Observation of latent tracks formed by heavy ion irradiation in poly(methyl methacrylate)

The irradiation effect in poly(methyl methacrylate) changed from main‐chain scission to crosslinking, depending on the stopping powers of the Au and Xe ions irradiated with high stopping powers. The latent ion tracks, including an end of the ion range in the polymer, were observed clearly by field emission and atomic force microscopies. Additionally, it was also observed that a crosslinked structure (formed by nuclear stopping) existed across an end of the ion range calculated by a Transport of Ion in Matters code, and it was different from that formed by electronic stopping. The nuclear stopping of the heavy ions can play an important role around the end of the ion range in the polymer. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 757–762, 2001     

Stopping power calculations of molecules for swift projectiles

Electronic stopping cross-section of compounds is calculated by considering velocity-dependent projectile and target electronic structure, and applying the Bragg addition rule. For velocity dependencies of projectile and target, we used previously developed stopping power formula by Tufan et al. In this work, we used two different screening functions. We calculated the electronic stopping cross-section of Al₂O₃, CO₂, and SiO₂ for O and Si ions. We compared our results with the other theoretical calculations and found 4–8% deviation.     

Round-robin test of medium-energy ion scattering for quantitative depth profiling of ultrathin HfO2/SiO2/Si films

Medium-energy ion scattering (MEIS) has been used for quantitative depth profiling with single atomic layer resolution to determine the composition, thickness, and interface structure of ultrathin films and nanoparticles. To assure the consistency of the MEIS analysis, an international round-robin test (RRT) with nominally 1-, 3-, 5-, and 7-nm thick HfO₂ films was conducted among 12 institutions. The measurements were performed at each participating laboratory under their own conditions, and the collected data were analyzed. For the data analysis, the Moliere potential, the stopping and range of ions in matter (SRIM) 95 and new fitted electronic stopping power and the Chu straggling were used. For analyzing the MEIS data from the magnetic sector and electrostatic analyzers, the neutralization corrections of Marion and Young for 100-keV H⁺ and He⁺ ions and of Armstrong for 400- to 500-keV He⁺ ions were used. The standard deviations were 5.3% for the composition, 15.3% for the thickness, and 13.3% for the Hf content, and they were improved to 7.3%, 4.5%, and 7.0% by using refitted electronic stopping powers based on the experimental data. Hence, this study suggests that correct electronic stopping powers are critical for quantitative MEIS analysis.     

Energy loss of scattered ions at glancing-angle incidence on the crystal surface

Experimental findings of glancing-angle scattering of energetic ions are reviewed with particular emphasis on the energy loss of ion during scattering. The position-dependent stopping power of the surface which is derived from the angular dependence of observed energy loss is explained in terms of single and distance electron excitations of surface electrons. For the ions with velocities less than the Fermi velocity of target valence electrons, it is shown that the stopping power of a surface is determined only by the elastic collisions of the ion with valence electrons. From the analysis of the energy losses of 12–30 keV He ions reflected from the (001) surface of SnTe, a method is proposed to derive the electron density distribution averaged over the plane parallel to the surface from this position-dependent stopping power.     

Light emission from sputtered aluminum atoms and ions produced by ion bombardment

Photon emission originating from sputtering of a polycrystalline aluminum surface under 1–10 keV ion (H⁺, He⁺, Ar⁺, Kr⁺ and Xe⁺) bombardment has been studied. Measured photon emission yields from the 3d ²D_3/2 resonance transition of sputtered excited Al atoms and calculated nuclear stopping powers are compared. The results demonstrate that elastic collisions play a major role in photon emission. Moreover, measurements of photon intensity as a function of the distance from the target surface show that decays of sputtered excited ions Al⁺ and Al²⁺ are faster than decays of excited Al atoms, and less affected by cascade repopulation and de-excitation of fast ions.     

