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Linear Dichroism in Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy is a useful tool for studying molecular orientation and for clarifying spectroscopic assignments. n-Alkane molecules can form single crystals with different polymorphs; orthorhombic and monoclinic in the case of even chain lengths between n-C28H58 and n-C40H82. These polymorphs exhibit different linear dichroism in their NEXAFS spectra; in particular a backbone-oriented transition contributes to the low-energy “C–H” band in the monoclinic polymorph.

The near edge X-ray absorption fine structure (NEXAFS) spectra of orthorhombic single crystals of n-octacosane (n-C28H58), recorded at room temperature (298 K) and at cryogenic temperatures (93 K),...

The origin of the exceptional stability of molecular glasses grown by physical vapor deposition (PVD) is not well understood. Differences in glass density have been correlated with thermodynamic stability for thin films of N,N'-Bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD) grown by PVD at specific substrate temperatures below the glass transition temperature. However, the relationship between the internal conformation of glass molecules and the thermodynamic properties of molecular glasses is not well studied. We use carbon 1s near edge x-ray absorption fine structure (NEXAFS) spectroscopy to examine different TPD sample preparations in which differences in the thermodynamic stability of the glass are known. Density functional theory simulations of the NEXAFS spectra of TPD allow us to attribute spectroscopic differences to changes in the internal conformation of the TPD molecule and relate this conformation to the stability of the TPD glass. This provides a direct experimental measurement of the internal conformation of molecules forming an organic glass.

Carbon 1s near edge X-ray absorption fine structure (NEXAFS) and UV–vis spectroscopy are used to examine differences between highly aggregated and poorly aggregated forms of the polymer poly(3-hexylthiophene) (P3HT), based on as-cast and annealed regiorandom and regioregular P3HT samples. UV–vis spectra show characteristic signatures of unaggregated P3HT in regiorandom P3HT, and of H-aggregation in regioregular P3HT samples. Distinct spectroscopic differences, including energy shifts, are observed in the NEXAFS spectra of aggregated P3HT relative to the unaggregated forms. These differences are reproduced with transition-potential density functional theory (TP-DFT) calculations which explore aggregation and molecular conformation. Differences in the NEXAFS spectra of P3HT are assigned to thiophene backbone twisting in the unaggregated forms of P3HT, and to various degrees of chain planarization in aggregated forms of P3HT that also correlate to the exciton bandwidth. These results open up the prospect of ...

NEXAFS spectroscopy has potential for study of packing and order in organic materials but only if intermolecular effects are understood. This work studies how π-π interactions between adjacent unsaturated groups affect their NEXAFS spectra, with a broader goal of building a general understanding of the role of intermolecular effects in NEXAFS spectroscopy. These effects are examined using paracyclophane (PCP) molecules, in which the benzene-benzene separation distance is controlled by varying the length of the alkyl groups separating the benzene rings. NEXAFS spectroscopy and density functional theory (DFT) simulations are used to examine spectroscopic changes related to the strength of these π-π interactions. A characteristic red shift is observed as adjacent benzene rings get closer together. This shift is attributed to Coulombic and orbital interactions between the adjacent benzene rings, mediated through overlapping π/π* orbitals.

The carbon 1s near edge X-ray absorption fine structure (NEXAFS) spectra of liquid phase n-alkanes have been measured for the first time. Clear trends are observed in the liquid phase spectra as a function of chain length. Specifically, the ‘C-H band’ in the NEXAFS spectra shifts to lower energy as the chain length increases. Characteristic differences between the gas, liquid and solid phase spectra are observed. Differences in the liquid phase n-alkane NEXAFS spectra are examined through molecular dynamics simulations coupled with density functional theory simulations of the NEXAFS spectra.

