Observations of the physical behaviour (motions) of the valence (frontier) electrons in the iconic benchmark hydride molecules CH4, NH3, and H2O are used to provide a direct experimental evaluation of hybridization and of the localized and delocalized orbital models of molecular electronic structure, after more than 80 years since the initial proposals of these theoretical concepts. Our previously published experimental measurements of valence (frontier) electron momentum probability distributions, made by electron momentum spectroscopy (EMS), are compared with those calculated using the localized molecular orbital (LMO) model and a range of hybrid orbital models, as well as with the delocalized canonical molecular orbital (CMO) and density functional theory Kohn–Sham orbital (KSO) models. In all cases, the electron momentum probability distributions calculated with LMOs and localized hybrid orbital type models are inconsistent with the experimental observations. In contrast, those calculated with the delocalized CMOs and KSOs are in very good agreement with the experimental measurements. These findings are of importance in those research applications such as reactivity, drug and novel material design, and also in molecular electronics, where the shapes and orientations of particular valence (frontier) orbital electron density probability distributions (and not the total electron densities) are considered to be key determining factors. These findings and their implications are also of pedagogical significance in chemistry and molecular physics.

Proton-Exchange Membrane Fuel Cell (PEM-FC) based engines are being developed rapidly for near-term implementation in hydrogen fueled, mass production, personal automobiles. Research is focused on understanding and controlling various degradation processes (carbon corrosion, Pt migration, cold start), and reducing cost by reducing or eliminating Pt catalyst. We are using soft X-ray scanning transmission X-ray microscopy (STXM) at the S 2p, C 1s, O 1s and F 1s edges to study a variety of issues related to optimization of PEM-FC materials for automotive applications. A method to efficiently and accurately measure perfluorosulfonic acid distributions was developed and is being used to better understand how different loadings and preparation methods affect the ionomer distribution in the cathode. Progress towards an environmental cell capable of controlling the temperature and humidity of a PEM-FC sample in the STXM is described. Methods for studying the 3D chemical structure of PEM-FC are outlined.

O 1s inner-shell excitation spectra of a number of vapor phase molecules containing peroxide bonds - hydrogen peroxide (H2O2), di-t-butylperoxide ((BuOBu)-Bu-t-Bu-t), benzoyl peroxide, ((C6H5(CO)O)(2)), luperox-F [1,3(4)-bis(tertbutylperoxyisopropyl)benzene], and analogous, non-peroxide compounds - water, t-butanol and benzoic acid have been measured. C 1s spectra are also reported. O 1s spectra of solid benzoic acid, di-t-butylperoxide and luperox-F recorded using a scanning transmission X-ray microscope, are also reported, and compared to the corresponding gaseous spectra. Spectral interpretation was aided by comparing the spectra of the peroxide and non-peroxide counterparts and with ab initio calculations. A characteristic O 1s -> sigma(O-O)* transition at 533.0(3) eV is identified in each peroxide species, which is absent in the corresponding non-peroxide counterpart species. The energy and intensity of the 533 eV peroxide feature is stable and thus useful for analysis of peroxides in mixtures, such as tracking residual peroxide initiators, or peroxides produced in fuel cells. (C) 2015 Elsevier B.V. All rights reserved.

