Monte Carlo simulation of bremsstrahlung emission by electrons

The basic components of Monte Carlo simulation of bremsstrahlung emission by electrons are presented. Various theoretical cross-sections that have been used in Monte Carlo codes are described and the emphasis is on the more accurate partial-wave cross-sections for which numerical databases are available. Sampling algorithms for a combination of numerical scaled energy-loss cross-sections and various analytical approximations to the intrinsic angular distribution are presented. Analogue simulation of the energy spectra and angular distribution of X rays from targets irradiated by electron beams is very inefficient and a simple variance-reduction technique, which is easy to implement and has proven to be particularly effective in speeding up these simulations, is described. Results from simulations of X-ray spectra with the general-purpose Monte Carlo code penelope are compared with experimental data for different materials and incident electrons with energies in the 20 keV to 1 GeV energy range.     

Fermi-shuttle processes in the electron emission by ion impact: Contribution to radiation damages

Secondary electrons, formed in biological tissues by high-energy particle impact, significantly contribute to the fragmentation of small molecules and to single- and double-strand brakes in DNA. Differential spectra of electrons emitted in the collisions of decelerating swift ions are of vital importance for estimating ion impact radiation damages. We demonstrate that the so-called Fermi-shuttle-type acceleration mechanism can produce a significant enhancement in the emission of high-energy secondary electrons. Double differential cross-sections for electron emission, measured in the collisions of N⁺ and N₂⁺ ions with Ar targets at 750 keV/u impact energy, clearly show this effect. The measured cross-sections are in good agreement with the theoretical results of CTMC calculations. Multiple scattering contribution to the Ar spectra above 300 eV is proved to be significant.     

Experimental results on 2–30 keV bremsstrahlung from thick and thin targets

The recent experimental investigations on electron bremsstrahlung produced from impact of 2–30 keV electrons with thick solid and thin gaseous targets are reviewed. The theoretical models describing the energy and angular distributions of bremsstrahlung photons are discussed with their brief outlines and formulations to explain the experimental data. The results on thick target bremsstrahlung (TTB) spectra produced by keV electrons have suggested that there is a need to develop a comprehensive theory for accounting the solid state effects. It is further noted that the prediction of the modified KKD formula gives a reasonable agreement with the TTB data, whereas a semi-empirical formula gives a better fit to the data for thick targets. The available experimental data for dependence of double differential cross-sections of emitted photons on impact energy and their emission angles for gaseous atoms and molecules exhibit a good agreement with the theoretical calculations of Kissel et al., [1983. Shape functions for atomic-field bremsstrahlung from electrons of kinetic energy 1–500 keV on selected neutral atoms 1<Z<92. Atom. Data Nucl. Data Tables 28, 381–460].     

Spectral properties and G-quadruplex DNA binding selectivities of a series of unsymmetrical perylene diimides

A series of new unsymmetrical perylene diimides have been synthesized to investigate their binding selectivities to G-quadruplex DNA structure, a unique four-stranded DNA motif, which is significant to the regulation of telomerase activity. The structures of the perylene diimides have been characterized by IR spectrophotometer, ¹H NMR, ¹³C NMR, MS, TGA and time-resolved instruments. Spectrochemical behaviors have been investigated by visible absorption and fluorescence emission spectra. The spectral characterization of the compounds has been investigated in five common organic solvents of different polarity and in water (in 170 mM phosphate buffer at pH 6). Marked red shifts of absorbance and fluorescence emission bands of the compounds in aqueous solution are compared with the other organic solutions. The fluorescence quantum yields are determined low in more polar solvents and also calculated to be about less than about 0.05 in aqueous solution because of the aggregation effects. Photodegradation rate constants (k_p) of the synthesized compounds have been compared under xenon lamp irradiation in acetonitrile solution.Binding abilities of the synthesized perylene diimides to different form of DNA strands have been investigated by visible absorption and fluorescence spectroscopy in the phosphate buffer solutions. Also, pH-dependent aggregation and G-quadruplex DNA binding selectivity of these ligands have been compared. Among these ligands, N-(2,6-diisopropylphenyl)-N′-(4-pyridyl)-perylene-3,4,9,10-tetracarboxylic diimide (PYPER) has been found to be the most selective interactive ligand for G-quadruplex formed in the G4′-DNA structure. PYPER has shown a significant selectivity to G4′-DNA which is comprised of d(TTAGGG) repeats, known as human telomeres, in the phosphate buffer at pH 6. The absorption maximum of the PYPER/G4′-DNA complex has given bathochromic shift of 7 nm with respect to the absorption maximum of DNA-free solution of PYPER in phosphate buffer at pH 6. Fluorescence quenching experiments between PYPER and G4′-DNA show that PYPER demonstrates about a 9.3-fold selectivity for binding to G4′-DNA versus ds-DNA base pairs with the bimolecular rate constant of 0.95 × 10¹² M⁻¹ s⁻¹.     

