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.

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.