Jorge Esteban Miraglia

Influence of the polarization in grazing scattering of fast helium atoms from LiF(001) surfaces

Fil: Gravielle, Maria Silvia. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Instituto de Astronomia y Fisica del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomia y Fisica del Espacio; Argentina

Simple correlated wavefunctions for the K-shell electrons of neutral atoms

Simple correlated wavefunctions considering two K-shell active electrons of neutral atoms from He to Xe are presented in this paper, describing a variational method subject to a local Hartree potential representing the presence of outer shell electrons. Three kinds of electron–electron correlation functions have been studied with rigorous observation of the exact behaviour of the wavefunctions at the electron–electron and electron–nucleus coalescence points (Kato cusp conditions). Global properties, such as the energies and virial coefficients, as well as local properties, such as spatial mean values, together with scaling laws with the nuclear charge for the variational parameters, are also analysed. We calculated the expansion of the functions in terms of bipolar spherical harmonics. Finally, comparisons are made with a more rigorous, fully quantal close-coupling method, which also includes the same Hartree potential for the outer electrons.

Neonization method for stopping, mean excitation energy, straggling, and for total and differential ionization cross sections of CH4, NH3, H2O and FH by impact of heavy projectiles

We propose a neonization method to deal with molecules composed by hydrides of the second row of the periodic table of elements: CH4 ,N H 3 ,O H 2 and FH. This method describes these ten-electron molecules as dressed atoms in a pseudo-spherical potential. We test it by covering most of the inelastic collisional magnitudes of experimental interest: ionization cross sections (total, single and double differential), stopping power, energy-loss straggling and mean excitation energy. To this end, the neonization method has been treated with different collisional formalisms, such as the continuum-distorted-wave–eikonal-initial-state, the first order Born, and the shell-wise local plasma approximations. We show that the present model reproduces the different empirical values with high reliability in the intermediate to high-energy region. We also include the expansion of the spherical wave functions in terms of Slater-type orbitals and the analytic expression for the spherical potentials. This makes it possible in the future to tackle present neonization strategy with other collisional models. (Some figures may appear in colour only in the online journal)

Electron-impact multiple ionization of Ne, Ar, Kr and Xe

This work describes the multiple ionization cross sections of rare gases by electron-impact. We pay special attention to the high energy region (0.1–10 keV) where the direct ionization is a minor contribution and the post-collisional electron emission dominates the final target charge state. We report here electron-impact single to sextuple ionization cross sections and total ionization cross sections including direct and post-collisional processes, even in the total values. We use the continuum distorted wave and the first Born approximations adapted to describe light-particle impact, i.e. energy, mass and trajectory corrections are incorporated, the latter by considering the electron-target potential and by using the Abel transformation. Auger-type post-collisional contributions are included in the multinomial expansion through experimental branching ratios after single ionization events. Tabulations of these experimental branching ratios for all the orbitals of the four targets are included. Present results are compared with the large amount of electron-impact experimental data available. We have obtained a good description of the multiple-ionization measurements at high energies, where the post-collisional ionization dominates. At intermediate energies, our theoretical results show the correct tendency, with the electron-impact ionization cross sections being far below the proton-impact ones.

Multivariable hypergeometric solutions for three charged particles

We present a new wavefunction which describes the ion - atom problem above the ionization threshold. This is an approximate solution of the Schrodinger equation for the three-body Coulomb problem that can be expressed in terms of a confluent hypergeometric function of two variables. The proposed wavefunction includes correlation among the motions of the three particles and verifies the correct Coulombic asymptotic behaviours.

Evidence of two-channel distortion effects in positronium formation reactions

The formation of ground-state positronium in collisions of positrons on hydrogen-like atoms is considered. In previous theoretical works, two-centre distorted wavefunctions were employed to approximate either the initial or the final channel. Here we report results obtained by means of the eikonal final state continuum distorted wave approximation for which asymptotically correct distorted wavefunctions are used for both the initial and final states of the scattering system. Comparison of the present theoretical total cross sections with experimental data reveals that distortion effects become important in both channels as the impact energy decreases. This work also shows that distorted-wave theories may be extended from their usual domain of high impact energies to lower ones.

Positron and electron-impact multiple-ionization

We present positron-impact multiple (single to quintuple) and total ionization cross sections of Ne, Ar, Kr, and Xe, covering an extended energy range from 50 eV to 7 keV. We improve on previous calculations of the ionization thresholds by adding the mean kinetic energy transferred to the target electrons. In this way the thresholds compare rather well with the experimental appearance energies for the different k-fold ionization cross sections. Present results include not only the novel energy threshold at low energies but also the post-collisional contribution at high energies. We performed a quite complete particle?antiparticle comparison by including our positron-impact results, new calculations for electron-impact ionization including the threshold correction, and a detailed compilation of experimental data. Present positron-impact multiple ionization cross sections are the first ones in such an extended energy region.

A collective model for inner shell ionization of very heavy targets

We present a theoretical study of the ionization of inner shells, such as L and M shells of Au, Pb and Bi or K-shell of Sb. The ionization is described using a collective response model, the shellwise local plasma approximation (SLPA). This model deals with each sub-shell of target electrons as an inhomogeneous electron gas with an ionization threshold. No parameters are included, just the theoretical wave functions of the ground state and the binding energies. The validity range is that of the perturbative approximation, high energies and asymmetric projectile target relation. In the case of Sb, known Hartree–Fock wave functions and energies are used. Instead, for Au, Pb and Bi, they were obtained in fully relativistic way by solving numerically the Dirac equation. The SLPA describes well the experimental data for K-shell ionization of Sb by O and F positive ions. However, it underestimates the data for C or lighter ions. Good results are obtained for M-shell ionization of Au and Bi above 2 MeV/amu, and for L-shell ionization of Au and Pb above 10 MeV/amu. For L- and M-shells, the SLPA tends to underestimate the data for energies below the range of validity of the model but approaches the experimental data for higher energies.

The Dielectric Formalism for Inelastic Processes in High-Energy Ion–Matter Collisions

Abstract In this chapter we analyze the possibilities and ranges of validity of the dielectric formalism to deal with correlated bound electrons in matter by using the shellwise local plasma approximation. This model describes the response of the electrons of the same binding energy as a whole (collectively), screening the interaction with the impinging ion. It considers separately each sub-shell of target electrons, with the corresponding dielectric response. The density of electrons and the energy gap are included explicitly by employing the Levine and Louie dielectric function. The goal of this chapter is to summarize and review the capability of this model to deal with fundamental magnitudes of the atomic collisions expressed as different moments of the energy loss: ionization cross sections (single or multiple, differential, and total), stopping power (and mean excitation energy), and energy loss straggling. This review covers a wide range of the collisions of ions with gases and solids, paying special attention to multi-electronic targets. The advantages and disadvantages of the model in comparison with independent electron ones, ranges of validity and future prospect will be considered.

Total electron yields and stopping power of protons colliding with NaCl-type insulator surfaces

We re-calculate electron yields and stopping power of protons colliding with surfaces of NaCl-type insulators. In this article, the projectile is considered to move taking into account the static and polarization potentials with all the individual ions forming the surface lattice unlike our previous work (Phys. Rev. A 75, 042904 (2007)) where the projectile was considered to move in an homogeneous planar potential. Substantial differences (up to forty percent of increment) have been found especially when the projectile incident angle approaches the critical one. We compare our prediction for electron yield and stopping power with the available experimental data for LiF, KI and KCl.