A cryo scanning transmission X-ray microscope, the cryo-STXM, has been designed and commissioned at the Canadian Light Source synchrotron. The instrument is designed to operate from 100 - 4000 eV (lambda = 12.4 - 0.31 nm). Users can insert a previously frozen sample, through a load lock, and rotate it plus minus 70 degrees in the beam to collect tomographic data sets. The sample can be maintained for extended periods at 92 K primarily to suppress radiation damage, and a pressure on the order of 10-9 Torr to suppress sample contamination. The achieved spatial resolution (30 nm) and spectral resolution (0.1 eV) are similar to other current soft X-ray STXMs, as demonstrated by measurements on known samples and test patterns. The data acquisition efficiency is significantly more favorable for both imaging and tomography. 2D images, 3D tomograms and 4D chemical maps of automotive hydrogen fuel cell thin sections are presented to demonstrate current performance and new capabilities, namely cryo-spectrotomography in the soft X-ray region., 56 pages

A new scanning transmission X-ray microscope (STXM) optimized for cryo-spectro-tomography with soft X-rays has been designed, built and commissioned at Canadian Light Source (CLS) beamline 10ID1 (130-2700 eV). It is controlled via a new python-based software package, pySTXM. A liquid N2 goniometer (Gatan 630, -80° to 80°), mounted on a computer controlled (x,y,Theta) tilt stage allows for spectro-tomographic measurements at cryogenic temperatures (-180°C) which reduces radiation damage. The CLS cryo-STXM is unique among the set of soft X-ray STXMs currently installed around the world. Details of the cryo-STXM design and examples of its performance will be presented.

Sulfur 1s NEXAFS spectroscopy is sensitive to sulfur speciation; however, the overlapping energy position of the white line in the spectra of thiols and disulfides can be problematic for speciation. We have acquired highly resolved sulfur 1s NEXAFS spectra of a group of disulfides with aliphatic and unsaturated ligand groups. Spectral transitions were assigned by the aid of Improved Virtual Orbital (IVO) ab initio calculations. These results confirm how molecular symmetry and the ligand group can affect the NEXAFS spectra, and show how the NEXAFS spectra of thiols and disulfides can be used for chemical speciation.

High-resolution sulfur 1s NEXAFS spectra of a series of sulfoxides and sulfones have been obtained, and are assigned with the aid of ab initio improved virtual orbital (IVO) calculations. Spectral differences between sulfones and sulfoxides are attributed to the sulfur oxidation state and the energy spacing between unoccupied molecular orbitals. The character of the white line in sulfones depends on the sulfur ligand groups, where an aromatic ligand causes a red shift and a broadening of the white line. These effects can pose challenges for speciation when the white line of sulfones overlaps with that of sulfoxide.

Sulfur plays an important role in the rubber industry as a vulcanizing agent, providing an enhancement in mechanical properties relative to unvulcanized rubber. The sulfur used in industry originates from natural and petroleum sources. Petroleum-based sulfur (PS), usually a by-product from a refinery process, was used as the vulcanizing sulfur in the present work. The chemical structure of PS was characterized and compared to that of a commercial rhombic sulfur (RS) using sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy. The results reveal that sulfur K-edge XANES spectroscopy has good sensitivity to the chemical structure of sulfur, and the chemical structures of sulfur in rubbers vulcanized with PS and RS are very similar. Furthermore, different types of rubbers vulcanized with PS or RS exhibit comparable mechanical properties. This work shows that PS can be used to replace RS as a vulcanizing agent for rubber.

We have quantitatively measured the angle dependence in the Silicon 1s X-ray absorption spectra of strained Si1−xGex thin films prepared by epitaxial growth on Si(1 0 0) substrates, through surface sensitive total electron yield detection. The linear dichroism difference extracted from these angle dependent X-ray absorption spectra is proportional to the degree of strain, as measured separately by Raman spectroscopy. This quantitative relationship provides a means to measure the compressive strain in Si1−xGex thin films. This strain-dependent X-ray absorption spectroscopy has the potential to realize a semiconductor strain metrology through high spatial resolution X-ray spectromicroscopy.

Photoelectron emission microscopy PEEM is a powerful tool to spectroscopically image dynamic surface processes at the nanoscale but is traditionally limited to ultra high or moderate vacuum conditions. Here, we develop a novel grapheme capped multichannel array sample platform that extends the capabilities of photoelectron spectromicroscopy to routine liquid and atmospheric pressure studies with standard PEEM setups. Using this platform, we show that graphene has only a minor influence on the electronic structure of water in the first few layers and thus will allow for the examination of minimally perturbed aqueous phase interfacial dynamics. Analogous to microarray screening technology in biomedical research, our platform is highly suitable for applications in tandem with large-scale data mining, pattern recognition, and combinatorial methods for spectro temporal and spatiotemporal analyses at solid liquid interfaces. Using Bayesian linear unmixing algorithm, we were able to discriminate between different X-ray induced water radiolysis scenarios and observe a metastable wetting intermediate water layer during the late stages of bubble formationformation, 20 pages, 4 figures

