Advanced Light Source In Berkeley Research Articles

Direct double ionization of the Ar+ M shell by a single photon

Author(s): Muller, A; Kilcoyne, ALD; Phaneuf, RA; Holste, K; Schippers, S; Kheifets, AS | Abstract: Direct double ionization of the Ar+(3p-1) ion by a single photon is investigated both experimentally and theoretically. The photon-ion merged-beams technique was employed at the Advanced Light Source in Berkeley, USA, to measure absolute cross sections in the energy range from 60 to 150 eV. In this range, three contributions to the double ionization of Ar+ are to be expected: the removal of two 3p electrons, of a 3s and a 3p electron, and of two 3s electrons. Among the possible mechanisms leading to double ionization, the TS1 (two-step one) process dominates in the near-threshold region. In TS1, a photoelectron is ejected and, on its way out, knocks out a secondary electron. This two-step mechanism is treated theoretically by multiplying the calculated cross section for direct single photoionization of a given subshell with the calculated (e,2e) ionization probability for the ejected photoelectron to knock off a secondary electron. The calculated cross section is in very good agreement with the experiment.

Photoionization of tungsten ions: experiment and theory for

Experimental and theoretical results are reported for single-photon single ionization of the tungsten ion . Absolute cross sections have been measured employing the photon-ion merged-beams setup at the Advanced Light Source in Berkeley. Detailed photon-energy scans were performed at 200 meV bandwidth in the 40–105 eV range. Theoretical results have been obtained from a Dirac-Coulomb R-matrix approach employing basis sets of 730 levels for the photoionization of . Calculations were carried out for the , J = 2, ground level and the associated fine-structure levels with J = 3 and 4 for the ions. In addition, cross sections have been calculated for the metastable levels . A very satisfying agreement of theory and experiment is found for the photoionization cross section of which is remarkable given the complexity of the electronic structure of tungsten ions in low charge states.

Synchrotron-based macromolecular crystallography module for an undergraduate biochemistry laboratory course.

This paper describes the introduction of synchrotron-based macromolecular crystallography (MX) into an undergraduate laboratory class. An introductory 2week experimental module on MX, consisting of four laboratory sessions and two classroom lectures, was incorporated into a senior-level biochemistry class focused on a survey of biochemical techniques, including the experimental characterization of proteins. Students purified recombinant protein samples, set up crystallization plates and flash-cooled crystals for shipping to a synchrotron. Students then collected X-ray diffraction data sets from their crystals via the remote interface of the Molecular Biology Consortium beamline (4.2.2) at the Advanced Light Source in Berkeley, CA, USA. Processed diffraction data sets were transferred back to the laboratory and used in conjunction with partial protein models provided to the students for refinement and model building. The laboratory component was supplemented by up to 2 h of lectures by faculty with expertise in MX. This module can be easily adapted for implementation into other similar undergraduate classes, assuming the availability of local crystallographic expertise and access to remote data collection at a synchrotron source.

Photoionization of tungsten ions: experiment and theory for and

Experimental and theoretical results are reported for single-photon single ionization of and W3+ tungsten ions. Experiments were performed at the photon–ion merged-beam setup of the Advanced Light Source in Berkeley. Absolute cross sections and detailed energy scans were measured over an energy range 20–90 eV at a bandwidth of 100 meV. Broad peak features with widths typically around 5 eV have been observed with almost no narrow resonances present in the investigated energy range. Theoretical results were obtained from a Dirac–Coulomb R-matrix approach. The calculations were carried out for the lowest-energy terms of the investigated tungsten ions with levels for and for . Assuming a statistically weighted distribution of ions in the initial ground-term levels there is good agreement of theory and experiment for ions. However, for ions at higher energies there is a factor of approximately two difference between experimental and theoretical cross sections.

Preface to the Special Issue of Topics in Catalysis on Ambient Pressure X-ray Photoelectron Spectroscopy

