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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. [ABSTRACT FROM AUTHOR]

The nature of the linear dichroism in the Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectra of linear n-alkanes is studied. The results show that failure to properly resolve spectroscopic features with differing angle dependence would lead to an erroneous measurement of the angle dependence of the NEXAFS spectra.

A study was conducted to analyze the sensitivity of near-edge X-ray absorption fine structure to differences in key chemical components of polyurethane polymers. The polymers were chosen to isolate particular chemical environments present in complex polyurethane polymers. Results indicated that differences in phenyl ring substitution promote significant variations in transition energies and character.

Thiophenic compounds are major constituents of fossil fuels and pose problems for fuel refinement. The quantification and speciation of these compounds is of great interest in different areas such as biology, fossil fuels studies, geology, and archaeology. Sulfur 1s Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy has emerged as a qualitative and quantitative method for sulfur speciation. A firm understanding of the sulfur 1s NEXAFS spectra of organosulfur species is required for these analytical studies. To support this development, the sulfur 1s NEXAFS spectra of simple thiols and thioethers were previously examined, and are now extended to studies of thiophenic and aromatic thioether compounds, in the gas and condensed phases. High-resolution spectra have been further analyzed with the aid of Improved Virtual Orbital (IVO) and Δ(self-consistent field) ab initio calculations. Experimental sulfur 1s NEXAFS spectra show fine features predicted by calculation, and the combination of experiment and calculation has been used to improve the assignment of spectroscopic features important for the speciation and quantification of sulfur compounds. Systematic differences between gas and condensed phases are also explored; these differences suggest a significant role for conformational effects in the NEXAFS spectra of condensed species. [ABSTRACT FROM AUTHOR]

We present high resolution oxygen K near-edge x-ray absorption spectra of the acenaphthenequinone (ANQ) derivative 3,8-dibromo-5,6-dichloro-ANQ (Br2Cl2-ANQ). The spectral features exhibit an almost identical vibronic fine structure compared to that shown by ANQ. The unequal distances of the vibronic levels as derived from the least-squares fit analysis of the vibronic progressions allows us to determine the anharmonicity of the excited state potentials involved. We conclude that a single vibrational progression couples to the resonant excitation of O 1s core electrons preferentially. Comparison of the two ANQ derivatives gives a clear indication that the vibronic mode corresponds to a C=O stretching mode, rather than coupling to a C-H mode as suggested previously. These conclusions are supported by density functional theory calculations. [ABSTRACT FROM AUTHOR]

The speciation and quantification of sulfur species based on sulfur K-edge x-ray absorption spectroscopy is of wide interest, particularly for biological and petroleum science. These tasks require a firm understanding of the sulfur 1s near-edge x-ray absorption fine structure (NEXAFS) spectra of relevant species. To this end, we have examined the gas phase sulfur 1s NEXAFS spectra of a group of simple thiol and thioether compounds. These high-resolution gas phase spectra are free of solid-state broadening, charging, and saturation effects common in the NEXAFS spectra of solids. These experimental data have been further analyzed with the aid of improved virtual orbital Hartree-Fock ab initio calculations. The experimental sulfur 1s NEXAFS spectra show fine features predicted by calculation, and the combination of experiment and calculation has been used to improve assignment of spectroscopic features relevant for the speciation and quantification of the sulfur compounds. [ABSTRACT FROM AUTHOR]

We have recorded the iron 2p and carbon 1s near-edge x-ray absorption fine structure (NEXAFS) spectra of decamethyl ferrocene (Fe 3d6) and ferrocenium hexafluorophosphate (Fe 3d5), two low spin compounds with closed- and open-shell electron configurations, respectively. These species have a similar covalent bonding environment but a different electron configuration, allowing us to examine the role of ligand-metal interactions and electron correlation responsible for their NEXAFS spectra. We have interpreted these spectra with the aid of ab initio configuration interaction (CI) calculations. The CI calculations indicate that double excitation is essential to interpret the Fe 2p NEXAFS spectra of not only the open-shell ferrocenium ion but also the closed-shell ferrocene species, even though the ground states of both species are well described within the Hartree–Fock single-configuration approximation. [ABSTRACT FROM AUTHOR]