A complex orientational morphology is observed when n-alkane thin films are vapor deposited on highly oriented pyrolytic graphite surfaces. Substrate temperature can be used to tune the orientation and morphology of n-alkane thin films. The molecular orientation changes from lateral to normal to the surface when the substrate temperature is raised sufficiently. Under specific substrate temperature conditions, the n-alkane molecules are aligned in the plane of the sample surface, in directions reflecting the 6-fold symmetry of the graphite substrate. A series of different morphologies, from uniform thin films to oriented bars, are observed as a function of chain length and substrate temperature. The systematic evolution of these oriented morphologies is mapped as a function of deposition conditions, and the kinetic and thermodynamic factors that govern the formation of different in-plane and normal domains are considered.

Deposition of mixtures of palmitic acid (C15H31COOH) and perfluorooctadecanoic acid (C17F35COOH) onto solid substrates gives rise to irregularly shaped, phase-separated domains under a variety of deposition conditions. The morphology and chemical composition of these phase-separated domains have been investigated using a combination of surface pressure-area isotherms, atomic force microscopy, X-ray photoemission electron microscopy, and confocal fluorescence microscopy imaging. While domain morphology and composition in 2D phase-separated mixed monolayer systems can typically be rationalized in terms of an interplay between line tension and dipole-dipole repulsion effects, it was found that for this system additional kinetic factors, including domain growth rates and the rate of dissolution of the fatty acid component into the aqueous subphase, also play a major role in controlling film properties. The potential importance of these effects for the controlled patterning of solid substrates is discussed.

We have examined the electronic structure of a series of metal arene monomers and polymers by near-edge X-ray absorption fine structure spectroscopy. We observe that electron-donating or -withdrawi...

The near-edge X-ray absorption fine structure (NEXAFS) spectra of poly(phenyl thioether) polymers with pendant Fe(Cp) iron moieties have been recorded at the carbon 1s, iron 2p, and phosphorus 1s and 2p edges. This study shows that solution photofragmentation of the organometallic polymer causes the removal of iron moieties and the preparation of the corresponding organic polymer: poly(phenyl thioether). Spectroscopic characterization of poly(phenyl thioether) prepared in this manner indicates the presence of iron oxide, likely Fe2O3, and hexafluorophosphate counterion trapped in the demetalated polymer matrix. A photolytic examination of the solid phase poly(phenyl thioether)−Fe(Cp) organometallic polymer shows that irradiation of this polymer in air leads to the formation of poly(phenyl thioether) and Fe2O3, while irradiation in vacuum leads to formation of iron species with a lower oxidation state. The ability to probe the oxidation state of this metal species is relevant to understanding how demetalat...

The structure and composition of phase-separated Langmuir–Blodgett monolayer films comprised of mixtures of arachidic acid (C 19 H 39 COOH) and perfluorotetradecanoic acid (C 13 F 27 COOH) were characterized using a combination of X-ray photoelectron emission microscopy (X-PEEM), secondary electron emission microscopy (SEEM) and atomic force microscopy (AFM). X-PEEM provides high lateral spatial resolution and is directly sensitive to the elemental and chemical (functional group) composition of these ultrathin films through the chemical sensitivity of NEXAFS spectroscopy; AFM provides high-resolution imaging, both in terms of lateral and vertical (height) film topography. SEEM provides additional structural and electronic information through work function and electron scattering effects. The combination is used for chemical mapping of the phase-separated domains in the monolayer film. Our results directly confirm previous AFM measurements that suggested that the discontinuous domains are enriched in arachidic acid, whereas the surrounding continuous domain is a mixture of both arachidic acid and perfluorotetradecanoic acid.