This Special Issue of Topics in Catalysis focuses on the application of Ambient Pressure X-ray Photoelectron Spectroscopy (APXPS) for the investigation of heterogeneous interfaces. The topics of the papers included in this issue range from heterogeneous catalysis, including electrocatalysis, to the chemistry of interfaces in the environment, to solid/liquid and liquid/vapor interfaces. This broad array of applications reflects the utility of APXPS for investigations in a wide range of disciplines. The first APXPS instrument was conceived and installed by the Siegbahn group in Uppsala in the early 1970s and applied in particular to the investigation of concentration gradients across liquid/vapor interfaces, a topic that is as important nowadays as it was then. Several other APXPS instruments were installed over the following decades: At Cardiff at the end of the 1970s, at Novosibirsk in the mid-1980s, and at Orono/ME at the end of the 1980s. All of these instruments used laboratory sources for the incident X-rays, and a set of differentially pumped apertures to separate the high pressure in the sample compartment from the vacuum in the electron analyzer, to minimize the path length of the electrons through the high pressure region and prevent arcing due to large potential differences in the electron analyzer at elevated pressure. The design of a differentially-pumped lens system at Berkeley Lab in the late 1990s helped to overcome the trade-off between efficient differential pumping using small apertures and highthroughput electron detection by placing electrostatic lenses between the differentially-pumped apertures, which increased the solid angle of electron collection in APXPS and thus the pressure limit from previously 1 mbar by at least an order of magnitude. The first two instruments at the Advanced Light Source in Berkeley and at BESSY in Berlin that used this design produced the vast majority of APXPS publications that appeared in the first decade of the twentieth century. At the end of that decade commercial APXPS instruments became available, many of which were installed at other 3rd generation synchrotron sources around the world. At the time of writing 11 synchrotrons either have already operating instruments or are in the process of installing them. The availability of instruments at synchrotrons has dramatically increased access to APXPS measurements for researchers from a wide range of disciplines, including heterogeneous catalysis, geosciences, atmospheric science and electrochemistry. Recently, a growing number of laboratory-based instruments have been installed; these instruments greatly benefit from the availability of small-spot, monochromatized X-ray anode sources. Future growth in the number of APXPS instruments is expected to be mainly due to the installation of additional laboratory-based instruments as opposed to those at synchrotrons. While the growth of the APXPS community and the breadth in the application of the technique over the past 15 years has been an important contribution to the development of new tools for the operando investigation of interfaces, there are many open problems that need to be addressed by the APXPS community over the coming years. Chief among these is the preparation of surfaces and interfaces with the same control over impurity levels and crystallography as one is used to from ultra-high vacuumbased surface science investigations. Sample preparation is in particular challenging for liquids with high vapor pressures, where exact control of the thermodynamic & Hendrik Bluhm hbluhm@lbl.gov

Single-photon single ionization of W+ ions: experiment and theory

Experimental and theoretical results are reported for photoionization of Ta-like (W+) tungsten ions. Absolute cross sections were measured in the energy range 16–245 eV employing the photon–ion merged-beam setup at the advanced light source in Berkeley. Detailed photon-energy scans at 100 meV bandwidth were performed in the 16–108 eV range. In addition, the cross section was scanned at 50 meV resolution in regions where fine resonance structures could be observed. Theoretical results were obtained from a Dirac–Coulomb R-matrix approach. Photoionization cross section calculations were performed for singly ionized atomic tungsten ions in their J = 1/2, ground level and the associated excited metastable levels with J = 3/2, 5/2, 7/2 and 9/2. Since the ion beams used in the experiments must be expected to contain long-lived excited states also from excited configurations, additional cross-section calculations were performed for the second-lowest term, J = 5/2, and for the 4F term, with J = 3/2, 5/2, 7/2 and 9/2. Given the complexity of the electronic structure of W+ the calculations reproduce the main features of the experimental cross section quite well.

Electron ionization, photoionization and photoelectron/photoion coincidence spectroscopy in mass-spectrometric investigations of a low-pressure ethylene/oxygen flame

Quantitative species data for the development and critical examination of combustion mechanisms are in high demand regarding the need for predictive combustion models that may assess the emission potential of current and emerging fuels. Mass spectrometric investigation is one of the often-used techniques to provide mole fractions of stable and reactive intermediates including radicals from specifically designed laboratory experiments. Molecular-beam mass spectrometry (MBMS) has been coupled with electron ionization (EI) and photoionization (PI) to determine the species compositions, and combinations of these techniques have been successful in the investigation of the combustion pathways in flames of numerous hydrocarbon, oxygenated and nitrogenated fuels. Photoelectron/photoion coincidence spectroscopy (PEPICO) has recently emerged as a novel diagnostics to be combined with flame-sampling mass spectrometry, and its potential as a complement of existing techniques is just about being explored. In a multi-laboratory investigation, the present study has thus combined four different MBMS spectrometers (in Bielefeld, Germany, the Advanced Light Source in Berkeley, USA, the Swiss Light Source in Villigen, Switzerland, and the SOLEIL synchrotron in St. Aubin, France) to study a rich premixed argon-diluted low-pressure (40mbar) ethylene–oxygen flame under comparable conditions. This was done with the aim of illustrating the respective properties and capabilities of the methods under these conditions, with an emphasis on the power offered by the synchrotron-based techniques, including PEPICO, for combustion chemistry studies. Examples include comparisons of selected species quantification as well as PEPICO spectra measured at different instruments.