Effect of temperature on the absorption spectra of the solvated electron in 1-propanol and 2-propanol: Pulse radiolysis and laser photolysis studies at temperatures up to supercritical condition

The absorption spectra of solvated electrons in 1-propanol and 2-propanol have been investigated from 22 to 270 °C at a fixed pressure of 7 MPa, by using both nanosecond pulse radiolysis and laser photolysis techniques. The results show that, even up to supercritical conditions, it is still possible to measure unambiguously the absorption spectra of solvated electron in these two propanols. The peak positions of the absorption spectra show a red-shift (shifts to longer wavelengths) as temperature increases, similar to water and other alcohols, but the temperature efficiency, dE_max/dT, of 1-propanol is larger than that of 2-propanol. In addition, in clear contrast to that of pulse radiolysis, in laser photolysis experiments, an increase in the maximum absorbance of the absorption spectra of solvated electron with temperature up to ∼200 °C is attributed to the increase of absorbance (CTTS absorption band) of I⁻ anion at 248 nm with increasing temperature.     

The energy spectrum of 662 keV photons in a water equivalent phantom

Investigation is made on the energy spectrum of photons originating from interactions of 662 keV primary gamma-ray photons emitted by a point source positioned at the centre of a water equivalent solid phantom of dimensions 19 cm×19 cm×24 cm. Peaks resulting from total energy loss (photopeak) and multiple and back scattering have been observed using a 51 mm×51 mm NaI(Tl) detector; good agreement being found between the measured and simulated response functions. The energy spectrum of the gamma photons obtained through the Monte Carlo simulation reveals local maxima at about 100 keV and 210 keV, being also observed in the experimental response function. Such spectra can be used as a method of testing the water equivalence of solid phantom media before their use for dosimetry measurements.     

Low-energy bulk plasmon of nickel

High-resolution electron energy loss spectroscopy has been used to study low-energy bulk excitation modes of planar-bound electrons (bulk plasmons) in nickel. We observed for the first time a bulk plasmon at about 1.2 eV, in agreement with dielectric theory. The behavior of its amplitude with the off-specular angle ensures the dipolar nature of such mode. On the other hand, the intensity of the plasmon peak is vanishing upon ion bombardment due to the sputtering-induced modification of dielectric function.     

Atomic arrangement of Al-induced clusters on Si(0 0 1) surface at high temperature

The atomic structure of the Al-induced clusters on Si(0 0 1) surface formed by the annealing of 0.5 ML Al/Si(0 0 1) at 500 °C has been studied using coaxial impact collision ion scattering spectroscopy (CAICISS). CAICISS results proposed that the Al atoms occupy the cave site (T4 site) and off-centered T4 site. To determine the structure of the Al-induced clusters definitely, classical ion-scattering trajectory simulations using scattering and recoiling imaging code (SARIC) have been performed for the recently proposed most possible four different cluster models (Bunk, Zotov, Kotlyar, and Zavodinsky model). Our CAICISS spectra and simulation results show that the Bunk model is the best plausible one among the models. As the results of the simulations, it is found that Al–Si dimers has been oriented on the topmost layer of the Si(0 0 1) surface with a bonding length (Δz) of 1.00 ± 0.05 Å.     

