Inelastic mean free path of low‐energy electrons in condensed media: beyond the standard models

The most established approach for ‘practical’ calculations of the inelastic mean free path (IMFP) of low‐energy electrons (~10 eV to ~10 keV) is based on optical‐data models of the dielectric function. Despite nearly four decades of efforts, the IMFP of low‐energy electrons is often not known with the desired accuracy. A universal conclusion is that the predictions of the most popular models are in rather fair agreement above a few hundred electron volts but exhibit considerable differences at lower energies. However, this is the energy range where their two main approximations, namely, the random‐phase approximation (RPA) and the Born approximation, may be invalid. After a short overview of the most popular optical‐data models, we present an approach to include exchange and correlation (XC) effects in IMFP calculations, thus going beyond the RPA and Born approximation. The key element is the so‐called many‐body local‐field correction (LFC). XC effects among the screening electrons are included using a time‐dependent local‐density approximation for the LFC. Additional XC effects related to the incident and struck electrons are included through the vertex correction calculated using a screened‐Hubbard formula for the LFC. The results presented for liquid water reveal that XC may increase the IMFP by 15–45% from its Born–RPA value, yielding much better agreement with available experimental data. The present work provides a manageable, yet rigorous, approach to improve upon the standard models for IMFP calculations, through the inclusion of XC effects at both the level of screening and the level of interaction. Copyright © 2015 John Wiley & Sons, Ltd.     

Inelastic scattering and energy loss of swift electron beams in biologically relevant materials

The inelastic mean free path and the stopping power of swift electrons in relevant biomaterials, such as liquid water, DNA, protein, lipid, carotene, sugar, and ice are calculated in the framework of the dielectric formalism. The Mermin Energy Loss Function – Generalized Oscillator Strength model is used to determine the energy loss function of these materials for arbitrary energy and momentum transfer using electron energy‐loss spectroscopy data as input. To ensure the consistency of the model, efforts are made so that both the Kramers–Kronig and f‐sum rules are fulfilled to better than 2%. Our findings indicate sizeable differences in the inelastic mean free path and stopping power among these biomaterials for low‐energy electrons. For example, at 100‐eV electron energy, the inelastic mean free path in protein is 25% smaller than for water and around 10% smaller than for the other biomaterials. The stopping power values of protein, DNA, and sugar are rather similar but 20% larger than for water. Taking into account these results, we conclude that electron interactions with living tissues at the nanometric scale cannot be reliably described using only liquid water as the surrogate of the biological target. Copyright © 2016 John Wiley & Sons, Ltd.     

A new fundamental parameter based calibration procedure for micro X-ray fluorescence spectrometers

Fundamental parameter based quantification of X-ray fluorescence (XRF) measurement data requires an accurate knowledge of the spectrometer parameters, including the spectral distribution of the excitation radiation. In case of micro-XRF where a polycapillary optic is utilized in the excitation channel this distribution is changed due to the transmission properties of the lens. A new calibration procedure, based on fluorescence data of thin standard samples, was developed to determine the excitation spectrum, i.e., the product of the X-ray tube spectrum and the transmission of the used X-ray optic of a micro-XRF setup. The calibration result was validated by the quantitative analyses of certified multi-element reference standards and shows uncertainties in the order of 2% for main components, 10% for minor elements and 25% for trace elements. The influence of secondary order effects like Coster-Kronig transitions and cascade effects is analyzed and the accuracy of fundamental parameters in common databases is discussed.     

Torsional Photoisomerization Proceeding Adiabatically Through a Volume‐Conserving Pathway in Uninhibited Fluid Media

Doing the hula twist? A photochemically stimulated inversion of an sp²‐hybridized oxygen atom upon simultaneous rotation of two adjacent bonds may be possible in a pure singlet excited potential energy surface in uninhibited fluid media (see scheme).

     

Thorium adsorption by oxidized biochar fibres derived from Luffa cylindrica sponges

Journal of Radioanalytical and Nuclear Chemistry - The present study deals with the preparation of oxidized biochar fibres derived from Luffa cylindrica sponges and the subsequent adsorption of...     

Triterpenic derivatives of Achillea alexandri-regis BORNM. & RUDSKI

Investigation of the aerial parts of Achillea alexandri-regis led to the identification of 19 chemical constituents: twelve 3-O-fatty acid esters of triterpene alcohols arnidiol (1-4), maniladiol (5-8) and 16beta-hydroxylupeol (9-12), alpha-amyrin, beta-amyrin, beta-sitosterol, 3,4-O-dicaffeoyl quinic acid, cinnamic acid, pinoresinol-beta-D-glucoside and rutin. Among them, compounds 3, 4, 8, 11 and 12 are new natural products. The structures of all compounds have been elucidated on the basis of their spectral and chemical data.