Inelastic mean-free path and mean escape depth of 10-140 eV electrons in SiO2 nanoparticles determined by Si 2p photoelectron yields.

We report on photoelectron spectra of SiO2 nanoparticles (d = 157 ± 6 nm) above the Si 2p threshold in the photon energy range 118-248 eV with electron kinetic energy 10-140 eV and analyze the photoelectron yield as a function of photon energy. Comparison of the experimental results with Monte-Carlo simulations on electron transport allows us to quantify the inelastic mean-free path and mean escape depth of photoelectrons in the nanoparticle samples. The influence of the nanoparticle geometry and electron elastic scattering on photoelectron yields is highlighted. The results show that the previously proposed direct proportionality of the photoelectron signal to the inelastic mean-free path or the mean escape depth does not hold for photoelectron kinetic energies below 30 eV due to the strong influence of electron elastic scattering. The present results deviate for photoelectron kinetic energies below 30 eV from the previously proposed direct proportionality of the photoelectron signal to the inelastic mean-free path or the mean escape depth, which is the result of a strong influence of electron elastic scattering. The presented inelastic mean-free paths and mean escape depths appear to be useful for the quantitative interpretation of photoemission experiments on nanoparticles and for modeling of the experimental results.

Size-dependent ion emission asymmetry of free NaCl nanoparticles excited by intense femtosecond laser pulses.

We report on asymmetric ion emission of size-selected NaCl nanoparticles (d = 100-600 nm) ionized by intense femtosecond laser pulses (λ = 800 nm, peak intensity ∼1013 W cm-2). Velocity map imaging indicates that a higher ion yield is observed in the propagation direction of the laser pulses than in the opposite direction. This asymmetric ion emission is found to be size-dependent and increases with particle size. This pronounced size dependence is interpreted in terms of discrete dipole simulations of the internal electric field in the nanoparticles, which reveal that the internal field is enhanced in the forward propagation direction of the laser pulses, occurring for nanoparticles >100 nm. The ion emission asymmetry is further found to depend on the peak intensity of the laser radiation. Nanoparticles of 100 nm show a symmetric distribution of ion emission, while the ion emission for 600 nm particles is found to become increasingly symmetric as the peak intensity is increased. In addition to single pulse ionization experiments, we explore the angular distribution of ion emission of resonantly heated NaCl nanoparticles using a pump-probe setup. Here, ion emission is found to be more symmetric for resonantly heated nanoparticles than for single pulse excitation. These differences are explained by the absorption mechanism, where the probe pulse in a dual pulse experiment can be efficiently absorbed by plasmonic excitation for suitable delays between both laser pulses.

Photoionization dynamics of cis-dichloroethene from investigation of vibrationally resolved photoelectron spectra and angular distributions.

The influence of vibronic coupling on the outer valence ionic states of cis-dichloroethene has been investigated by recording photoelectron spectra over the excitation range 19-90 eV using plane polarized synchrotron radiation, for two polarization orientations. The photoelectron anisotropy parameters and electronic state branching ratios derived from these spectra have been compared to theoretical predictions obtained with the continuum multiple scattering approach. This comparison shows that the photoionization dynamics of the Ã2B2, B̃2A1, C̃2A2, and D̃2B1 states, all of which are formed through the ejection of an electron from a nominally chlorine lone-pair orbital, exhibit distinct evidence of the Cooper minimum associated with the halogen atom. While retaining a high degree of atomic character, these orbital ionizations nevertheless display clear distinctions. Simulations, assuming the validity of the Born-Oppenheimer and the Franck-Condon approximations, of the X̃2B1, Ã2B2, and D̃2B1 state photoelectron bands have allowed some of the vibrational structure observed in the experimental spectra to be assigned. The simulations provide a very satisfactory interpretation for the X̃2B1 state band but appear less successful for the Ã2B2 and D̃2B1 states, with irregularities appearing in both. The B̃2A1 and C̃2A2 state photoelectron bands exhibit very diffuse and erratic profiles that cannot be reproduced at this level. Photoelectron anisotropy parameters, ß, have been evaluated as a function of binding energy across the studied photon energy range. There is a clear step change in the ß values of the Ã2B2 band at the onset of the perturbed peak intensities, with ß evidently adopting the value of the B̃2A1 band ß. The D̃2B1 band ß values also display an unexpected vibrational level dependence, contradicting Franck-Condon expectations. These various behaviors are inferred to be a consequence of vibronic coupling in this system.

An experimental and theoretical study of the photoelectron spectra of cis-dichloroethene: Valence shell vertical ionization and vibronic coupling in the low-lying cationic states.