PELDOR on an exchange coupled nitroxide copper(II) spin pair

Transition metal ions play an important role in the design of macromolecular architectures as well as for the structure and function of proteins and oligonucleotides, which makes them interesting targets for spectroscopic investigations. In combination with site directed spin labelling, pulsed electron–electron double resonance (PELDOR or DEER) could be a well-suited method for their characterization and localization. Here, we report on the synthesis and full characterization of a copper(II) porphyrin/nitroxide model system bearing an extended π-conjugation between the spin centres and demonstrate the possibility to disentangle the dipolar through space interaction from the through bond exchange coupling contribution even in the presence of orientational selectivity and conformational flexibility. The simulations used are based on the known experimental and spin Hamiltonian parameters and on a structural model as previously employed for similar systems. The mean exchange coupling of +4(1) MHz (antiferromagnetic) is in agreement with the value of |J| = 3(1) MHz determined from room temperature continuous wave electron paramagnetic resonance (EPR). Thus, as long as the pulse excitation bandwidths are large versus the spin–spin coupling, X-band PELDOR measurements in combination with explicit time trace simulations allow for disentangling the sign and magnitude of through bond electron–electron exchange from the through space dipolar interaction D.     

Hierarchically organized titanium dioxide nanostructured electrodes: Quantum-sized nanowires grown on nanotubes

Titanium dioxide nanotube electrodes were fabricated by anodization of titanium and decorated with quantum-sized rutile nanowires (2 nm in diameter) by chemical bath deposition. The length of the nanotubes (120 nm in external diameter) was varied between 4 and 10 μm by changing the anodization time. The hierarchically organized electrodes present good mechanical properties and an enhanced capacity for reversible charge accumulation. The photoelectrocatalytic properties of such electrodes have been tested by photo-oxidizing both water and oxalic acid, turning out to be superior to those of bare nanotubes, which are ascribed to an enhanced interfacial area while keeping the favorable transport properties (for both electrons and chemicals) typical of nanotube electrodes.     

Electron attachment rate constant measurement by photoemission electron attachment ion mobility spectrometry (PE-EA-IMS)

Photoemission electron attachment ion mobility spectrometry (PE-EA-IMS), with a source of photoelectrons induced by vacuum ultraviolet radiation on a metal surface, has been developed to study electron attachment reaction at atmospheric pressure using nitrogen as the buffer gas. Based on the negative ion mobility spectra, the rate constants for electron attachment to tetrachloromethane and chloroform were measured at ambient temperature as a function of the average electron energy in the range from 0.29 to 0.96 eV. The experimental results are in good agreement with the data reported in the literature.     

Mutual and self-diffusion of charged porphyrines in aqueous solutions

We have investigated the diffusion properties for an ionic porphyrin in water. Specifically, for the {tetrasodium tetraphenylporphyrintetrasulfonate (Na₄TPPS) + water} binary system, the self-diffusion coefficients of TPPS⁴⁻ and Na⁺, and the mutual diffusion coefficients were experimentally determined as a function of Na₄TPPS concentration from (0 to 4) · 10⁻³ mol · dm⁻³ at T = 298.15 K. Absorption spectra for this system were obtained over the same concentration range. Molecular mechanics were used to compute size and shape of the TPPS⁴⁻ porphyrin. We have found that, at low solute concentrations (<0.5 · 10⁻³ mol · dm⁻³), the mutual diffusion coefficient sharply decreases as the concentration increases. This can be related to both the ionic nature of the porphyrin and complex associative processes in solution. Our experimental results are discussed on the basis of the Nernst equation, Onsager–Fuoss theory and porphyrin metal ion association. In addition, self-diffusion of TPPS⁴⁻ was used, together with the Stokes–Einstein equation, to determine the equivalent hydrodynamic radius of TPPS⁴⁻. By approximating this porphyrin to a disk, we have estimated structural parameters of TPPS⁴⁻. These were found to be in good agreement with those obtained using molecular mechanics. Our work shows how the self-diffusion coefficient of an ionic porphyrin in water is substantially different from the corresponding mutual-diffusion coefficient in both magnitude and concentration dependence. This aspect should be taken into account when diffusion-based transport is modelled for in vitro and in vivo applications of pharmaceutical relevance.     

