Simulation of energy spectra of slow He ions scattered from a copper surface

Scattering of low energy (2 keV) He⁺ ions from a polycrystalline copper surface is studied. Experimental Time-Of-Flight (TOF) spectra are presented for different values of the angle of incidence α and a fixed scattering angle θ = 129°. Using Monte Carlo simulation, based on the TRIM (TRansport of Ions in Matter [6]) code, energy spectra are calculated and compared to experimental data. The influence of different combinations of interaction potential and inelastic energy loss on the spectrum shape and on the angular distribution of the scattering yield is considered in the simulation. Simulations show very good agreement with the experiment for all investigated geometries. An analysis of the experimental yield of scattered particles in different parts of the spectrum is performed to gain information on the relative contributions of surface single scattering and multiple scattering.     

Analysis of the Auger neutralization of He at Cu surfaces in low energy ion scattering

Recently, strong crystal effects in P⁺ were observed for He⁺ and Cu in the Auger neutralization regime, with differences in the ion fraction by up to a factor of three non-equivalent Cu surfaces. In this contribution, it is shown that these findings can quantitatively be described within the jellium model assuming perpendicular velocity scaling of P⁺.     

Kinetic electron emission in the interactions of slow ions with MgO surfaces

We report experimental energy distributions and yields of electrons emitted from MgO surfaces under the impact of slow noble gas and sodium singly charged ions at varying incident energies.

At impact energies below 1 keV, electron spectra are nearly independent of ion type and energy. A tail of high-energy electrons is observed to grow at higher impact energies.

The results are explained in terms of promotion of oxygen-2p electrons during binary projectile-oxygen collisions populating continuum and excitonic states. Excitons can significantly contribute to electron emission due to the negative electron affinity of the surface.     

Charge exchange of medium energy H and He ions emerging from solid surfaces

Charge exchange of medium energy H and He ions emerging from clean solid surfaces is studied extensively using a toroidal electrostatic analyzer with an excellent energy resolution. The charge distributions of He ions scattered from sub-monolayers near a surface are non-equilibrated, resulting in a surface peak even for poly-crystal solids. By solving simultaneous rate equations numerically, we derive electron capture and loss cross sections for Ni and Au surfaces. Based on a free electron gas model, non-equilibrated He⁺ fractions dependent on emerging angle reveals uniform electronic surfaces for metals and corrugated surfaces for Si and graphite with covalent bonds. It is also found that equilibrium charge fractions of H⁺ are independent of surface materials (Z₂) and in contrast equilibrium He⁺ fractions depend pronouncedly on Z₂. The data obtained are compared with semi-empirical formulas.     

Molecular dynamics study of kinetic electron emission induced by slow sodium ions incident on gold surfaces

Electron excitation and emission phenomena, due to Na⁺ ion impact on Au (1 0 0) surfaces, are studied at incident projectile energies below the threshold for kinetic electron emission. The trajectories and velocities of the projectile and the target atoms are simulated with molecular dynamics. This information are used to calculate the energy loss by electronic stopping as a series of discrete events, localized in space and time, that are treated as sources of excitation energy. The diffusion of the energy deposited by the projectile into the solid is converted into electron yield as proposed by Duvenbeck and coworkers [14]. The results show similar trends to available experimental data.     

Convolution approximation for the energy loss, ionization probability and straggling of fast ions

In this work we describe an extension of the convolution approximation for the ionization probability and energy-loss straggling as a function of the impact parameter for swift ions. Analytical formulas for these quantities are derived and compared to full first-order Born calculations. The physical inputs of the model are the electron density and oscillators strengths of the target as well as the screening function of the projectile (in the case of dressed ions). A very good agreement is obtained for all impact parameters. In addition, we propose a general schema to add contributions from distant and close collisions. In this way physical processes arising from large and small impact parameters can be easily included into a single expression valid for all impact parameters. This model is then used to investigate the projectile-charge q dependence of ionization, stopping and straggling cross-sections.     