Motivated by fundamental molecular physics and by atmospheric and planetary sciences, the valence excitations of N-2 gas have seen several decades of intensive study, especially by electron-energy-loss spectroscopy (EELS). It was consequently surprising when a comparison of nonresonant inelastic x-ray scattering (NIXS) and nonresonant EELS found strong evidence for violations of the first Born approximation for EELS when leaving the dipole scattering limit. Here we reassess the relative strengths of the constituent resonances of the lowest-energy excitations of N-2, encompassed by the so-called Lyman-Birge-Hopfield (LBH) band, expanding on the prior, qualitative interpretation of the NIXS results for N-2 by both quantifying the generalized oscillator strength of the lowest-energy excitations and also presenting a time-dependent density functional theory (TDDFT) calculation of the q dependence of the entire low-energy electronic excitation spectrum. At high q, we find that the LBH band has an unexpectedly large contribution from the octupolar w(1)Delta(u) resonance exactly in the regime where theory and EELS experiment for the presumed-dominant a(1)Pi(g) resonance have previously had substantial disagreement, and also where the EELS results must now be expected to show violations of the Born approximation. After correcting for this contamination, the a(1)Pi(g) generalized oscillator strength from the NIXS results is in good agreement with prior theory. The NIXS spectra, over their entire q range, also find satisfactory agreement with the TDDFT calculations for both bound and continuum excitations.

In situ Scanning Transmission X-ray Microscopy (STXM) with humidity control at the 10ID-1 spectromicroscopy beamline at the Canadian Light Source was used to study the effect of humidity on individual SnO2 coated carbon nanotubes (SnO2/CNT). O 1s STXM image stacks of individual SnO2 coated CNT were measured at controlled relative humidity (RH). At high RH = 0.78, 10-50 nm of condensed water was found adsorbed on individual SnO2/CNT as determined by spectral curve fitting to O 1s image sequences. Coating variations among individual SnO2/CNT were found. This is consistent with O 1s NEXAFS spectra extracted from the same individual SnO2/CNT. For some entangled SnO2/CNT regions, as much as 200 nm condensed liquid water was detected, indicating strong interaction between SnO2 and water. (C) 2010 Elsevier B.V. All rights reserved.

We present measurements of the spectra of electrons with energy between 0.6 and 2.25 keV elastically and inelastically scattered from Ar and Ne over large angles (from 3 degrees to 135 degrees). The intensity of the first loss feature [np -> (n + 1)s], relative to that of the elastic peak, was determined and compared with the results of relativistic distorted-wave calculations (for the energy loss part) and a relativistic optical potential method (for the elastic peak). Good agreement was found. The distorted-wave calculations are compared with first Born calculations. At small angles, both theories coincide and estimates of the optical oscillator strength are obtained. However, at large angles, the first Born approximation predicts negligible intensity, in strong contrast to the distorted-wave theory and the experimental data. The implications of these results for the interpretation of measurements of the generalized oscillator strength are discussed.

Bound-state, valence electronic excitation spectra of N-2 are probed by nonresonant inelastic x-ray and electron scattering. Within usual theoretical treatments, dynamical structure factors derived from the two probes should be identical. However, we find strong disagreements outside the dipole scattering limit, even at high probe energies. This suggests an unexpectedly important contribution from intramolecular multiple scattering of the probe electron from core electrons or the nucleus. These effects should grow progressively stronger as the atomic number of the target species increases.

Received 28 July 2010DOI:https://doi.org/10.1103/PhysRevLett.105.069903©2010 American Physical Society

Over the past four years we have measured quasi-elastic electron scattering spectra from molecules and atoms at large momentum transfer (100° angle, 2.25keV incident energy, ∼20a.u.). The peak positions agree completely with those predicted by classical conservation of momentum and energy, assuming the electron scatters from each atom independently. However the peak intensities do not agree with expectations, particularly for light elements. According to classical electron Compton scattering, quasi-elastic peak intensities should be proportional to nuclear charge squared. However, our recent study [Phys. Rev. Lett. 100 (2008) 043204] found a significant deviation (∼30%) in the intensities of the H versus D signals relative to this prediction. Here we present new quasi-elastic electron scattering data for H2/D2, Ar/H2, Ar/D2 and He/H2 and Ar/He mixtures. The new H2/D2 data confirm the earlier result – quasi-elastic scattering by H is low by ∼31(4)% compared with D. More significantly, when compared to He the quasi-elastic scattering intensity by H is 48(6)% lower and that for D is 30(3)% lower relative to that expected from Compton scattering theory. When compared to the quasi-elastic signal from Ar, H shows a 63(6)%, D shows a 45(5)% and He shows a 35(8)% reduced intensity as compared to that expected from Compton scattering theory. When cross-compared all the results are internally consistent, confirming that quasi-elastic scattering intensities for light elements are anomalously low compared to both classical electron Compton scattering predictions and a recent quantum mechanical treatment within the first Born approximation [J. Chem. Phys. 130 (2009) 144303]. The reason for the anomalously low QEES intensities for light elements is unknown at this time.