Theoretical rate constants of super-exchange hole transfer and thermally induced hopping in DNA

Recently, the electronic properties of DNA have been extensively studied, because its conductivity is important not only to the study of fundamental biological problems, but also in the development of molecular-sized electronics and biosensors. We have studied theoretically the reorganization energies, the activation energies, the electronic coupling matrix elements, and the rate constants of hole transfer in B-form double-helix DNA in water. To accommodate the effects of DNA nuclear motions, a subset of reaction coordinates for hole transfer was extracted from classical molecular dynamics (MD) trajectories of DNA in water and then used for ab initio quantum chemical calculations of electron coupling constants based on the generalized Mulliken-Hush model. A molecular mechanics (MM) method was used to determine the nuclear Franck-Condon factor. The rate constants for two types of mechanisms of hole transfer-the thermally induced hopping (TIH) and the super-exchange mechanisms-were determined based on Marcus theory. We found that the calculated matrix elements are strongly dependent on the conformations of the nucleobase pairs of hole-transferable DNA and extend over a wide range of values for the "rise" base-step parameter but cluster around a particular value for the "twist" parameter. The calculated activation energies are in good agreement with experimental results. Whereas the rate constant for the TIH mechanism is not dependent on the number of A-T nucleobase pairs that act as a bridge, the rate constant for the super-exchange process rapidly decreases when the length of the bridge increases. These characteristic trends in the calculated rate constants effectively reproduce those in the experimental data of Giese et al. [Nature 2001, 412, 318]. The calculated rate constants were also compared with the experimental results of Lewis et al. [Nature 2000, 406, 51].     

Curvature of the lanthanide contraction: An explanation

A number of studies have shown that for isostructural series of the lanthanides (elements La through Lu), a plot of equivalent metal-ligand bond lengths versus atomic number differs significantly from linearity and can be better fit as a quadratic equation. However, for hydrogen type wave functions, it is the inverse of the average distance of the electron from the nucleus (an estimate of size) that varies linearly with effective nuclear charge. This generates an apparent quadratic dependence of radius with atomic number. Plotting the inverse of lanthanide ion radii (the observed distance minus the ligand size) as a function of effective nuclear charge gives very good linear fits for a variety of lanthanide complexes and materials. Parameters obtained from this fit are in excellent agreement with the calculated Slater shielding constant, k.     

Variational response-function formulation of vibrational circular dichroism

The atomic axial tensor (AAT) of vibrational circular dichroism is expressed as the frequency derivative at zero frequency of a linear response function for operators referencing a nuclear displacement and a magnetic field. This is used in the density matrix-based quasienergy derivative Lagrangian approach of Thorvaldsen et al. [J. Chem. Phys., 2008, 129, 214108] to express the AAT in a form where the need to solve response equations for the nuclear displacements is removed, significantly reducing the computation cost compared to existing formulations. The density matrix-based quasienergy derivative Lagrangian approach also allows us straightforwardly to use London atomic orbitals to remove the gauge-origin dependence and to account for the atomic orbitals' dependence on the nuclear coordinates. The formalism is entirely based on atomic-orbital density and integral matrices and therefore amenable to linear scaling for sufficiently sparse matrices and given a linearly scaling response solver.     

Explaining the periodic table, and the role of chemical triads

Some recent work in mathematical chemistry is discussed. It is claimed that quantum mechanics does not provide a conclusive means of classifying certain elements like hydrogen and helium into their appropriate groups. An alternative approach using atomic number triads is proposed and the validity of this approach is defended in the light of some predictions made via an information theoretic approach that suggests a connection between nuclear structure and electronic structure of atoms.     

Proton stopping powers: Binary encounter calculations based on accurate speed distributions for target electrons

Atomic and molecular electronic stopping powers for medium energy protons (≈ 10 keV-10 MeV) have been calculated using the binary-encounter approximation in conjunction with (1) either an energy or maximum impact parameter cut-off based on minimum excitation energies; and (2) ab initio electronic speed distributions. The maximum impact parameter approach yields good agreement with experiment for inert gases and closed-shell polyatomic molecules comprised of first-row atoms.     