Structural and Optical Properties of Luminescent Silicon Carbonitride Thin Films

A series of silicon carbonitride (SiCN) thin films, a relatively unstudied material, were fabricated by electron cyclotron resonance plasma enhanced chemical vapour deposition (ECR PECVD) located at McMaster University. For comparison, a set each of silicon nitride and silicon carbide samples were also fabricated. To study the effect of introducing rare earth ions into SiCN host matrix, we synthesised a series of SiCN co-doped with Ce and Tb. This work investigates the influence of film composition and annealing conditions on the luminescence and structure of rare earth doped and undoped SiCN thin films through photoluminescence (PL), X-ray emission (XES), X-ray absorption near edge structure (XANES), and X-ray diffraction (XRD) experiments. The XES measurements were measured on Bl. 8.0.1 at the Advanced Light Source in Berkeley, USA. XANES has been carried out on the high resolution spherical grating monochromator (SGM) 11ID-1, soft X-ray microcharacterization beamline (SXRMB) 06B1-1, and variable line spacing plane grating monochromator (VLS PGM) beamline at the Canadian Light Source (CLS) synchrotron facility. PL emission was measured at the room temperature using a 17 mW 325 nm HeCd laser as the excitation source. For both doped and un-doped thin films, the XANES measurements were recorded at the N K-edge, C K-edge, and Si L3,2-edge while total fluorescence yield (TFY), total electron yield (TEY), and partial fluorescence yield (PFY) were simultaneously measured. Both the XES and XANES spectra were calibrated using the same reference materials. For doped sample the XANES spectra of Ce N5,4-edge and Ce M5,4-edge are also measured. The XES measurements included the Ce N5,4, Si L3,2, C K, and N K spectra. XRD also was used to determine the local bonding coordination of rare earth ions when incorporated in amorphous as well as crystalline structures. Film composition was determined using Rutherford backscattering spectrometry (RBS) and elastic recoil detection (ERD) analysis carried out at the Tandetron Laboratory at Western University. The thickness of film varying from 170 to 200 nm allowed the conduction of ion beam and X-ray absorption spectroscopy measurements without interference from the substrate. The variable angle spectroscopic (VASE) ellipsometer was employed to investigate the thickness and optical constants of the films.The PL spectra of SiCN samples annealed at the lower temperatures consist of a broad peak ranging from 400 to 700 nm. Bright Ce3+ and Tb3+ related emission lines are observed for the doped films annealed in nitrogen at 1200°C for one hour. TEM analysis reveals the formation of randomly distributed nanocrystals in doped SiCN films following 1200°C annealing, while the undoped SiCN samples did not appear to show any crystalline structure at any temperature. Comparisons of the XANES and XES results indicated that in a the doped SiCN films, structural re-ordering occurs at the higher annealing temperatures leading to the formation of Si clustering along with a decrease in Si-N bonding. Conversely, little to no difference was observed in the electronic structure of undoped SiCN films following annealing at different temperatures. This suggests that the electronic structure of the un-doped SiCN films may not change with respect to the Si and N atoms.

Can Laue microdiffraction be used to solve and refine complex inorganic structures?

The white-beam Laue diffraction experiment is an attractive alternative to the more conventional monochromatic one for single-crystal structure analysis, because it takes full advantage of the X-ray energy spectrum of a synchrotron source and requires no rotation of the crystal in the beam. Therefore, it could be used for structural characterizations underin situoroperandoconditions. The potential of Laue diffraction was recognized and exploited by the protein community many years ago, and the methodology, which involved positioning and rotating the crystal in the beam, has been successfully applied to the determination of both protein and small-molecule crystal structures. Here, it is proposed that the specificities of Laue diffraction are exploited to study randomly oriented stationary microcrystals of inorganic materials. In order to determine the best strategy for collecting a reasonable quantity of data from stationary crystals, a series of simulations on four model structures for three experimental setups have been performed. It is shown that the structures of the four samples can be solved with the dual-space method inSHELX, even though the data sets are highly incomplete and much of the low-resolution part is missing. The experimental setup and data collection strategy for measuring such microcrystals have been developed on BL12.3.2 at the Advanced Light Source in Berkeley. The intensities of reflections with one and two harmonics can be extracted reliably by exploiting the tunable low-energy threshold of a Pilatus detector. In this way, the number of usable reflections can be increased from 75 to 95%. Such Laue microdiffraction data have been measured and used successfully to refine the structures of the model samples.