The soft X-ray spectromicroscopy beamline (SM) at the Canadian Light Source (CLS) is a dedicated spectromicroscopy facility, consisting of an elliptically polarized undulator (EPU), a beamline based on a collimated PGM optimized for 100–2000 eV range and two end stations: scanning transmission X-ray microscope (STXM) and roll-in X-ray photoemission electron microscope (X-PEEM, from Elmitec GmbH). The overall system has achieved its design parameters with an on-sample flux of ∼108 ph/s@R=3000, 0.5 A in STXM and ∼1012 ph/s@R=3000, 0.5 A in the PEEM, in each case at a spatial resolution exceeding 40 nm. It can also provide an energy resolving power above 10,000. A careful EPU calibration procedure enables advanced polarization measurements.

A practical synthesis of racemic heptahelicene has been develeped being based on key [2+2+2] cycloisomerization of bis[2-(but-3-yn-1-yl)-1-naphthyl]acetylene under CpCo(CO)2/PPh3 or CpCo(C2H4)2 catalysis. The application of the Ni(cod)2 catalyst with (-)-(Sa)-(2'-methoxy-1,1'-binaphthalen-2-yl)diphenylphosphane resulted in enantioselective triyne cyclization to provide (+)-7,8,11,12-tetrahydroheptahelicene in 40% ee. Optically pure (-)-(M)- and optically highly enriched (+)-(P)-heptahelicene were obtained on a milligram scale by resolution of racemate by chiral HPLC on a semipreparative Whelk-O1 column.

We have acquired high-resolution carbon 1s near-edge X-ray absorption fine structure (NEXAFS) spectra of methane, ethane, propane, isobutane, and neopentane. These experimental measurements are complemented by high-quality ab initio calculations, performed with the improved virtual orbital approximation. The degree and character of Rydberg-valence mixing in the preedge of the NEXAFS spectra of these species is explored. Significant Rydberg-valence mixing only occurs when there are excited states of valence sigma(C-H) character that have the appropriate symmetry to interact with excited states of Rydberg character. Our results show that this mixing is only present when there are C-H bonds to the core excited carbon atom.

Carbon 1s (C−R) → π*CC electronic transitions originating from the substituent-bonded carbon atom of a benzene ring show distinctive chemical shifts in their near-edge X-ray absorption fine structure (NEXAFS) spectra. We have systematically explored these chemical shifts through ab initio calculations and carefully calibrated experimental data for a wide range of molecules containing substituted benzene rings. The systematic disparity between experimental and calculated transition energies was used to develop a semiempirical correction for this class of transitions, allowing us to map calculated transition energies onto a corrected, experimental energy scale. The correction method was applied to a large set of calculated core C 1s (C−R) → π*CC transition energies, and used to prepare a chemically wide-ranging NEXAFS correlation diagram for the “C−R π* band”. We demonstrate the usefulness of this correlation diagram for the analytical application of NEXAFS spectroscopy and microscopy to organic materials.

X-ray photoemission electron microscopy (X-PEEM) using synchrotron radiation illumination in the C 1s, N 1s and O 1s regions has been used to characterize a phase segregated polystyrene/polymethylmethacrylate (PS/PMMA) polymer thin film, and to map the adsorption of fibrinogen (a blood plasma protein) on this surface from both isotonic, buffered, and low ionic strength, unbuffered aqueous solutions at varying fibrinogen concentrations. The concentration dependence of the coverage correlates with independent, non-spatially resolved measurements using 125 I-radiolabeled protein. At low concentrations (

We present a high-resolution C K- and O K-NEXAFS study for the large aromatic molecule acenaphthenequinone (ANQ) in the condensed phase. This is the largest molecule for which a distinct vibronic fine structure at the O K-edge has been resolved so far. The most prominent results are the appearance of only one predominant vibrational mode, a C–H stretching mode, in the O 1s NEXAFS spectra and the analysis of the anharmonicity of the excited state potential.

Materials of commercial significance in the rubber industry are usually multi-component systems composed of several elastomers and various fillers. Elucidating the complex morphology that can arise from blending and understanding how this affects the various properties are essential. A technique advantageous to the study of multi-component elastomeric systems is Scanning Transmission X-ray Microscopy (STXM). STXM utilizes the chemical sensitivity of Near Edge X-ray Absorption Fine Structure (NEXAFS) and combines with relatively high spatial resolution and low beam damage to allow the successful characterization of multi-component materials that may be difficult or impossible with other techniques. An overview of the technique and example applications for the rubber industry is presented.