Plasmon excitation and electron promotion in the interaction of slow Na ions with Al surfaces

We studied the energy distributions of electrons emitted in the interaction of slow Na⁺ ions with polycrystalline Al surfaces. To study sub-threshold plasmon excitation we performed measurements as a function of emission angle which showed the excitation of bulk plasmons, confirming the kinetic nature of the excitation process.

Electron spectra show narrow transition lines due to Auger decay in vacuum of sodium projectiles excited in the 2p-shell by electron promotion in collisions with Al target atoms. Several previously unidentified transition lines could be attributed to the autoionization of doubly excited projectiles.     

Simulation study of carbon impurity dynamics on tungsten surfaces exposed to hydrogen ions

The depth profile of C impurity deposited on a W target exposed to H⁺ and C⁺ impurities at a concentration of C: 0.8% has been calculated in terms of segregation, diffusion and chemical erosion. For the segregation, the Gibbsian model has been used. For the diffusion, a concentration dependent diffusion model (C in WC and/or C) has been utilized. For the chemical erosion, the chemical erosion yield much lower than that for the H-C system has been applied. The calculated depth profiles at 653 K and 913 K are in good agreement with the XPS data. The agreement indicates that there is a significant contribution of segregation, which shifts the maximum C concentration to the top surface in the depth profiles. On the other hand, there are little contributions from diffusion and chemical erosion, which are related closely to formation of WC in the target.     

Effect of energy loss in the simulation of slow ion scattering by a solid surface

As previously shown [B. Arezki, Y. Boudouma, P. Benoit-Cattin, A.C. Chami, C. Benazeth, K. Khalal, M. Boudjema, J. Phys.: Condens. Mat. 10 (1998) 741, K. Khalal-Kouache, A.C. Chami, M. Boudjema, P. Benoit-Cattin, C. Benazeth, Y. Boudouma, Nucl. Instr. and Meth. B 183 (2001) 279], energy spectra of scattered particles from amorphous solid surface can be deduced from angular distributions of the total path length of the projectiles. In preceding papers [K. Khalal-Kouache, A.C. Chami, M. Boudjema, P. Benoit-Cattin, C. Benazeth, Y. Boudouma, Nucl. Instr. and Meth. B 183 (2001) 279, K. Khalal-Kouache, A. Mekhtiche, A.C. Chami, M. Boudjema, Surf. Coat. Technol. 201 (2007) 8420], a model based on the transport theory was used to simulate the scattering of slow ions by a solid target. Angular distributions of the total path length were computed and compared, in absolute value, to those simulated using a Monte Carlo code. A good agreement was obtained except for the highest values of the total path length L. In the transport theory, the variation of the scattering cross section versus energy during the slowing down of the particle was not taken into account. The purpose of this contribution is to take into account this effect within the model. The obtained results are found to be in agreement with Monte Carlo computation for angular distributions of the total path length and for energy spectra of the scattered ions.     

Separation of potential and kinetic electron emission from Si and W induced by multiply charged neon and argon ions

The total secondary electron emission yields, γ_T, induced by impact of the fast ions Ne^q+ (q = 2-8) and Ar^q+ (q = 3-12) on Si and Ne^q+ (q = 2-8) on W targets have been measured. It was observed that for a given impact energy, γ_T increases with the charge of projectile ion. By plotting γ_T as a function of the total potential energy of the respective ion, true kinetic and potential electron yields have been obtained. Potential electron yield was proportional to the total potential energy of the projectile ion. However, decrease in potential electron yield with increasing kinetic energy of Ne^q+ impact on Si and W was observed. This decrease in potential electron yield with kinetic energy of the ion was more pronounced for the projectile ions having higher charge states. Moreover, kinetic electron yield to energy-loss ratio for various ion-target combinations was calculated and results were in good agreement with semi-empirical model for kinetic electron emission.     