Magnetic Microstructure of Rolled‐Up Single‐Layer Ferromagnetic Nanomembranes

The magnetic microstructure of rolled-up magnetic nanomembranes is revealed both theoretically and experimentally. Two types of nanomembranes are considered, one with a non-magnetic spacer layer and the other without. Experimentally, by using different materials and tuning the dimensions of the rolled-up nanomembranes, domain patterns consisting of spiral-like and azimuthally magnetized domains are observed, which are in qualitative agreement with the theoretical predictions.

Using a non-monochromatic microbeam for serial snapshot crystallography

The new X-ray free-electron laser source (SwissFEL) that is currently being developed at PSI will provide a broad-bandpass mode with an energy bandwidth of about 4%. By using the full energy range, a new option for structural studies of crystalline materials may become possible. The proof of concept of broad-bandpass diffraction presented here is based on Laue single-crystal microdiffraction and the experimental setup on BL12.3.2 at the Advanced Light Source in Berkeley. Diffraction patterns for 100 randomly oriented stationary crystallites of theMFI-type zeolite ZSM-5 were simulated assuming several bandwidths, and the statistical and structural results are discussed. With a 4% energy bandwidth, the number of reflection intensities measured in a single shot is significantly higher than with monochromatic radiation. Furthermore, the problem of partial reflection measurement, which is inherent to the monochromatic mode with stationary crystals, can be overcome.

K-shell photoionization of Be-like B+ ions

Absolute cross sections for K-shell photoionization of Be-like B+ ions were measured employing the photon-ion merged-beam technique at the Advanced Light Source in Berkeley. The results are found to be in fair agreement with R-matrix calculations using L-S coupling.

High-resolution photoionization of Xe+ ions: Experiment and theory

High-resolution photoionization measurements of Xe+ ions have been performed at the Advanced Light Source in Berkeley, California, USA. The experimental cross sections are compared with results from Dirac-Coulomb R-matrix calculations.

Photoionization of singly and multiply charged tungsten ions

Relative cross sections for photoionization of Wq+ ions q = 1, ..., 5 were measured at energies up to 90 eV employing the photon-ion merged-beam technique at the Advanced Light Source in Berkeley. While the cross sections for the lower charge states are dominated by broad resonance features in the range of 4f and 5p excitations, the cross section for W5+ shows much narrower peak features.

Morphology and microstructure of magnetite and ilmenite inclusions in plagioclase from Adirondack anorthositic gneiss

It is well known that oriented iron and titanium oxide inclusions occur in pyroxenes and plagioclase of anorthosites and granulites, and they are attributed to exsolution at subsolidus conditions. The oxides occur as needles or platelets. In this study, we determine the morphology of oxide needles as well as their orientation in plagioclase (An 30–35) in anorthosite gneiss from the Adirondack mountains (New York). The investigation was done with electron backscatter diffraction (EBSD) in a scanning electron microscope, as well as Laue diffraction with a microfocus synchrotron X-ray beam at the Advanced Light Source in Berkeley. It was observed that the needle direction is [110] in magnetite and [1010] in ilmenite. The needle direction is consistently parallel to [001] of plagioclase. Furthermore, (111) of magnetite and (0001) of ilmenite are sub-parallel to (120) and (120) of plagioclase. We note that for directions [110] in the magnetite structure and [1010] in ilmenite, O atoms are close-packed, and (111) and (0001) are close-packed planes, correspondingly. In plagioclase, [001] is a direction with open channels as well as approximate alignment of Si tetrahedral edges, thus providing nucleation sites with a coincidence lattice relationship. (120) and (120) in this triclinic mineral are planes with approximate tetrahedral sides so that the relationship is structurally plausible. From Laue diffraction, we can determine that the magnetite needle axis is subject to an extensional stress, most likely attained during cooling of the inclusions within the plagioclase host.

Magnetic soft x-ray imaging of vortex core dynamics

Soft x-ray microscopy offers high spatial and temporal resolution imaging with element specific magnetic contrast. As such, it is an ideal method for studying nanoscale spin dynamics, such as vortex core dynamics. At XM-1, the full field soft x-ray transmission microscope at the Advanced Light Source in Berkeley, a technique has been developed for pinpointing vortex dynamics without time resolution. In addition, a phase-locked setup has been used to conduct time resolved experiments of vortex core dynamics. The samples in this study were 100nm thick, 2μm diameter Ni80Fe20 disks. Analysis of nontime resolved images suggested that resonant vortex core dynamics were excited by ac magnetic fields close to 340MHz. This behavior was confirmed with time resolved imaging and gyrotropic motion of the vortex core was observed.