High energy resolution C 1s near-edge X-ray absorption fine structure (NEXAFS) spectra of ethylene-1-alkene copolymers with systematic variations in comonomer content and thus systematic changes in branch length, branching ratio, and degree of crystallinity are presented. Spectral changes of the r �–H /Rydberg and r �–C features in these ideal model systems provide unambiguous experimental evidence for intermolecular interactions with profound effects on the spectral intensity, but only very small energy shifts. Ab initio calculations reproduce the experimental results in detail. The intermolecular interaction observed suggests that interpretation of NEXAFS spectra based on calculations of isolated molecules can be insufficient even in relatively weakly interacting macromolecules. 2003 Elsevier Science B.V. All rights reserved. The interpretation and use of near edge X-ray absorption fine structure spectra of macromolecules is often based on molecular analogues, a semi-quantitative building block or finger print approach, as well as calculations based on isolated small molecules [1,2]. Explicit in this approach is the assumption that the character of the bonding and antibonding orbitals in single molecules are the most fundamental aspect, while intermolecular interactions and matrix effects are generally considered to be negligible. Occasionally, bandstruc

Near edge X-ray absorption fine structure (NEXAFS) microscopy has evolved into a powerful characterization tool for polymeric materials. The foundation of this utility depends crucially on the sensitivity of NEXAFS to the specific chemical structure of the polymer. Furthermore, for quantitative compositional analysis, reliable reference spectra with known energy resolution and calibrated energy scale are required. We report a set of NEXAFS spectra from 24 common polymers that represent a range of chemical functionalities in order to create a database of calibrated polymer NEXAFS spectra to be used for compositional analysis. These spectra illustrate the sensitivity of NEXAFS spectroscopy to the polymer composition, illustrating the potential of NEXAFS for chemical analysis.

Scanning transmission X-ray microscopy (STXM) and atomic force microscopy have been used to study the morphology and chemical composition of macrophase-segregated block copolymers in plaque formulations based on water-blown flexible polyurethane foams. Although there has been a large body of indirect evidence indicating that the observed macrophase-segregated features in water-rich polyurethane foams are due principally to urea components, this work provides the first direct, spatially resolved spectroscopic proof to support this hypothesis. The STXM results are consistent with a segregation model where urea segments segregate, forming enriched phases with the majority of the polyether- polyol and urethane groups at the chain ends of the urea hard segments. Chemical mapping of the urea, urethane, and polyether distribution about the urea-rich segregated phases showed that the urea concentration changes gradually (across several hundred nanometers) in a butylene oxide-based foam. This mapping also showed the urea-rich segregated phases present as a partial network in an ethylene oxide/propylene oxide sample.

The physical and chemical effects of the soft X-ray irradiation of polymers have been systematically evaluated for photon energies just above the C 1s binding energy. This exposure causes radiation damage in the form of the loss of mass and changes to the chemical structure of the polymers. These effects are evident in the Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra of the exposed polymers, posing a fundamental limit to the sensitivity of NEXAFS spectroscopy for chemical microanalysis. Quantitative understanding of the chemistry and kinetics of radiation damage in polymers is necessary for the successful and validated application of NEXAFS microscopy. This paper outlines a method for quantifying this radiation damage as a function of X-ray dose, and applies these methods to characterize the loss of mass and loss of carbonyl group functionality from a diverse series of polymers. A series of simple correlations are proposed to rationalize the observed radiation damage propensities on the basis of the polymer chemical structure. In addition, NEXAFS spectra of irradiated and virgin polymers are used to provide a first-order identification of the radiation chemistry.

A new soft X-ray scanning transmission X-ray microscope (STXM) optimized for cryo spectro-tomography was designed and commissioned at the Canadian Light Source (CLS). The instrument was required to achieve ultra high vacuum and be compatible with in-situ plasma cleaning. It also required a scintillator detector, and the design of this detector had to evolve to meet these environmental requirements. The scintillator deposition technique, and the suppression of background by introduction of an edge filter are also presented.