Stopping power and energy loss straggling of thin Formvar foil for 0.3–2.7 MeV protons and alpha particles

Stopping power and energy loss straggling data for protons (¹H⁺) and alpha particles (⁴He⁺) crossing Formvar thin polymeric foils (thickness of ～0.3 μm) have been measured in the energy range (0.3–2.7) MeV by using the indirect transmission technique. The determined stopping power data were compared to SRIM-2010, PSTAR or ASTAR calculation codes and then analyzed in term of the modified Bethe–Bloch theory to extract the target mean excitation and ionization potential 〈I〉. A resulting value of 〈I〉≈(69.2±1.8) eV was deduced from proton stopping data. The measured straggling data were corrected from surface roughness effects due to target thickness inhomogeneity observed by the atomic force microscopy (AFM) technique. The obtained data were then compared to derived straggling values by Bohr's and Bethe–Livingston's classical theories or by Yang's empirical formula. A deviation of ～40%–80% from the Bohr's straggling value has been observed for all reported energies, suggesting that the Bohr theory cannot be correctly applied to describe the electronic energy loss straggling process with the used low thickness of Formvar foil. The inner-shell contribution of target electrons to energy loss process is also advanced to explain the observed deviation from experiment in case of He⁺ ions. Finally, the reliability of Bragg's additivity rule was discussed in case of stopping power and straggling results.     

离子质电比和相差异因子对离子半径的综合标度

根据万有引力势与电势的关系式和系统的质电比(单位电量的质量)Sr的物理意义, 研究了离子半径r与离子的Sr和相差异因子的关系. 对于阳离子, r与lgSr和相差异因子呈线性关系; 对于稳定构型阴离子, r与Sr和相差异因子也存在定量关系. 采用回归分析方法, 给出稳定构型和非稳定构型阳离子半径计算公式, 以及稳定构型阴离子半径计算公式. 从相关系数R和回归方程的显著性检验(F)都可说明r与Sr和相差异因子密切相关, 其中拟合的96种元素的138种阳离子半径数据与具有代表性参考值相比, 平均绝对误差为2.0 pm, 相对误差为2.5%. 并预测出较为合理的稀有气体等30种离子半径数据. 同时给出一条获取离子半径(包括复杂离子)数据的新途径.     

On the performance of two-component energy-consistent pseudopotentials in atomic Fock-space coupled cluster calculations

The four-component atomic intermediate-Hamiltonian Fock-space coupled cluster (IHFSCC) code of Landau et al. [J. Chem. Phys. 115, 6862 (2001)] has been adapted to two-component calculations with relativistic pseudopotentials of the energy-consistent variety. Recently adjusted energy-consistent pseudopotentials for group 11 and 12 transition elements as well as group 13 and 14 post-d main group elements, which were fitted to atomic valence spectra from four-component multiconfiguration Dirac-Hartree-Fock calculations, are tested in IHFSCC calculations for ionization potentials, electron affinities, and excitation energies of a variety of atoms and ions. Where comparison is possible, the deviations from experimental data are in good agreement with those found in previously published IHFSCC all-electron calculations: experimental data are usually reproduced within a few hundred wavenumbers.     

Analyte ionization in the inductively coupled plasma

Spatially resolved ion-atom emission intensity ratios for Sr, Ca, Mg, Cd and Zn have been measured at rf power settings of 1.00, 1.25, 1.50, 1.75 and 2.0 kW at a vertical height of 16 mm above the load coil. Measured values of electron density have been used to construct a theoretical local thermal equilibrium (LTE) framework, and ion-atom emission intensity ratios calculated from this framework have been compared to experimentally measured values. The measured ion-atom emission intensity ratios were found to be within an order of magnitude of these calculated LTE ratios.The experimental degree of ionization for these five elements was determined for the various rf input powers. These values have been compared to the analagous LTE values. Both degree of ionization and departure from LTE were found to be strongly correlated with the ionization potential of the element.The radial spatial dependence of the degree of ionization for Cd at an rf power of 1.25 kW has been measured for aerosol flow rates of 0.6, 0.8 and 1.21 m⁻¹ for vertical heights of 4, 8, 12, 16 and 20 mm above the load coil. The spatial distribution of electron number density was measured at an rf power of 1.25 kW and at aerosol flow rates of 0.6, 0.8 and 1.21 m⁻¹ and a correlation between degree of ionization and electron density identified. Finally the relative concentration of Cd ions has been calculated from ion spatial emission profiles and plasma operating conditions which produce a maximum in the ion density identified.     

