Ion-solid Collisions Research Articles

Experimental and theoretical L-shell ionization cross sections of heavy atoms by impact of Si ions

We present a theoretical and experimental study of the subshell resolved L-shell ionization of relativistic targets such as 73Ta, 78Pt, 90Th, and 92U. The measurements of x-ray production cross sections by (84–140 MeV) Si+q ions (q = 8; 12), were held at the Inter-University Accelerator Centre of New Delhi. Multiple-hole fluorescence and Coster-Kronig yields were used to obtain the Li(i = 1–3) ionization cross sections from the measured x-ray production cross sections of Lℓ, Lα, and Lβ, Lη, and Lγ lines. The experimental results are compared with ab initio theoretical calculations by means of the shell-wise local plasma approximation (SLPA). This model uses the quantum dielectric formalism to obtain the total ionization cross sections from an initial ground state. The wave functions and binding energies of the different targets were obtained by solving the fully relativistic Dirac equation using the HULLAC code package. These calculations are based on first order perturbation theory with a central field, including Breit interaction and quantum electrodynamics corrections. The present SLPA ionization cross sections of the L-shell are found to be independent of the charge state of the Si ions. The experimental observations display also quite similar character if the correct mean projectile charge state inside the target is used for including the multiple ionization effect during ion-solid collisions. A general good agreement between the experimental measurements and full theoretical calculations supports the reliability of present results. The comparison also includes the well-known ECPSSR and ECUSAR semi empirical approximations. We noted that the ECUSAR results agree well with the SLPA, while the ECPSSR cross sections are rather low.

Contribution of molecular orbital promotion to inelastic energy losses in ion-solid collisions

A model for estimating the contribution of autoionization state formation to inelastic energy losses in collisions of keV-energy ions with solids is proposed. Using the suggested scaling for L and M shell excitation it is possible to estimate the ionization cross sections and the contribution of the ionization processes to the electronic stopping power dE/dx. The model predicts a threshold behavior of dE/dx. In the cases studied the formation of autoionization states gives important contribution to the value of dE/dx.

Employing X-ray absorption technique for better detector resolution and measurement of low cross-section events

Abstract The versatility of X-ray absorption technique is experimentally employed for enhancing the detector resolution and to rejuvenate the low probable transitions obscured in the pile-up region, during a beam-foil spectroscopy experiment. The multiple aluminum absorber layers (10 μm each) are used to suppress the pile-up contribution drastically and to restore a weak transition which is about 1.38 × 104 times weaker than a one-electron-one-photon transitions viz. Kα and Khα. The weak line is possibly originating from a two-electron-one-photon transition in He-like Ti. Further, the transitions, which were obscured in the spectra due to high intensity ratio, are revived by dissimilar line intensity attenuation using this technique. The measured lifetimes of Kα line with and without intensity attenuation match well within error bar. The present technique finds potential implications in understanding the structure of multiple-core-vacant ions and other low cross section processes in ion-solid collisions.

Signature of triply excited Li-like V states in ion–solid collisions

The transitions originating from triply excited, doubly autoionizing states of Li-like V formed in beam-foil experiments detected within a set of blended spectroscopic profiles have been assigned tentatively by the Hartree–Fock calculations including relativistic corrections and multi-configuration interactions as perturbations. The x-ray decay channels from the triply excited states such as 3p34S, 2p2np4S (n∼12) through radiative transitions to the ground state via two or more steps have been observed.

Experimental evidence of beam-foil plasma creation during ion-solid interaction

Charge state evolution of the energetic projectile ions during the passage through thin carbon foils has been revisited using the X-ray spectroscopy technique. Contributions from the bulk and the solid surface in the charge changing processes have been segregated by measuring the charge state distribution of the projectile ions in the bulk of the target during the ion–solid interaction. Interestingly, the charge state distribution measured in the bulk exhibits Lorentzian profile in contrast to the well-known Gaussian structure observed using the electromagnetic methods and the theoretical predictions. The occurrence of such behavior is a direct consequence of the imbalance between charge changing processes, which has been seen in various cases of the laboratory plasma. It suggests that the ion-solid collisions constitute high-density, localized plasma in the bulk of the solid target, called the beam-foil plasma. This condensed beam-foil plasma is similar to the high-density solar and stellar plasma which may have practical implementations in various fields, in particular, plasma physics and nuclear astrophysics. The present work suggests further modification in the theoretical charge state distribution calculations by incorporating the plasma coupling effects during the ion–solid interactions. Moreover, the multi-electron capture from the target exit surface has been confirmed through comparison between experimentally measured and theoretically predicted values of the mean charge state of the projectile ions.