The Nitrogen 1s near edge X-ray absorption fine structure (NEXAFS) of gallium nitride (GaN) shows a strong natural linear dichroism that arises from its anisotropic wurtzite structure. An additional spectroscopic variation arises from lattice strain in epitaxially grown GaN thin films. This variation is directly proportional to the degree of strain for some spectroscopic features. This strain variation is interpreted with the aid of density functional theory calculations., Comment: 20 pages, 6 figures, http://www.sciencedirect.com/science/article/pii/S0169433214015980 or Applied Surface Science 316, 232 (2014)

The merits of a polymer characterization technique, near edge X-ray absorption fine structure (NEXAFS) spectromicroscopy, are demonstrated through the characterization of a multilayer polymer film with partially unknown chemical composition. The combination of chemical speciation through NEXAFS spectroscopy with the high spatial resolution available in X-ray microscopy allows the characterization of polymeric materials not possible with conventional techniques. Analysis of a multilayer with layers as thin as 4 μm has yielded results that differ from those previously obtained by infrared microscopy. Layers below the spatial resolution limit of infrared microscopy were characterized.

We report the visualization and quantitative analysis of the cross-link structure in model core/shell hydrogel polymers on the microscopic scale, in a fully swollen state, using soft X-ray microscopy. The cross-link density in these materials and their microscopic or even nanoscopic variation critically influence materials characteristics, yet the cross-link density is difficult to characterize by conventional methods. By the use of soft X-ray microscopy, one can investigate these materials in a fully swollen state and thus directly visualize and quantitatively determine the cross-link structure on a microscopic scale. Materials that were cross-linked by different methods were shown to give rise to differently shaped profiles. Abrupt and gradient cross-link density profiles have been investigated, and the spatial variation in their cross-link density has been determined quantitatively.

The interfacial properties of a homopolymer (polybutadiene (PB)) and a terpolymer (brominated poly(isobutylene- co - p -methylstyrene) (BIMS)) are reported. Neutron reflectivity was used to study the interfacial structure. The results were complemented by scanning transmission X-ray microscopy and atomic force microscopy, which were used to probe the morphology of these binary blends. Our results show that the interfacial behavior of these elastomeric blends is a direct function of the BIMS chemical composition. The interfacial width decreased with increasing bromide functionality. At levels below 8 mol%, the para -methylstyrene concentration had a less pronounced effect on the compatibility and interfacial characteristics. We also studied the effect of styrene butadiene random copolymers on the miscibility of the PB/BIMS blends. The results showed that styrene-butadiene rubber (SBR) was not fully miscible with BIMS and PB on an individual basis, but addition of relatively small amount of SBR enhances the compatibilization of the PB/BIMS interface. Self-consistent field (SCF) modeling was used to determine the optimum copolymer composition. The calculations are consistent with the experimental results.

The effects of carbon black and pyrogeneous silica fillers on the interfacial properties of a homopolymer (polybutadiene, (PB)) and a terpolymer (brominated poly(isobutylene-co-p-methylstyrene), (BIMS)) are reported. Neutron reflectivity (NR) was used to study the interfacial structure. The results are complemented by scanning transmission X-ray microscopy (STXM) and lateral force microscopy (LFM), which were used to probe the morphology and surface lateral force. Small-angle neutron scattering (SANS) was used to characterize the size and surface properties of the filler aggregates in elastomers. Our results show that the interfacial behavior of PB/BIMS is more sensitive to carbon black than to silica. The interfacial broadening is significantly slowed down with addition of merely CB ) 0.01 (volume fraction) carbon black fillers. This volume level is much lower than that used in bulk rubbers (CB g 0.1). Pyrogeneous silica has a less pronounced effect on the interfacial characteristics. When both carbon and silica are incorporated into the PB layer, the effect of carbon black is offset by silica fillers.

This Letter presents the high-resolution C 1s near-edge X-ray absorption fine structure (NEXAFS) spectra of hydrogenated and deuterated polystyrene. The differences between these spectra provide unambiguous evidence for the presence of a significant vibronic contribution to the shape and structure of the C 1s(C–H) →1π ∗ C  C transition in polystyrene. High-resolution NEXAFS spectra, spectroscopic simulations and ab initio calculations are used to help resolve the relative contributions of chemical shifts and vibronic excitation to the shape of the characteristic C 1s(C–H) →1π ∗ C  C transition.