Chemical dynamics, molecular energetics, and kinetics at the synchrotron

Scientists at the Chemical Dynamics Beamline of the Advanced Light Source in Berkeley are continuously reinventing synchrotron investigations of physical chemistry and chemical physics with vacuum ultraviolet light. One of the unique aspects of a synchrotron for chemical physics research is the widely tunable vacuum ultraviolet light that permits threshold ionization of large molecules with minimal fragmentation. This provides novel opportunities to assess molecular energetics and reaction mechanisms, even beyond simple gas phase molecules. In this perspective, significant new directions utilizing the capabilities at the Chemical Dynamics Beamline are presented, along with an outlook for future synchrotron and free electron laser science in chemical dynamics. Among the established and emerging fields of investigations are cluster and biological molecule spectroscopy and structure, combustion flame chemistry mechanisms, radical kinetics and product isomer dynamics, aerosol heterogeneous chemistry, planetary and interstellar chemistry, and secondary neutral ion-beam desorption imaging of biological matter and materials chemistry.

Spatial resolution limits for synchrotron-based spectromicroscopy in the mid- and near-infrared

Spatial resolution tests were performed on beamline 1.4.4 at the Advanced Light Source in Berkeley, CA, USA, a third-generation synchrotron light source. This beamline couples the high-brightness synchrotron source to a Thermo-Electron Continumicrom XL infrared microscope. Two types of resolution tests were performed in both the mid-IR and near-IR. The results are compared with a diffraction-limited spot size theory. At shorter near-IR wavelengths the experimental results begin to deviate from diffraction-limited so a combined diffraction-limit and electron-beam-source-size model is employed. This description shows how the physical electron beam size of the synchrotron source begins to dominate the focused spot size at higher energies. The transition from diffraction-limited to electron-beam-size-limited performance is a function of storage-ring parameters and the optical demagnification within the beamline and microscope optics. The discussion includes how different facilities, beamlines and microscopes will affect the achievable spatial resolution. As synchrotron light sources and other next-generation accelerators such as energy-recovery LINACs and free-electron lasers achieve smaller beam emittances, beta-functions and/or energy spreads, diffraction-limited performance can continue to higher-energy beams, perhaps ultimately into the extreme ultraviolet.

Soft x-ray imaging of spin dynamics at high spatial and temporal resolution

Soft x-ray microscopy provides element specific magnetic imaging with a spatial resolution down to 15 nm. At XM-1, the full-field soft x-ray microscope at the Advanced Light Source in Berkeley, a stroboscopic pump and probe setup has been developed to study fast magnetization dynamics in ferromagnetic elements with a time resolution of 70 ps, which is set by the width of the x-ray pulses from the synchrotron. Results obtained from a 2 μm×4 μm×45 nm rectangular permalloy sample exhibiting a seven domain Landau pattern reveal dynamics up to several nanoseconds after the exciting magnetic field pulse. Domain wall motion, a gyrotropic vortex motion, and a coupling between vortices in the rectangular geometry are observed.

A simple method for determining linear polarization and energy calibration of focused soft X-ray beams

Although critical to quantitative linear dichroism studies of molecular orientation, the degree of linear polarization of focused soft X-ray beams delivered by X-ray microscopes has not been previously measured. Here, we present a scaled-down version of a recently developed technique in which the π ∗ near edge X-ray absorption fine structure (NEXAFS) resonance of highly oriented pyrolytic graphite (HOPG) is used to probe the electric field intensity in each direction and hence deduce the degree of linear polarization of the incident X-ray beam. Applying this technique to the soft X-ray microscope at beamline 5.3.2 of the Advanced Light Source in Berkeley, CA, yielded a measured value of 79 ± 11 %, for the first Stokes parameter of 0.79 ± 0.11 or as a Stöhr P factor of 0. 89 ± 0.06 . It is expected that the error margin could be significantly reduced via the use of an in-vacuum rotation actuator. We have also calibrated the energy of the graphite exciton to be 291.65 ± 0.025 eV, improving the utility of graphite as an energy calibration standard for NEXAFS and allowing the convenience of both energy calibration and polarization determination with a single inexpensive sample.