Extension of charge-state-distribution calculations for ion-solid collisions towards low velocities and many-electron ions

Extension of charge-state-distribution calculations for ion-solid collisions towards low velocities and many-electron ions

Angular distribution and recoil effect for 1 MeV Au+ ions through a Si3N4 thin foil

The Stopping and Range of Ions in Matter (SRIM) code has been widely used to predict nuclear stopping power and angular distribution of ion–solid collisions. However, experimental validation of the predictions is insufficient for slow heavy ions in nonmetallic compounds. In this work, time-of-flight secondary ion mass spectrometry (ToF-SIMS) is applied to determine the angular distribution of 1MeV Au ions after penetrating a Si3N4 foil with a thickness of ∼100nm. The exiting Au ions are collected by a Si wafer located ∼14mm behind the Si3N4 foil, and the resulting 2-dimensional distribution of Au ions on the Si wafer is measured by ToF-SIMS. The SRIM-predicted angular distribution of Au ions through the Si3N4 thin foil is compared with the measured results, indicating that SRIM slightly overestimates the nuclear stopping power by up to 10%. In addition, thickness reduction of the suspended Si3N4 foils induced by 1MeV Au ion irradiation is observed with an average loss rate of ∼107atoms/ion.

Damage profiles and ion distribution in Pt-irradiated SiC

Single crystalline 6H-SiC samples were irradiated at 150K with 2MeV Pt ions. The local volume swelling was determined by electron energy loss spectroscopy (EELS), and a nearly sigmoidal dependence on irradiation dose is observed. The disorder profiles and ion distribution were determined by Rutherford backscattering spectrometry (RBS), transmission electron microscopy, and secondary ion mass spectrometry. Since the volume swelling reaches 12% over the damage region at high ion fluence, the effect of lattice expansion is considered and corrected for in the analysis of RBS spectra to obtain depth profiles. Projectile and damage profiles are estimated by SRIM (Stopping and Range of Ions in Matter). When compared with the measured profiles, the SRIM code predictions of ion distribution and the damage profiles are underestimated due to significant overestimation of the electronic stopping power for the slow heavy Pt ions. By utilizing the reciprocity method, which is based on the invariance of the inelastic energy loss in ion–solid collisions against interchange of projectile and target atom, a much lower electronic stopping power is deduced. A simple approach, based on reducing the density of SiC target in SRIM simulation, is proposed to compensate the overestimated SRIM electronic stopping power values, which results in improved agreement between predicted and measured damage profiles and ion ranges.

Shower approach in the simulation of ion scattering from solids

An efficient approach for the simulation of ion scattering from solids is proposed. For every encountered atom, we take multiple samples of its thermal displacements among those which result in scattering with high probability to finally reach the detector. As a result, the detector is illuminated by intensive "showers," where each event of detection must be weighted according to the actual probability of the atom displacement. The computational cost of such simulation is orders of magnitude lower than in the direct approach, and a comprehensive analysis of multiple and plural scattering effects becomes possible. We use this method for two purposes. First, the accuracy of the approximate approaches, developed mainly for ion-beam structural analysis, is verified. Second, the possibility to reproduce a wide class of experimental conditions is used to analyze some basic features of ion-solid collisions: the role of double violent collisions in low-energy ion scattering; the origin of the "surface peak" in scattering from amorphous samples; the low-energy tail in the energy spectra of scattered medium-energy ions due to plural scattering; and the degradation of blocking patterns in two-dimensional angular distributions with increasing depth of scattering. As an example of simulation for ions of MeV energies, we verify the time reversibility for channeling and blocking of 1-MeV protons in a W crystal. The possibilities of analysis that our approach offers may be very useful for various applications, in particular, for structural analysis with atomic resolution.

