Inelastic Energy Loss Research Articles

On the origin of Ne 2+ production in low-energy Ne + scattering from Al and Si surfaces

We report an investigation on the inelastic energy loss of doubly charged Ne 2+ ions produced in the single scattering of Ne + from Al and Si surfaces. The inelasticity of 86 ± 5eV found in the binary encounter agrees well with the energy needed to excite two electrons from Ne + to Ne 2+*. Our results suggest that direct decay or autoionization decay of Ne +** formed subsequently by capturing one electron from the solid is the main mechanism for the production of Ne 2+ detected in ion-scattering spectroscopy.

Monte Carlo calculations for the design of Mott scattering spin polarimeters

Using the Monte Carlo method, we have calculated, for 50 keV electrons incident on a gold target, the dependence of the effective Sherman asymmetry function and scattered intensity on the target thickness and inelastic energy loss window for scattering angles from 90° to 180°. Our results show that, when the gold target is thicker than 700 Å or the inelastic energy loss window is larger than 1200 eV, the scattered intensity is maximum at a scattering angle of about 120°, and the effective Sherman function is almost constant over a wide range of scattering angles. Thus, for Mott scattering spin polarimeters, the electron detectors should be positioned at ±120°, and the larger the collection angle for scattered electrons, the higher the efficiency of the Mott polarimeter.

Ionization phenomenon in high-density gaseous and liquid argon in corona discharge experiments

Experimental investigations of corona discharge in Ar for a wide region of fluid density from a rare gas up to a liquid have been carried out. Corona current has been observed for a point cathode voltage above the threshold value . Values of have been measured for a wide range of fluid density N and for different cathode radii . The obtained dependences of on N and have been analysed using the Townsend - Meek breakdown criterion. For low-density gas the calculations of have been carried out by using universal data on ionization coefficient . The data of for low-density gas are in good agreement with these calculations. Deviation from the `gaseous' dependence has been obtained in measurements carried out with a continuous increasing of the fluid density. The onset corona voltage in dense supercritical gas and in liquid is appreciably smaller than the values calculated from the gaseous Paschen curve. To explain this disagreement a model of impact ionization in liquid was developed. An assumption was made that inelastic energy losses of fast electrons by excitation of atoms without their ionization are absent in liquid. The expression for the ionization coefficient in liquid Ar has been obtained and it was used for calculation of in liquid. The calculated values are in agreement with the experimental data obtained in liquid Ar.

Inelastic energy loss in low-energy ion-surface collisions

Recent measurements showed a previously unobserved sharp thresholdlike inelastic excitation occurring in low-energy ion-metal surface collisions when varying the energy of the incident ion. Using a relativistic Dirac-Fock-Slater approach we deduce that the inelastic energy loss of the ion beam can be traced to the fast diabatic promotion of specific molecular levels to the continuum.

Intermixing at Ge/Si(001) interfaces studied by surface energy loss of medium energy ion scattering

Intermixing of Ge(0.15 ML, 1 ML)/Si(001) grown at 400°C was studied by a new method of surface energy loss spectroscopy of medium energy ion scattering. We measured the inelastic energy losses of scattered ions from Ge for 100.0 keV H + incidence as a function of exit angle. In the previous work for 50 and 100 keV H + backscattered from the Si(001)2×1-Sb surface [Phys. Rev. B in press], we showed that the surface stopping cross-section was expressed by extending the bulk stopping region from the top atomic plane toward the vacuum side by d, which was given by 2 d ρ= σ ( σ is the areal density of top surface and ρ is the bulk atomic density) as the first approximation. The above approximation was applied to analysis of the Ge/Si(001) interface. The present analysis revealed that the deposited Ge atoms were distributed over the first to third atom layers with a ratio of 4:3:1 or 4:2:2 for both 0.15 ML and 1 ML coverage. This result is consistent with that reported by Sasaki et al. [Appl. Surf. Sci. 82–83 (1994) 387] using Auger electron and X-ray photoelectron diffraction.