Monte Carlo bicanonical ensemble simulation for sodium cation hydration free energy in liquid water

A series of bicanonical ensemble Monte Carlo (BC MC) simulations has been performed to calculate Na⁺ hydration Gibbs energy in aqueous solution. The hydration Gibbs energy of Na⁺ ion in aqueous solution is the difference between formation free energies of Na⁺ (H₂O)_n and (H₂O)_n clusters at n → α. The convergence of the hydration free energy to bulk water value is fast, and the results at n = 60 turned out to be in good agreement with experimental ones and those calculated using free energy perturbation method [1]. The ion–water interaction has been described by Aqvist's pair potential [1] and SPC model [2] has been used for water–water interactions. The behaviour of the absolute Gibbs energy, the entropy, the internal energy of the clusters and the development of hydration shells’ structure with the increase of the number of water molecules are discussed.     

Ultrafast interfacial electron transfer from the excited state of anchored molecules into a semiconductor

Ultrafast heterogeneous electron transfer (HET) from the excited singlet state of the large organic chromophore perylene into the inorganic semiconductor rutile TiO₂ was investigated with femtosecond time-resolved two-photon photoemission (TR-2PPE). The strength of the electronic interaction between the chromophore and the semiconductor was varied by inserting different anchor/bridge groups that functioned either as electronic wire or electronic tunnelling barrier. Both anchor groups, i.e. carboxylic and phosphonic acid, formed strong chemical bonds at the TiO₂ surface. The perylene chromophore with the different anchor/bridge groups was adsorbed from solution in a dedicated ultra-high-vacuum (UHV) chamber. The adsorption geometry of the chromophore perylene was determined from angle and polarization dependent two-photon photoemission (2PPE) signals and was found to be very different for the two different anchor/bridge groups. The measured adsorption geometries are compatible with recent DFT (density functional theory) calculations by P. Persson and co-workers [M. Nilsing, S. Lunell, P. Persson, L. Ojamäe, Phosphonic acid adsorption at the TiO₂ anatase (1 0 1) surface investigated by periodic hybrid HF-DFT computations, Surf. Sci. 582 (2005) 49–60]. Two different processes contributed to the TR-2PPE transients, firstly electron transfer from the chromophore to the electronic acceptor states on the surface and secondly escape of the electrons from the surface into the bulk of the semiconductor. The latter escape process was measured separately by making the interfacial electron injection process instantaneous when the chromophore catechol was employed in place of the perylene compounds. The thus measured electron escape behavior was governed by the same time constants that have recently been predicted by Prezhdo and coworkers from time dependent DFT calculations [W.R. Duncan, W.M. Stier, O.V. Prezhdo, Ab initio nonadiabatic molecular dynamics of the ultrafast electron injection across the Alizarin-TiO₂ interface, J. Am. Chem. Soc. 127 (2005) 7941–7951]. The HET times derived from the 2PPE transients showed very good agreement with HET times measured via transient absorption (TA) on anatase TiO₂ layers. The measured energy distribution of the 2PPE signals for the injected electrons suggests that a high density of electronic acceptor states is operative in both systems and is spread over an at least 1 eV wide energy range. The acceptor states are tentatively identified with surface states created through the formation of chemical bonds between the anchor groups of the organic molecules and surface atoms of the semiconductor.     

Geometric shielding corrections for calculation of electron scattering by CO2, C2H2, CHCl3, CH2Cl2, CH3Cl,CHF3, CH2F2 and CH3F at 30–5000 eV

Taking into account the changes of the geometric shielding effect in a molecule as the incident electron energy varies, an empirical fraction, which depends on the energy of the incident electrons, the target's molecular dimension and the atomic and electronic numbers in the molecule, is presented. Using this empirical fraction, a new formulation of the additivity rule is proposed. Using the new additivity rule, the total cross sections of electron scattering by CO₂, C₂H₂, CHCl₃, CH₂Cl₂, CH₃Cl, CHF₃, CH₂F₂ and CH₃F are calculated at the Hartree–Fork level at 30–5000 eV. The quantitative total cross sections are compared with those obtained by experiments and other theories, and good agreement is obtained over a wide energy range, especially above 100 eV.     