The successful application of X-ray spectromicroscopy to chemical analysis of polymers is reviewed and a detailed application to quantitative analysis of polyurethanes is presented. Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy is the basis of chemical sensitive X-ray imaging, as well as qualitative and quantitative micro-spectroscopy. These capabilities are demonstrated by a review of recent work, and by presentation of new results outlining a methodology for quantitative speciation of polyurethane polymers. C 1s inner-shell excitation spectra of a series of molecular and polymeric model compounds, recorded by gas phase inelastic electron scattering (ISEELS) and solid phase NEXAFS techniques, are used to understand the spectroscopic basis for chemical analysis of polyurethanes. These model species contain the aromatic urea, aromatic urethane (carbamate) and aliphatic ether functionalities that are the main constituents of polyurethane polymers. Ab initio calculations of several of the model molecular compounds are used to support spectral assignments and give insight into the origin and relative intensities of characteristic spectral features. The model polymer spectra provide reference standards for qualitative identification and quantitative analysis of polyurethane polymers. The chemical compositions of three polyurethane test polymers with systematic variation in urea/urethane content are measured using the spectra of model toluene diisocyanate (TDI) urea, TDI-carbamate, and poly(propylene oxide) polymers as reference standards.

The sensitivity of near-edge X-ray absorption fine structure (NEXAFS) to differences in key chemical components of polyurethane polymers is presented. Carbon 1s NEXAFS spectra of polyurethane polymers made from 4,4‘-methylene di-p-phenylene isocyanate (MDI) and toluene diisocyanate (TDI) isocyanate monomers illustrate that there is an unambiguous spectroscopic fingerprint for distinguishing between MDI-based and TDI-based polyurethane polymers. NEXAFS spectra of MDI and TDI polyurea and polyurethane models show that the urea and carbamate (urethane) linkages in these polymers can be distinguished. The NEXAFS spectroscopy of the polyether component of these polymers is discussed, and the differences between the spectra of MDI and TDI polyurethanes synthesized with polyether polyols of different molecular composition and different molecular weight are presented. These polymer spectra reported herein provide appropriate model spectra to represent the pure components for quantitative microanalysis.

A number of tert-butyl-substituted, cyclic, saturated and unsaturated diaminocarbene, diaminosilylene, and diaminogermylene compounds were investigated using inner shell electron energy loss spectroscopy (ISEELS) and ab initio calculations. These compounds, each of which contains a divalent group 14 element (C, Si, Ge), are of particular interest since they are stable indefinitely, and thus, they are readily accessible for detailed spectroscopic analysis. The C 1s and N 1s spectra of the hydrogenated tetravalent analogues of the saturated carbene, of di-tert-butyldiazabutadiene, and of di-tert-butyldiazabutane were also obtained to assist with spectral interpretation. Our analysis shows that there is significant π-allyl delocalization over the N−EII−N fragment in all three species. Further, in the unsaturated species there is additional aromatic delocalization. Both theory and experiment indicate that the divalent center of the carbene is qualitatively different from that in the germylenes or silylenes.

Oscillator strengths for C 1s inner-shell excitation of the isomeric xylenes ( ortho -, meta - and para -) have been derived from electron energy loss spectra recorded under scattering conditions dominated by electric dipole transitions. The lineshape of the C 1s→ π * transition is found to be dependent on the substitution pattern, with para -xylene exhibiting two components, ortho -xylene a single asymmetric peak, and meta -xylene a single symmetric peak at an instrumental resolution of 0.35 eV. Improved virtual orbital ab initio calculations were carried out on all three species, providing spectral shapes in reasonable agreement with experiment. The calculations indicate that the transition responsible for the splitting which is observed only in the C 1s spectrum of para -xylene is associated with the methyl-substituted ring carbons, for which there is a somewhat larger chemical shift in the para - relative to the ortho - or meta -isomers. These results are compared to other recent studies of C 1s spectroscopy of di-substituted benzenes (dimethylphthalates and nitroanilines) in order to investigate trends in the sensitivity of C 1s spectroscopy to isomeric substitution patterns.

The silicon 1s, 2p, and 2s, carbon 1s, and nitrogen 1s gas-phase core excitation spectra are reported for two stable divalent silylenes, Si(NRCHCHNR) and Si(NRCH2CH2NR) (R = tBu), and for the two analogous tetravalent dihydridosilane molecules, H2Si(NRCHCHNR) and H2Si(NRCH2CH2NR) (R = tBu). The nature of the excited states arising from one-electron core → π* and core → σ* excitations at different sites on the silylene ring are examined by relating the changes in the spectra of these compounds to differences in their molecular structure. The variations in the intensities of core → π* transitions in these spectra probe the extent of delocalization in the π-electron manifold and, thus, allow us to investigate the nature of π stabilization in heterocycles containing divalent silicon. Low-lying transitions to a π*(Si−N) level observed in all core edge spectra strongly support the existence of π-delocalization in the CC unsaturated silylene. Ab initio calculations are used to confirm spectral assignments.