The valence shell photoelectron spectrum of cis-dichloroethene has been studied both experimentally and theoretically. Photoelectron spectra have been recorded with horizontally and vertically plane polarized synchrotron radiation, thereby allowing the anisotropy parameters, characterizing the angular distributions, to be determined. The third-order algebraic-diagrammatic construction approximation scheme for the one-particle Green's function has been employed to compute the complete valence shell ionization spectrum. In addition, the vertical ionization energies have been calculated using the outer valence Green's function (OVGF) method and the equation-of-motion coupled-cluster, with single and double substitutions for calculating ionization potentials (EOM-IP-CCSD) model. The theoretical results have enabled assignments to be proposed for most of the structure observed in the experimental spectra, including the inner-valence regions dominated by satellite states. The linear vibronic coupling model has been employed to study the vibrational structure of the lowest photoelectron bands, using parameters obtained from ab initio calculations. The ground state optimized geometries and vibrational frequencies have been computed at the level of the second-order Møller-Plesset perturbation theory, and the dependence of the ionization energies on the nuclear configuration has been evaluated using the OVGF method. While the adiabatic approximation holds for the X̃2B1 state photoelectron band, the Ã2B2, B̃2A1, and C̃2A2 states interact vibronically and form a complex photoelectron band system with four distinct maxima. The D̃2B1 and Ẽ2B2 states also interact vibronically with each other. The potential energy surface of the D̃2B1 state is predicted to have a double-minimum shape with respect to the out-of-plane a2 deformations of the molecular structure. The single photoelectron band resulting from this interaction is characterized by a highly irregular structure, reflecting the non-adiabatic nuclear dynamics occurring on the two coupled potential energy surfaces forming a conical intersection close to the minimum of the Ẽ2B2 state.

Surface Composition of Free Mixed NaCl/Na2SO4 Nanoscale Aerosols Probed by X-ray Photoelectron Spectroscopy.

The local chemical surface composition of unsupported mixed solid NaCl/Na2SO4 aerosols ( d ∼ 70 nm) is studied by X-ray photoelectron spectroscopy. The solid aerosols are generated by drying aqueous droplets containing mixtures of the two salts in different mole fractions. The mole fraction of these salts is found to deviate at the solid aerosol surface significantly from the initial droplet composition. The minority species in the droplets are found to be enhanced at the surface of the solid mixed aerosols. This surface enhancement is rationalized in terms of the nucleation/crystallization process, where the salts evidently do not cocrystallize, rather than each salt forms pure crystal moieties. Characteristic variations of the surface ion concentration as a function of the mole fraction of the salts in the initial droplet are observed in the nanometer size regime. This is unlike core-shell architectures previously found in mixed micron salt aerosols, indicating that aerosol models derived from micron-sized aerosols are evidently not fully reliable to describe the surface composition of nanosized aerosols. Furthermore, surface enhancement of the minority component in mixed NaCl/Na2SO4 aerosols is also different from previous results on surface segregation of mixed NaCl/NaBr aerosols, where one of the anionic species is surface segregated for all mole fractions, which was explained in terms of the ability of the involved salts to cocrystallize and forming solid solutions. The present results rather indicate that mixed NaCl/Na2SO4 aerosols do not cocrystallize. Electron microscopy of deposited mixed salt aerosols reveals mostly a cubic structure of pure NaCl aerosols, whereas mixed salt aerosols are found to show a grainy structure composed of multiple small crystals which supports the present findings obtained from photoelectron spectroscopy.

Photoelectron angular distribution from free SiO2 nanoparticles as a probe of elastic electron scattering.

In order to gain quantitative information on the surface composition of nanoparticles from X-ray photoelectron spectroscopy, a detailed understanding of photoelectron transport phenomena in these samples is needed. Theoretical results on the elastic and inelastic scattering have been reported, but a rigorous experimental verification is lacking. We report in this work on the photoelectron angular distribution from free SiO2 nanoparticles (d = 122 ± 9 nm) after ionization by soft X-rays above the Si 2p and O 1s absorption edges, which gives insight into the relative importance of elastic and inelastic scattering channels in the sample particles. The photoelectron angular anisotropy is found to be lower for photoemission from SiO2 nanoparticles than that expected from the theoretical values for the isolated Si and O atoms in the photoelectron kinetic energy range 20-380 eV. The reduced angular anisotropy is explained by elastic scattering of the outgoing photoelectrons from neighboring atoms, smearing out the atomic distribution. Photoelectron angular distributions yield detailed information on photoelectron elastic scattering processes allowing for a quantification of the number of elastic scattering events the photoelectrons have undergone prior to leaving the sample. The interpretation of the experimental photoelectron angular distributions is complemented by Monte Carlo simulations, which take inelastic and elastic photoelectron scattering into account using theoretical values for the scattering cross sections. The results of the simulations reproduce the experimental photoelectron angular distributions and provide further support for the assignment that elastic and inelastic electron scattering processes need to be considered.

First in-flight synchrotron X-ray absorption and photoemission study of carbon soot nanoparticles.