The alignment of β-sheets within spider dragline silk fibers is an important factor in their tensile strength and extensibility. We are using linear dichroism of the C 1s → π*amide transition measured using scanning transmission X-ray microscopy (STXM) to generate quantitative maps of the orientation parameters with 30 nm spatial resolution. Here we have extended these measurements from dry samples to samples with partial or full hydration. A device for monitoring and controlling the humidity of a sample in the STXM is described and used to measure the effect of saturated humidity on a section of N. clavipes dragline spider silk. The microstructure and distributions of molecular orientation change considerably with hydration in ways consistent with the supercontraction observed in free standing dragline spider silk. The STXM results are compared to infrared and Raman microscopy results.

A significant anomaly in the ratio of the cross sections for quasielastic scattering of D(2) to H(2) in a 50:50 mixture has been reported recently [Phys. Rev. Lett. 100, 043204 (2008)]. In particular, the interpretation of quasielastic scattering provided in that work predicts that the signals should be based on the elemental content and not on the molecular structure, and thus the spectrum for HD and that for the 50:50 mixture should be effectively the same, aside from minor line shape differences. In fact the signal from H in the mixture was low by 31(4)% (or that from D was correspondingly high) when compared to the signal from H in HD. Here, an attempt is made to explain this anomaly by scattering theory calculations based on the first Born and Born-Oppenheimer approximations. The relative contributions of translational, vibrational, and rotational excitations are computed for several temperatures. The computed signals are compared to the experimental spectra of HD and the 50:50 H(2)/D(2) mixture and to pure H(2) and D(2), which were reported previously. Good agreement is found for the spectral line shapes. The predicted peak positions are also found to be in good agreement with experiment except for a slight shift in the case of D(2). However the anomalous cross section ratio of D(2) to H(2) experimentally observed in the 50:50 mixture could not be explained by this approach.

Quasielectron electron scattering from gaseous H2, D2, a 50:50 mixture of H2 and D2, and HD is investigated with 2.25 keV impact energy and a momentum transfer variant Planck's over 2piq of 19.7 a.u. The energy transfer is less than the dissociation energy. The spectral positions of the H and D recoil peaks agree with Rutherford scattering theory. Surprisingly, in the spectrum of the 50:50 H2-D2 mixture, the integrated intensity of the H peak is 31%+/-4% lower (as compared to that of D) than predicted by Rutherford scattering, despite equal screening of nuclear charges by the electrons. In contrast, the ratio of scattering intensities from H and D in HD agrees with the predictions of Rutherford scattering. Comparison is made with neutron Compton scattering results from the same systems, but at higher energy transfers causing bond breaking. Possible theoretical explanations are outlined.

Electron energy loss spectra of gas phase carbon disulfide have been recorded under dipole and strongly non-dipole conditions in the region of S 2p, and C 1s excitation. Inner-shell triplet states have also been observed by measuring near threshold C 1s energy loss spectra using a wide range of angles (4–20°) and impact energies (130–1300eV). The optical oscillator strength (OOS) and generalized oscillator strength profiles (GOS) have been calculated for vertical excitation from the ground X1Σg+ electronic state to several C(1s) and S(2p) inner-shell electronic excited states of CS2 molecule, using high level ab initio (HF-CI) calculations.

