms, molecules and clusters all constitute building blocks of macroscopic matter. Therefore, understanding the electronic and geometrical properties of such systems is the key to understanding the properties of solid state objects.

In this thesis, some atomic, molecular and cluster systems (clusters of O₂, CH₃Br, Ar/O₂, Ar/Xe and Ar/Kr; dimers of Na; Na and K atoms) have been investigated using synchrotron radiation, and in the two last instances, laser light. We have performed x-ray photoelectron spectroscopy (XPS) on all of these systems. We have also applied ultraviolet photoelectron spectroscopy (UPS), resonant Auger spectroscopy (RAS) and near-edge x-ray absorption spectroscopy (NEXAFS) to study many of the systems. Calculations using ab initio methods, namely density functional theory (DFT) and Møller-Plesset perturbation theory (MP), were employed for electronic structure calculations. The geometrical structure was studied using a combination of ab initio and molecular dynamics (MD) methods.

Results on the dissociation behavior of CH₃Br and O₂ molecules in clusters are presented. The dissociation of the Na₂ molecule has been characterized and the molecular field splitting of the Na 2p level in the dimer has been measured. The molecular field splitting of the CH₃Br 3d level has been measured and the structure of CH₃Br clusters has been determined to be similar to the structure of the bulk solid. The diffusion behavior of O₂, Kr and Xe on large Ar clusters, as a function of doping rate, has been investigated. The shake-down process has been observed from excited states of Na and K. Laser excited Na atoms have been shown to be magnetically aligned. The shake-down process was used to characterize the origin of various final states that can be observed in the spectrum of ground-state K.

Current research on the Earth’s upper atmosphere requires molecular parameters of unprecedented detail and accuracy. State-of-the-art models of the vacuum-ultraviolet (VUV) absorbing properties of the atmosphere call for absorption cross sections with details on the scale of the Doppler linewidths (i.e., 0.1 to 0.15 cm−1 at 295 K). As a consequence, spectroscopic data at resolving powers of the order of 106 are needed. Current particular needs are for ultrahigh resolution absorption cross section data for some bands of NO and O2 in the UV to VUV region. To meet some of these requirements, we have used a UV Fourier transform (FT) spectrometer at Imperial College (IC) to measure the Herzberg I bands of O2 [1,2] and the δ (0,0) band of NO [3]

Although X-ray Absorption Spectra (XAS) were first measured in the 1930’s, it was not until synchrotron radiation became available in the 1970’s that it became a feasible technique and the direct dependence of the XAS on the local structure of the absorbing element recognized. By a straightforward deconvolution, the Extended X-ray Absorption Fine Structure (EXAFS) region of the XAS provides the types and numbers of and distances to neighboring atoms within 3-4 Å of the selected absorber. In addition, transitions to bound and quasi-bound final states produce features in the X-ray Absorption Near Edge Structure (XANES) region which can provide information on the electronic states and occupancies of the absorber and the three-dimensional arrangement of its near neighbors. Its ability to provide direct, structural information for any selected element in a sample in any physical state has made XAS a tool of increasing importance for deter mining local structures in metalloproteins, minerals, catalysts, and other non-crystalline systems.