Decay of coupled plasmon - phonon modes in heavily doped semiconductors

For comparison with results obtained in recent phonon pulse experiments, we have calculated the relative magnitudes of different decay mechanisms for coupled plasmon - phonon modes in heavily doped GaAs. We find the dominant mechanism for energy loss by the coupled modes to be the direct excitation of bare phonons through second-order dipole moment and third-order anharmonic interactions, resulting in a lifetime of less than 1 ps for the particular material used in the experiments. Other mechanisms, notably lattice viscosity, electron - hole pair creation, rigorously forbidden at absolute zero, and the excitation of two electron - hole pairs, a higher-order effect but allowed at zero temperature, all give significantly smaller contributions to the decay rate. However, collision damping of the electrons through impurity scattering gives a lifetime which is much shorter than the above value, around 0.1 ps, but we note that this is an elastic scattering time, in contrast with the inelastic energy loss process. Linewidths in Raman scattering experiments yield the total scattering rate, which is dominated by the elastic process; currently only the phonon pulse measurements can give information directly on the inelastic process.

Inelastic energy loss of fast N2 scattered from Pd(111)

The dissociation dynamics of fast molecular nitrogen scattered off Pd(111) surfaces is dominated by elastical processes. Modelling of this system by means of classical trajectory calculations is therefore a useful approach. In contrast, experimental results reveal a strong influence of inelastic energy loss processes especially on the time-of-flight distributions of the surviving molecules. Here we present the results obtained after incorporating inelastic energy losses in the code according to a simple model of the friction at the surface.

Energy distributions of particles transmitted through free foils at oblique incidence

Energy loss distributions of particles transmitted through free mylar foil of 25 μg/cm 2 thickness for 10–25 keV protons incident at angles 0 ≤ θ 0 ≤ 60° are measured. An analytical treatment is made using the Fokker-Planck approximation for the kinetic equation. Different factors that can affect energy loss distributions were taken into account: energy straggling during inelastic collisions, path length fluctuations due to multiple eleastic collisions and foil thickness fluctuations. A simple formula for the energy spectra FWHM is derived comprising all aforesaid factors. It is shown that the dependence of FWHM on these factors as function of θ 0 is different. So possible contribution of each factor can be evaluated from angle measurements of FWHM. Comparison of the analytical result with the experiment as well as computer simulations using TRIM-like code shows that the main contribution to measured FWHM is due to fluctuations of the inelastic energy losses.

Inelastic processes in keV H + and He + scattering from Cu(110)

Measurements and calculations for the kinetic electron emission from keV protons and He + ions grazingly incident on Cu(110) are discussed. The electron energy distributions are shown to be dominated by the emission of the localized 3d electrons of Cu during the atom-atom collisions at the surface. This mechanism gives rise to inelastic energy losses of the projectiles; the excitation of the 3d electrons of Cu to energies just above the Fermi level are shown to contribute significantly to the total energy loss. In case of H it is shown that also the contribution of the excitation of the 1s electron of H may be important. In case of He this is not the case due to the larger binding energies of the 1s electrons.

Backscattering of light atomic particles from solids

Thermonuclear plasma-wall interaction is accompanied by intensive backscattering of light ions and atoms, which influences strongly the mass and energy transfer between the plasma and construction materials. Backscattering of atomic particles from solids plays an important role in the modification of materials by ion beams and also as a means of studying the near surface region by low energy ion scattering spectroscopy. In this report, the reflection of medium energy ions and atoms from thick targets has been studied both analytically and via computer simulations. A new method of solving the kinetic equation, with an appropriate boundary condition, has been developed, which enables simple and effective expressions to be obtained for the total particle energies and total reflection coefficients. The elastic scattering is described by realistic potentials, while inelastic energy losses are accounted for in the local density approximation. The analytical results compare well with the corresponding Monte Carlo calculations.