The Si 1s and 2p solid state photoabsorption (total electron yield) spectra of triphenylsilanol, hexaphenyldisiloxane, and hexaphenyldisilane and the Si 1s spectra (total ion yield) of gaseous trimethylsilanol, hexamethyldisiloxane, hexamethyldisilane, and trimethylmethoxysilane have been recorded using synchrotron radiation. These spectra are compared to inner shell electron energy loss spectra of gaseous triphenylsilanol, hexaphenyldisilane, trimethylmethoxysilane, hexamethyldisiloxane, and hexamethyldisilane in the Si 2p and C 1s regions, measured under scattering conditions where electric dipole transitions dominate (2.5 keV residual energy, θ ≤ 2°). Comparison of the Si 1s and Si 2p spectra of the Ph3Si−X and Me3Si−X species shows there are low-lying transitions at Si which occur exclusively in the Ph3Si−X species. These transitions are attributed to (Si 1s-1,π*Si-Ph) and (Si 2p-1,π*Si-Ph) states in which the core excited electron is delocalized across the Si−C(phenyl) bond into the π* levels of the ...

The C 1s X-ray absorption spectra (XAS) of poly(diallyl phthalate), poly(diallyl isophthalate), and poly(ethylene terephthalate) (PET) have been recorded using transmission detection. The phthalate segments of these polymers are isomers with different patterns of substitution (ortho, meta, para) of the methyl carboxylate groups on the phenyl ring. The C 1s and O 1s electron energy loss spectra (EELS) of the corresponding isomeric monomers, dimethyl phthalate, dimethyl isophthalate, and dimethyl terephthalate, have also been recorded in the gas phase using inelastic electron scattering under conditions dominated by electric dipole transitions. Good agreement is found in overall shape and in the energies of the spectral features of the same isomer in monomeric (EELS) versus polymeric (XAS) form. Ab initio calculations are used to provide a detailed interpretation of the spectra, in particular the origin of the isomeric variations. The analytical potential for using inner shell excitation spectroscopy to ide...

The C 1s and O 1s X-ray absorption spectra of poly(ethylene terephthalate) (PET) have been recorded using transmission, fluorescence, and electron yield detection. The corresponding electron energy loss spectra (EELS) have been recorded in a scanning transmission electron microscope. These results are compared to the C 1s and O 1s spectra of gas phase 1,4-dimethyl terephthalate (the monomer of PET) recorded using EELS. The comparison of monomer and polymer materials in different phases and with different techniques has aided the understanding of the relative strengths and limitations of each technique as well as assisting the spectral interpretation. Good agreement is found in the overall shape and the energies of the spectral features. Relatively minor differences in intensities can be understood in terms of the properties of the individual spectroscopic techniques. The critical dose for radiation damage by 100 keV electrons incident on PET at 100 K is found to be (1.45 ± 0.15) × 103 eV nm-3. In contrast...

The epitaxial growth of organic molecules can lead to the formation of complex orientated morphologies. In previous work, we studied the kinetic and thermodynamic factors that drive the epitaxial growth of n-alkane thin films on HOPG(0001) and NaCl(001) by physical vapor deposition. A wide variety of morphologies are observed as a function of deposition conditions (substrate temperature, n-alkane chain length, etc.). In the current study we examine how a modified substrate (Au deposited on a HOPG(0001) or NaCl(001) substrate) affects the epitaxial growth of n-C36H74 (50 nm thick) relative to the uncoated substrates. This “indirect epitaxy”, in which the patterned attractive forces of the substrate are transferred through a thin metal film, can tailor the conditions for epitaxial growth. The observation of fourfold symmetry for n-alkane growth on Au/NaCl(001) and sixfold symmetry for n-alkane growth on Au/HOPG(0001) demonstrates indirect epitaxy over a wide range of substrate temperatures during deposition.