The carbon 1s near edge x-ray absorption fine structure (NEXAFS) spectra of simple gaseous alkane molecules differ from the spectra of the same alkane molecules in the condensed phase. The origin of these large, systematic differences is poorly understood. The NEXAFS spectra of gaseous alkanes are interpreted as a progression of core→Rydberg transitions with distinctive vibronic structure. The interpretation of the NEXAFS spectra of condensed phase alkanes is varied. Specifically, the degree of Rydberg character in the preedge core excited states of condensed alkanes is controversial. We determined the character of core excited states in condensed alkanes with a combination of experiment and computational study. From this, we have determined the nature of matrix effects for these species. The high-resolution carbon 1s NEXAFS spectrum of gaseous neopentane is dramatically different from its condensed phase spectrum, a striking illustration of the dramatic spectroscopic changes that occur upon condensation. High quality ab initio calculations of a cluster designed to model the solid phase environment provide definitive evidence for the reduction of Rydberg character and support the assignment of σC–H* valence character in the preedge features in the NEXAFS spectra of condensed alkanes. [ABSTRACT FROM AUTHOR]

Byline: Stephen G. Urquhart, Uday D. Lanke, Juxia Fu Soft X-ray spectromicroscopy is emerging as a powerful method for the chemical and structural analysis of nanostructured organic materials. X-ray spectromicroscopy combines the chemical and structural sensitivity of Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy with the high spatial resolution of X-ray microscopy. Linear Dichroism (LD), the anisotropic absorption of linearly polarised radiation by an oriented molecule, is observed in NEXAFS spectra. LD-NEXAFS offers excellent sensitivity to molecular orientation, and can be used to characterise molecular order in materials at high spatial resolution. Technical developments in X-ray microscopy and synchrotron undulator sources have enhanced X-ray Linear Dichroism Microscopy (XLDM) studies of organic materials. This paper will review the state of XLDM for studies of oriented organic materials, its relationship to other techniques, and the prospects for the study of nanoscale organic materials with new spectromicroscopy facilities, such as those at the Canadian Light Source.

The nitrogen 1s near-edge X-ray absorption fine structure (NEXAFS) spectra of several amino acids and other model compounds were acquired and their experimental measurements complemented with ab initio calculations of isolated molecules and molecular clusters to unambiguously characterize the chemical origin of the considerable variation. It is observed that the differences observed between the zwitterionic and un-ionized forms of amino acids arise directly from the structural difference, which leads to a change in the degree of Rydberg-valence mixing.

The high-resolution carbon 1s near-edge X-ray absorption fine structure (NEXAFS) spectra of methane, ethane, propane, isobutane, and neopentane and the results of high-quality ab initio calculations on these species are presented. The results of ab initio calculations are used to characterize the degree and nature of Rydberg-valence mixing in these spectra and to identify the structural requirements under which significant Rydberg-valence mixing occurs.

The study complies a set of reliable experimental C 1s near-edge X-ray absorption fine structure (NEXAFS) or Inner Shell Electron Energy Loss (ISEELS) spectra of substituted benzene molecules with a wide range of subtituents. The study explores these chemical shifts through ab initio calculations and carefully calibrated experimental data for a wide range of molecules containing substituted benzene rings.

A complex orientational morphology is observed when n-alkane thin films are vapor deposited on highly orientedpyrolyticgraphite surfaces. Substrate temperature can be used to tune the orientationand morphology of n-alkane thin films. The molecularorientation changes from lateral to normal to the surface when thesubstrate temperature is raised sufficiently. Under specific substratetemperature conditions, the n-alkane molecules arealigned in the plane of the sample surface, in directions reflectingthe 6-fold symmetry of the graphite substrate. A series of differentmorphologies, from uniform thin films to oriented bars, are observedas a function of chain length and substrate temperature. The systematicevolution of these oriented morphologies is mapped as a function ofdeposition conditions, and the kinetic and thermodynamic factors thatgovern the formation of different in-plane and normal domains areconsidered. [ABSTRACT FROM AUTHOR]

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.