Incident-angle dependence of ion-induced electron emission from Al & Co films using swift heavy ion

The interaction of charged particles with solid surfaces leads to ion induced electron emission (IIEE). The study of emission of electrons has been a subject of interest in the field of ion-solid collisions. In the present experiment we have measured electron emission yields for Al and Co targets as a function of angle of incidence in the range 0°–30° using swift heavy ions. We observed that the inverse cosine law is not followed in case of swift heavy ions. The possible reasons for deviation from inverse cosine law are discussed. The aim of this work is to show how the variation of the electron yield with the angle of ion incidence reveals proportionality between the yield and the amount of the energy deposited in the electronic excitation near the target surface.

Formation of the Circular Rydberg States in Ion-Solid Collisions

We observed the circular Rydberg states populating H-like Fe ions during fast ion-solid collisions. Time-resolved X-ray spectra obtained with 164 MeV -->5626Fe ions colliding with a carbon foil do not show any lines due to H-like Fe ions at small delay times until 600 ps. However, Ly? and Ly? transitions appear after a while and attain maximum intensity at a delay of 920 ? 5 ps. Such a delay is attributed to successive cascading from the circular Rydberg levels, which has important implications for understanding the origin of radio recombination lines from interstellar space.

K-shell ionization of low Z elements in ion–solid collisions and applicability of local plasma approximation

K-shell vacancy production cross sections are measured using X-ray technique, in collisions of highly charged fluorine ions with various solid targets such as, Cl, K, Fe and Cu, at energies from 50 to 110 MeV. The experimental data is compared with an ab initio model based on local plasma approximation (LPA) and the usually employed ECPSSR. A detailed comparison with the LPA model is presented as a function of generalized perturbation strength.

Production and transport of long-lifetime excited states in preequilibrium ion-solid collisions

A complete experimental study of the production and transport of long-lifetime excited states has been done for ${\mathrm{Ar}}^{18+}$ on solid C targets, at a velocity of $23\phantom{\rule{0.3em}{0ex}}\mathrm{a.u.}$, and for a range of thickness allowing us to vary the transport conditions from single collision to equilibrium $(3.5\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}201\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{g}∕{\mathrm{cm}}^{2})$. A systematic determination of ${\mathrm{Ar}}^{17+}$ Rydberg $\ensuremath{\ell}$- and $2s$-state populations has been performed using the x-ray spectroscopy technique. Results are compared with predictions of different transport simulations (developed on either a quantum or a classical phase space), which take into account multiple collisions and the strong polarization induced by the incoming ion (the wakefield). Using the continuum distorted-wave approximation for modeling the initial capture process, very good agreement is found between experimental Rydberg-state populations and theoretical approaches limited to the effect of multiple collisions. On the contrary, the transport of the metastable $2s$ state exhibits strong sensitivity to Stark mixing induced by the wakefield. The limitation of each theoretical approach is discussed with respect to the different experimental observables.

Beam-single and beam-two-foil experimental facility to study physics of highly charged ions

A facility for lifetime measurement of metastable states in highly charged ions using the beam-foil technique with a single-foil and a two-foil target has been developed. In the two-foil technique, one foil moves with respect to the other and the option of varying the thickness of the fixed foil online has been implemented. A holder with multiple foils is used as a fixed target, and moved along x⁠, y⁠, and θ⁠, the angle of rotation with respect to beam direction along the z axis. Using this facility, the He-like 1s2pP2o3 and Li-like 1s2s2pP5∕2o4 titanium lifetimes have been measured and compared with earlier values. In addition to this, the processes which occur when excited states collide with carbon foils of different thicknesses have also been investigated. Preliminary results suggest the scope of studying intrashell transitions during ion-solid collision using this setup. In this article, the setup is described in detail and representative results are briefly discussed.

Fermi-shuttle acceleration of electrons in ion–matter interaction

Large electron yields, compared to theoretical predictions, have often been observed in the high-energy part of the electron spectra in collisions of energetic ions with atomic, molecular or solid targets. The relative enhancement of the electron emission yield at high energies can be especially strong in ion–solid collisions. In this work, following a brief overview, recent experimental evidences are presented for Fermi-shuttle type accelerating electron scattering sequences in ion–atom collisions. Signatures for double (projectile–target, P–T), triple (projectile–target–projectile, P–T–P) and quadruple (P–T–P–T) scattering sequences have been found in different collision systems. Our new results indicate the presence of even higher-order scattering contributions in the collisions of few keV energy N + ions with inert gas atoms. The observations support that high-energy electrons produced by accelerating scattering sequences may play a significant role in ion–solid collisions.