An (e, e+ion) spectrometer has been constructed that has the capability to measure time-of-flight (TOF) mass spectra of gases in coincidence with energy selected inelastically scattered electrons, as a function of impact energy, electron energy loss and electron scattering angle. Relative to earlier dipole-regime implementations of the (e, e+ion) technique, this spectrometer can be used to study molecular fragmentation under both dipole and non-dipole electronic excitation and ionization conditions. The spectrometer uses a position-sensitive electron detector and a TOF tube positioned at 90° with respect to the electron impact and scattering plane. The TOF design makes it possible either to extract all ions from the interaction region, or to discriminate preferentially for ions that have kinetic energy along the axis of the TOF tube, thus allowing one to collect spectra with maximum efficiency, or to study the dynamics of the production of the molecular and fragment ions. The design and construction of the spectrometer is described, along with preliminary results for ionization in the S 2p excitation and ionization region of SF6.

Quasielastic electron scattering from gaseous species at high momentum transfer was recently reported for the first time [Cooper et al., J. Electron Spectrosc. Relat. Phenom. 155, 28 (2007)]. The first results for CH(4) and CD(4) were well explained by a classical electron Compton scattering picture in which the electron scatters independently from each atom rather than the molecule as a whole. However, an alternative possible interpretation in terms of nondipole molecular vibrational excitation is suggested by previously published quantum mechanical calculations on high momentum transfer electron scattering from diatomic molecules [Bonham and de Souza, J. Chem. Phys. 79, 134 (1983)]. In order to determine which of these two interpretations best fits the experimental results, we have measured the quasielastic spectra of gaseous 2-methylpropane, ethylene, methane, and two isotopically substituted methanes, CH(2)D(2) and CD(4), at a momentum transfer of approximately 20 a.u. (2.25 keV impact energy and 100 degrees scattering angle). The experimental spectra are found to be composed of as many peaks as there are different atomic isotopes in the molecule (two for CH(4), C(2)H(4), 2-methylpropane, and CD(4) and three for CH(2)D(2)). The peak positions are predicted accurately by the independent atom electron Compton scattering model, and the relative intensities are in reasonable agreement. The experimental results thus support classical electron Compton scattering as the origin of the signal.

The generalized oscillator strength profiles in the momentum transfer range (K) of (2a.u.−2<K2<30a.u.−2) for the most intense resolved S 2p, and C 1s transitions of carbon disulfide (CS2) are presented. Optical oscillator strengths and generalized oscillator strength profiles have been calculated for vertical excitation from the ground X1Σg+ electronic state to several C(1s) and S(2p) inner-shell electronic excited states of CS2, using high level ab initio (HF-CI) calculations. The experimental and computed GOS profiles of CS2 are compared and found to be generally in reasonable agreement.

The inner-shell (C 1s, N 1s, O 1s) excitation spectra of dimethoxymethanone, diphenoxymethanone, and a series of α,γ-dicarbonyl compounds – 2,4-pentanedione, N-acetylacetamide, malonamide, acetyl anhydride, dimethyl malonate, diethyl malonate, 2-imidodicarbonic diamide, di-t-butyl iminodicarboxylate and dimethyldicarbonate – have been recorded in the gas phase with inner shell electron energy loss spectroscopy in the scattering regime dominated by electric dipole transitions. All spectra are presented on absolute oscillator strength intensity scales. They are interpreted with the aid of chemical series systematics and with the help of ab initio calculations. As found in a recent study of the X-ray absorption spectra of condensed carbonyl compounds [S.G. Urquhart, H. Ade, J. Phys. Chem. B 106 (2002) 8531], there is a very systematic correlation of the C 1s→πCO∗ transition energy and the relative oxidation at the carbonyl carbon, as expressed by a suitable oxidation index such as the sum of the atomic numbers or Pauling electronegativities of the elements bonded to the carbonyl carbon. Although the calculations clearly show there is delocalization between the two carbonyl groups, this had no detectible influence on the inner shell spectra. The absence of signals associated with π∗ delocalization is explained in terms of core hole localization and symmetry effects.