Stopping power and energy straggling of protons in graphite and amorphous carbon obtained from a resonance in BS spectra

Backscattering (BS) spectra with a sharp 4.8-MeV resonance for carbon targets have been measured using proton beams in an energy range 4.85-6.1 MeV per 100-keV step. By systematic analyses of the resonance peak profiles, values of stopping power and energy straggling have been deduced for proton energies from 0.8 to 3.4 MeV which corresponds to a penetration depth of 88 μm. In particular, to investigate the difference in stopping power and straggling caused by target inhomogeneity, we used two target materials which were highly oriented pyrolytic graphite (HOPG, 2.26 g/cm³) as a homogeneous material and amorphous carbon (1.73 g/cm³) as an inhomogeneous material. We describe a method of measuring stopping power and straggling using a resonance in the BS spectra. The stopping powers obtained are compared with the values determined by SRIM-2006. Moreover, collision straggling and a density straggling due to the inhomogeneity of the target materials are evaluated from the width broadening of resonance peaks.     

Fragmentation of water on swift He ion impact

Charge exchange and fragmentation are the usual results in ion-molecule collision systems, and the specifics of the fragmentation process determine the chemical destiny of the target system. In this paper, we report recent progress on calculations of the fragmentation patterns for the model system He²⁺ + H₂O for projectile energies of a few keV. The calculations are obtained using the electron-nuclear dynamics (END) method for solution of the time-dependent Schrödinger equation.     

Monte Carlo calculation of electron Rutherford backscattering spectra and high-energy reflection electron energy loss spectra

The electron Rutherford backscattering spectra and high-energy reflection electron energy loss spectra have been calculated by a Monte Carlo method for bulk solids and overlayer/substrate systems. The simulation model is mainly based on the use of Mott cross section for elastic scattering and the use of Penn’s dielectric functional approach to the electron inelastic scattering inside the solid. Moreover, it has further considered the recoil energy loss of energetic electrons and the thermal vibration of atoms with an isotropic distribution of the velocity direction. The calculated energy loss spectra for Al/Pt and Al/Mo agree with the experimental spectra quite well. The signals owing to different kinds of atoms can be separated by taking the scatter for the maximum-scattering angle event along an electron trajectory as the Rutherford backscattering atom, enabling a theoretical estimation of the peak intensity ratio. Furthermore, the simulation has also indicated that the multiple scattering is the dominant process to the quasi-elastic and energy loss of the electrons and is responsible mainly to the difference on the peak intensity between the linear model and experiment.     

Possible diamond-like nanoscale structures induced by slow highly-charged ions on graphite (HOPG)

The interaction between slow highly-charged ions (SHCI) of different charge states from an electron-beam ion trap and highly-oriented pyrolytic graphite (HOPG) surfaces is studied in terms of modification of electronic states at single-ion impact nanosize areas. Results are presented from AFM/STM analysis of the induced-surface topological features combined with Raman spectroscopy. I-V characteristics for a number of different impact regions were measured with STM and the results argue for possible formation of diamond-like nanoscale structures at the impact sites.     

L-subshell ionization of heavy elements by S ions with energy of 0.4-3.8 MeV/amu

The L-shell X-ray production cross sections have been measured for sulphur ions in the energy range of 12.8-120 MeV for Au and Bi elements. The experimental L X-ray spectra were analyzed using the method that takes into account the multiple ionization in outer shells. The L-subshell ionization cross sections have been obtained from measured X-ray production cross sections for resolved Lα_1,2 Lγ₁ and Lγ_2,3 transitions using the L-shell fluorescence and Coster-Kronig yields modified by the multiple ionization effects in the M and N shells. The results are compared with the predictions of ECUSAR theory, which is the modified ECPSSR approach describing both direct ionization and electron-capture processes and the semiclassical approximation (SCA) calculations for direct ionization. These approaches were modified by the L-subshell coupling effects within the “coupled-subshell model” (CSM). Both modified approaches are in good agreement with the data. Remaining discrepancies are discussed in terms of the L-shell decay rates modified for the multiple ionization effects.     