Sputtering: the material erosion tool

The erosion of materials with energetic ions is an indispensable tool in the laboratory and the industry. The ion–solid collision process leading to the ejection of atoms contains fascinating physics and materials science of surfaces. In this article, different aspects of sputtering relevant to applications are discussed. Current research is emphasized and future directions are outlined.

Defect production, multiple ion–solid interactions and amorphization in SiC

Recent progress in the atomic-scale simulations of fundamental damage production processes in SiC is reviewed, which includes the displacement threshold energy surface, the primary damage state and statistics of defect production, multiple ion–solid collision events and structural evolution in SiC. The threshold energy surface, E d, appears to be highly anisotropic, and the results of molecular dynamics (MD) simulations, in conjunction with experimental studies, suggest that E d values of 20 eV for C and 35 eV for Si should be used in Kinchin–Pease calculations. The Si displacement cascades with energies up to 50 keV show that the surviving defects are dominated by C interstitials and vacancies, consistent with experimental observations. The defect production efficiency decreases with increasing recoil energy, but the number and size of clusters or complex domains formed at the end of cascades are very small, independent of cascade energy. A large number of 10 keV displacement cascades were randomly generated in a model crystal to simulate multiple ion–solid interaction and damage accumulation. The coalescence of clusters represents an important mechanism leading to the complete amorphization of SiC, and the relative disorder and swelling behavior show an excellent agreement with experimental observations. HRTEM images simulated from the MD cell reveal the microstructural evolution of multiple ion–solid collision events, and provide atomic-level interpretations of experimentally observed features in SiC.

Atomic-scale simulations of multiple ion–solid interactions and structural evolution in silicon carbide

Molecular dynamics (MD) were employed in atomic-level simulations of fundamental damage production processes due to multiple ion–solid collision events in cubic SiC. Isolated collision cascades produce single interstitials, vacancies, antisite defects, and small defect clusters. As the number of cascades (or equivalent dose) increases, the concentration of defects increases, and the collision cascades begin to overlap. The coalescence of defects and clusters with increasing dose is an important mechanism leading to amorphization in SiC and is consistent with the homogeneous amorphization process observed experimentally in SiC. The driving force for the crystalline– amorphous (c–a) transition is the accumulation of both interstitials and antisite defects. High-resolution transmission electron microscopy (HRTEM) images of the defect accumulation process and loss of long-range order in the MD simulation cell are consistent with experimental HRTEM images and disorder measurements. Thus, the MD simulations provide atomic-level insights into the interpretation of experimentally observed features associated with multiple ion–solid collision events in SiC.

Theory andmeasurement of absolute doubly differential cross sections ofbinary encounter electron ejection in collisions of swift heavyions with solids

Measurements of the momentum distribution of high-energyelectrons emitted in weakly relativistic ion-solid collisions arecompared with a transport theory which is based on therelativistic electron impact approximation for electronproduction and which accounts for angular deflection and energyloss but also energy straggling of the transmitted electrons.The measurements on electron ejection by 45 MeV u-1Ni28+ projectiles colliding with carbon foils of thickness10-8300 µg cm-2 were made on an absolute scale,providing a sensitive test of the transport theory. The theory givesa satisfactory description of earlier (relative) data from13.6 MeV u-1 Ar18+ impact on carbon. Absolute singlydifferential cross sections for Ni28+ agree within 10% in the case of thin targets, but the width of thebinary encounter peak is underestimated by theory. For thethickest targets the measured doubly differential cross sectionsare overpredicted by up to one order of magnitude, particularlyat forward emission angles, but peak shapes are well reproduced.

Many-electron aspect of the time-dependent description of ion-atom and ion-solid collisions

We present a method which solves the time-dependent Dirac equation for complicated ion-atom systems in the Dirac-Fock-Slater approximation including the many-electron effects in the inclusive probability description. Various gas-gas and gas-solid target collision systems are discussed. The effect of excitation and tranfer during the passage within the solid allows us to understand the gas-solid target systems. A many-particle ab initio calculation of K MO X-rays are presented for the system Cl 16+ on Ar.