High-temperature transport and stability of SrFe1−xNbxO3 − δ

The conductivity is measured in the series of solid solutions SrFe_1 ? xNb_xO_3 ? δ, where x = 0.05, 0.1, 0.2, 0.3, 0.4, within the oxygen partial pressure limits 10^?18–0.5 atm and temperature range 650–950 °C. The contributions to the total conductivity from oxygen ions, electrons and electron holes are obtained based on their different pressure dependences. The doped derivative with x = 0.1 is found to be a singular composition where ion conductivity attains a maximal value while activation energy for ion transport is minimal. This peculiar behavior is attributed to formation of favorable microstructure in the oxide. The deeper doping results in deterioration of ion transport, which is explained by oxygen vacancy filling. It is shown that replacement of iron for niobium favors enhanced thermodynamic stability towards reduction. The oxygen permeability is evaluated from the conductivity data, and it achieves rather high values in the doped derivatives. These oxides can, therefore, be recommended for further evaluation as oxygen separating membrane materials for partial oxidation of natural gas.     

Electron and positron elastic scattering in gaseous and liquid water: A comparative study

In the present work, we present both theoretical differential and total cross sections for the elastic scattering process of positrons and electrons in liquid and vapour water for energies ranging from 10 eV to 10 keV. The calculations are performed in the partial-wave formalism by means of a complex interaction potential taking into account static potential as well as fine effects like exchange and polarization contributions. The theoretical results obtained in this free-parameter quantum-mechanical treatment are compared to available experimental data and good agreement is generally observed. Moreover, quantitative differences are reported between the positron and electron scattering, in vapour as well as in liquid water.     

Solvated electrons at elevated temperatures in different alcohols: Temperature and molecular structure effects

The absorption spectra of solvated electrons in pentanol, hexanol and octanol are measured from 22 to 200, 22 to 175 and 50 to150 °C, respectively, at a fixed pressure of 15 MPa, using nanosecond pulse radiolysis technique. The results show that the peak positions of the absorption spectra have a red-shift (shift to longer wavelengths) as temperature increases, similar to water and other alcohols. Including the above mentioned data, a compilation of currently available experimental data on the energy of absorption maximum (E_max) of solvated electrons changed with temperature in monohydric alcohols, diols and triol is presented. E_max of solvated electron is larger in those alcohols that have more OH groups at all the temperatures. The molecular structure effect, including OH numbers, OH position and carbon chain length, is investigated. For the primary alcohols with same OH group number and position, the temperature coefficient increases with increase in chain length. For the alcohols with same chain length and OH numbers, temperature coefficient is larger for the symmetric alcohols than the asymmetric ones.     

Magnetic and spectroscopic properties of gadolinium tripodal Schiff base complex

Gadolinium(III) tripodal Schiff base (tris(((5-chlorosalicylidene)amino)ethyl)amine) complex has been obtained and investigated by infrared spectroscopy (IR), magnetic susceptibility, and electron paramagnetic resonance (EPR) methods. Comparison of IR bands in ligand and gadolinium complex confirmed the formation of the gadolinium complex and allowed to propose its structure. Both electron ionization and electron spray molecular spectroscopy spectra confirmed the [1:1] proportion of a ligand to metal in gadolinium tripodal Schiff base complex sample. IR spectroscopy and TG–DTA excluded the presence of water molecule in the metal coordination sphere. X-ray powder analysis applying Fullprof computer program has shown that the investigated sample was monophase with the monoclinic symmetry of the unit cell having the lattice constants: a = 10.028(4) Å, b = 13.282(5) Å, c = 21.20(1) Å and β = 101.58(4)°. Space group P21/c, Z = 4. EPR spectra of the complex have been registered in the 4–300 K temperature range. Each spectrum has been fitted using EPR–NMR computer program and the values of the spin-Hamiltonian parameters at each temperature have been calculated. Temperature dependence of the integrated intensity of the EPR spectrum allowed revealing the magnetic interactions in the spin system of this compound. Comparison of the temperature dependence of dc magnetic susceptibility (χ) and EPR susceptibility (χ_EPR) showed significant differences between these quantities due to the presence of short-lived clusters with a non-magnetic ground state.