Impact and energy deposition of slow, highly charged ions on a solid surface

A plasma region in nanometer scale may be created by a highly charged ion impact on solid surface. The charge imbalance leads to enormous electric fields and may further induce Coulomb explosion due to electrostatic repulsion in the region. Thus, the highly charged ion is thus expected to be a powerful tool to induce surface modification in the nanometer scale. The Coulomb explosion model is applied in order to interpret the interaction mechanism and to understand the impact and energy deposition of highly charged ions on a solid surface, and to obtain the energy deposited by the ion. The energy deposition ratio is dependent on the material and charge. A high temperature and high pressure environment will be formed by the deposited energy, causing the atoms to swell up and a hillock nano-defect to be formed on surface. The height of hillock is estimated from the Coulomb explosion.     

Z2 structure and gas-solid effect in the stopping of slow ions

A recent claim by Paul of a systematic gas-solid difference in stopping cross sections for ions such as nitrogen and oxygen in the velocity range v ? v₀ is studied on the basis of existing experimental data. We find that all existing data support the commonly known Z₂ structure which, by and large, follows the valence structure of the target material. Existing experimental evidence is not found to support a specific gas-solid difference in the velocity range under consideration. The possibility of such an effect due to a gas-solid difference in charge state is rejected on theoretical grounds. Data for compound gases and solids are found to be well described by the Bragg additivity rule.We have also studied nitrogen/helium and oxygen/helium stopping ratios which determine the so-called effective-charge ratio. Taking into account the scatter of experimental data, we do not find clear evidence against Northcliffe’s assumption of a stopping ratio independent of Z₂ and common for gases and solids in the considered velocity range, although the absolute value appears too high.     

The influence of the charged back side on the transmission of highly charged ions through PC nanocapillaries

We have measured the fraction of the ions transmitted through nanocapillaries with their initial charge state for 200 keV Xe⁷⁺ ions impact on a polycarbonate (PC) foil with a thickness of 30 μm and a diameter of 150 nm. An Au film was evaporated on both the front and back side. It is found that more than 97% of the transmitted ions remain in their initial charge state. Then, the transmitted ion fraction and the characteristic tilt angle of 40 keV Xe⁷⁺ ions through this foil and another one with the same thickness and diameter, but evaporated by Au only on the front side, were measured. By comparing the results of these two foils, the influence of the ions deposited in the capillary exit region on the transmitted ion fraction and the characteristic tilt angle is studied. In comparison with the foil evaporated by Au on both sides, the maximum transmitted ion fraction of the foil evaporated by Au on the front side only is nearly 4 times smaller. Also, the characteristic tilt angle is slightly decreased. These results are discussed within the models for the guiding effect.     

Density effects on the guided transmission of 3 keV Ne ions through PET nanocapillaries

Experiments for guided transmission of 3 keV Ne⁷⁺ ions through nanocapillaries in insulating PET polymers are reported. The ion guiding was studied for a two types of PET samples which consist of 200 nm capillaries with densities of and . The width of the emission profile and the fraction of transmitted ions were measured as a function of the capillary tilt angle. For the high capillary density the profile width of the transmitted ions is independent of the tilt angle in agreement previous studies. However, for the low-density sample the profile width was found to increase by a factor of 2 as the tilt angle increases from 0° to 8°. The results for the fraction of transmitted ions are used to evaluate the guiding angle, which specifies the guiding power of the material. The guiding powers were found to be equal for the two samples. The present results are discussed in terms of scaling laws, which have recently been established.     

Quantum interference in grazing scattering of swift He atoms from LiF(0 0 1) surfaces: Surface eikonal approximation

This work deals with the interference effects recently observed in grazing collisions of few-keV atoms with insulator surfaces. The process is studied within a distorted-wave method, the surface eikonal approximation, based on the use of the eikonal wave function and involving axial channeled trajectories with different initial conditions. The theory is applied to helium atoms impinging on a LiF(0 0 1) surface along the 〈1 1 0〉 direction. The role played by the projectile polarization and the surface rumpling is investigated. We found that when both effects are included, the proposed eikonal approach provides angular projectile spectra in good agreement with the experimental findings.