Shannon information entropy of fractional occupation probability as an electron correlation measure in atoms and molecules

A new method to determine electron correlation energy is presented for atoms and molecules. This method is based on Shannon information entropy that is obtained by fractional occupation probabilities of natural atomic orbitals. It is indicated that the Shannon entropy increases as the number of electrons increases and thus can be considered as a possible measure for the electron correlation in atomic and molecular systems. For neutral atoms and singly charged positive ions we proposed an expression for correlation energy with explicit dependence on the Shannon entropy and atomic number. The obtained correlation energies have been used to compute the first ionization potentials of the ground state of the main group elements from hydrogen through krypton. The calculated ionization potentials are in reasonably good agreement with their corresponding experimental values.We also developed the additivity scheme to find a connection between Shannon entropy and molecular correlation energy. The estimated molecular correlation energies show an excellent agreement with those obtained by elaborate G3 method with R² = 0.990.     

Vicinage effects in the stopping power of H 3 + beams in amorphous carbon

The stopping power of H ₃ ⁺ molecular beams incident on amorphous carbon foils is analyzed as a function of the orientation of the molecule relative to its direction of motion. The electronic stopping power was calculated within the dielectric formalism, taking into account the dynamics of the Coulomb repulsion among the molecular partners. At intermediate velocities, ν ? 2 a.u., the stopping ratio for the perpendicular orientation is greater than for the parallel one, while the opposite behaviour is shown for higher velocities. The stopping ratio for randomly oriented H ₃ ⁺ molecules was evaluated for different target thicknesses, resulting in larger values for the thinner targets and the higher energies. Comparison of our results with available experimental data shows a reasonable agreement.     

Delta-electron emission in fast heavy ion — atom collisions: observations of new phenomena and breakdown of common scaling laws

Double differential cross sections for the emission of Delta-electrons have been measured in fast uranium-rare gas collisions. The well-known Binary Encounter peak reveals unexpected structures for certain observation angles and its intensity increases towards smaller angles, which is in contradiction to results and scaling laws obtained by experiments with light ion impact. The observed dependencies are fairly well described by recent calculations in the framework of IA and CTMC. From systematic experimental as well as theoretical studies we can derive that the potential of the partially stripped projectile ion gives rise to rainbow and glory scattering of the target electron in the field of the projectile. The rainbow scattering is observed in the laboratory frame as pronounced interference structures, whereas the glory scattering is responsible for the steep increase of the cross sections for binary-encounter electrons towards small laboratory ejection angles. The observed effects have a dramatic influence on the commonq ² scaling laws derived from experiments with light ions. Furthermore, since the binary-encounter electrons ejected at forward angles have approximately twice the projectile velocity, these new phenomena have an important influence on the electronic stopping power of heavy ions and therefore have to be taken into account for the investigation of radiation damage by these ions e.g. in biological matter.     

Quantitative elastic recoil detection (ERD)

Elastic recoil detection with heavy ions is a well suited method to measure depth profiles of light and medium heavy elements in thin films. Due to known Rutherford scattering cross sections and stopping powers of ions in matter, measurements can be quantified with an accuracy of about 1%. The transformation of energy spectra to quantitative elemental depth profiles, however, is generally a non trivial task and therefore a transofrmation algorithm has been developed based on a similar one for RBS analysis and realised in the program KONZERD. The transformation procedure allows a fast conversion from raw spectra to concentration profiles for classical ERD measurements as well as for high resolution measurements with a depth resolution better than 1 nm.     

Effect of ion energy on the implantation products of benzene

Solid benzene has been implanted with inert-gas ions with energies ranging from 16 to 100 keV. The small variation observed in the relative concentration of the products has been found to depend mainly on the fact that the relative weight of the nuclear and electronic stopping mechanisms is changing along the energy range explored.