Many studies have been conducted on the environmental impacts of combustion generated aerosols. Due to their complex composition and morphology, their chemical reactivity is not well understood and new developments of analysis methods are needed. We report the first demonstration of in-flight X-ray based characterizations of freshly emitted soot particles, which is of paramount importance for understanding the role of one of the main anthropogenic particulate contributors to global climate change. Soot particles, produced by a burner for several air-to-fuel ratios, were injected through an aerodynamic lens, focusing them to a region where they interacted with synchrotron radiation. X-ray photoelectron spectroscopy and carbon K-edge near-edge X-ray absorption spectroscopy were performed and compared to those obtained for supported samples. A good agreement is found between these samples, although slight oxidation is observed for supported samples. Our experiments demonstrate that NEXAFS characterization of supported samples provides relevant information on soot composition, with limited effects of contamination or ageing under ambient storage conditions. The highly surface sensitive XPS experiments of airborne soot indicate that the oxidation is different at the surface as compared to the bulk probed by NEXAFS. We also report changes in soot's work function obtained at different combustion conditions.

Interatomic scattering in energy dependent photoelectron spectra of Ar clusters.

Soft X-ray photoelectron spectra of Ar 2p levels of atomic argon and argon clusters are recorded over an extended range of photon energies. The Ar 2p intensity ratios between atomic argon and clusters' surface and bulk components reveal oscillations similar to photoelectron extended X-ray absorption fine structure signal (PEXAFS). We demonstrate here that this technique allows us to analyze separately the PEXAFS signals from surface and bulk sites of free-standing, neutral clusters, revealing a bond contraction at the surface.

A fresh look at the photoelectron spectrum of bromobenzene: A third-order non-Dyson electron propagator study.

The valence-shell ionization spectrum of bromobenzene, as a representative halogen substituted aromatic, was studied using the non-Dyson third-order algebraic-diagrammatic construction [nD-ADC(3)] approximation for the electron propagator. This method, also referred to as IP-ADC(3), was implemented as a part of the Q-Chem program and enables large-scale calculations of the ionization spectra, where the computational effort scales as n(5) with respect to the number of molecular orbitals n. The IP-ADC(3) scheme is ideally suited for investigating low-lying ionization transitions, so fresh insight could be gained into the cationic state manifold of bromobenzene. In particular, the present IP-ADC(3) calculations with the cc-pVTZ basis reveal a whole class of low-lying low-intensity two-hole-one-particle (2h-1p) doublet and quartet states, which are relevant to various photoionization processes. The good qualitative agreement between the theoretical spectral profile for the valence-shell ionization transitions generated with the smaller cc-pVDZ basis set and the experimental photoelectron spectrum measured at a photon energy of 80 eV on the PLÉIADES beamline at the Soleil synchrotron radiation source allowed all the main features to be assigned. Some theoretical aspects of the ionization energy calculations concerning the use of various approximation schemes and basis sets are discussed.

The influence of the bromine atom Cooper minimum on the photoelectron angular distributions and branching ratios of the four outermost bands of bromobenzene.

Angle resolved photoelectron spectra of the X̃(2)B1, Ã(2)A2, B̃(2)B2, and C̃(2)B1 states of bromobenzene have been recorded over the excitation range 20.5-94 eV using linearly polarized synchrotron radiation. The photoelectron anisotropy parameters and electronic branching ratios derived from these spectra have been compared to theoretical predictions obtained with the continuum multiple scattering approach. This comparison shows that ionization from the 8b2 orbital and, to a lesser extent, the 4b1 orbital is influenced by the Cooper minimum associated with the bromine atom. The 8b2 and 4b1 orbitals are nominally bromine lone-pairs, but the latter orbital interacts strongly with the π-orbitals in the benzene ring and this leads to a reduced atomic character. Simulations of the X̃(2)B1, B̃(2)B2, and C̃(2)B1 state photoelectron bands have enabled most of the vibrational structures appearing in the experimental spectra to be assigned. Many of the photoelectron peaks exhibit an asymmetric shape with a tail towards low binding energy. This asymmetry has been examined in the simulations of the vibrationally unexcited peak, due mainly to the adiabatic transition, in the X̃(2)B1 state photoelectron band. The simulations show that the asymmetric profile arises from hot-band transitions. The inclusion of transitions originating from thermally populated levels results in a satisfactory agreement between the experimental and simulated peak shapes.

The frequency content of gait.

We address amplification of noise in double differentiation of position histories for dynamic analysis of gait. Measurements of the frequency domain characteristics of signal and noise are required to quantitatively assess errors in raw, filtered, and dynamic gait data. The results of a simple technique to determine the frequency content of gait using a population of 12 subjects and a total of 30 gait records is presented.

Gait analysis--precise, rapid, automatic, 3-D position and orientation kinematics and dynamics.

A fully automatic optoelectronic photogrammetric technique is presented for measuring the spatial kinematics of human motion (both position and orientation) and estimating the inertial (net) dynamics. Calibration and verification showed that in a two-meter cube viewing volume, the system achieves one millimeter of accuracy and resolution in translation and 20 milliradians in rotation. Since double differentiation of generalized position data to determine accelerations amplifies noise, the frequency domain characteristics of the system were investigated. It was found that the noise and all other errors in the kinematic data contribute less than five percent error to the resulting dynamics.