Mobilities of Li+ in Ne and in N2 and Na+ in SF6: Effect of inelastic energy loss

Ion mobility measurements were made for Li+ in Ne and in N2 and Na+ in SF6 using a variable-temperature drift tube. The measurements were made by two different methods: variable-E/N method at constant T and variable-T method at constant E/N, where E, N, and T are the electric field strength, the gas number density, and the gas temperature, respectively. Two datasets were compared at the same effective temperature Teff, as calculated from the Wannier equation. For Li+ in Ne, the two datasets are put on a single curve in the range 150 K<Teff<10 000 K, indicating that the scaling rule holds well. However, for Li+ in N2 and Na+ in SF6 there exists a mobility difference between the two datasets, which is attributed to inelastic energy loss in the variable-E/N measurement. Using the modified Wannier equation developed by Viehland et al., we determined the inelastic energy loss factor for Na+ in SF6.

Impact-parameter dependence of inelastic energy losses in slow atom-atom collisions

A quasiclassical expression has been derived for the inelastic energy losses in atom-atom collisions below the Bohr velocity as a function of the impact parameter. A collision of slow atoms is treated on the basis of the Thomas-Fermi statistical model. The gas of atomic electrons is characterized by the space dependent value of the Fermi energy. The inelastic energy losses are believed to be due to the electron momentum transfer between colliding partners. The excitation of the Fermi gas at each point of the phase space is assumed to be small compared with the Fermi energy. The projectile trajectory shape is accounted for classically by means of the Thomas-Fermi-Firsov potential. It has been found that the Coulomb repulsion between the nuclei of the projectile and the target atom decreases greatly the contribution of small impact-parameter collisions in the limiting case of low reduced energies.

Inelastic energy loss of recoiled hydrogen ions in low-energy He +, Ne + and Ar + collisions with hydrogenated silicon surface

Inelastic excitation processes are reported for recoiled hydrogen ions produced by He +, Ne + and Ar + ion beams with an energy range from 400 to 1500 eV at hydrogenated silicon surface. Such inelastic excitation processes are observed as displacements in the recoiled hydrogen ion energy from the position of the elastic recoiling. The observed displacements for the recoiled H + ions are 25 eV in the Ne + ion incidence, and 20 eV in the Ar + ion incidence, which are independent of the incident beam energy. In case of the He + ion incidence, the displacement is not distinguished. As a possible inelastic excitation processes relating to the observed displacement, an electron promotion model that both the neutralized incident ions and the recoiled hydrogen are simultaneously excited to form ground state ions is suggested, where electrons in both atoms are promoted above the Fermi level of the surface during hard collisions.

1 keV hydrogen implantation in a-Si and c-Si: Physical vs computational modeling

We have used the codes MARLOWE, CRYSTAL-TRIM, TRIM-SP and TRIM-95, to simulate our experimental data on the penetration of 1-keV hydrogen ions in a-Si and in c-Si in “random” and channeled directions. The dominant energy loss mechanism is inelastic for most of the path length of the penetrating ion. The effect of using an impact parameter dependent inelastic energy loss using the Oen-Robinson (OR) model or a drag force as in the LSS and ENR theories was investigated. In the case of c-Si, the data can be reproduced with the OR model using a single set of physical parameters, whereas in the case of a-Si only partial agreement was found. Even though elastic energy loss is not dominant, simultaneous collisions have to be taken into account. Also, the effect of the computationally required maximum impact parameter (imposed by the binary collision approximation) was examined.

Monte Carlo simulation of time-of-flight spectra of coaxial impact collision ion scattering spectroscopy applied to MoS 2(0001) and SrTiO 3(001)

We have synthesized a Monte Carlo program to simulate the time-of-flight (TOF) spectra of coaxial impact collision ion scattering spectroscopy (CAICISS). This program is based on the binary collision model considering inelastic energy loss and follows the incoming and outgoing trajectories for each incident ion from a top surface up to 300 monolayers (MLs) below a surface. The present simulation reproduced well the observed overall TOF spectra from MoS 2(0001), whose surface structure was already known. It was also found that the ions backscattered from the atoms located in the depths from top surface down to 150th atom layer contributed to the TOF spectrum. In order to extract quantitative information on surface structures from CAICISS spectra, it is essential to estimate accurately the background originated from the backscattering at the depths. The CAICISS measurement followed by the Monte Carlo simulation was applied to structure analysis of a TiO 2-terminated SrTiO 3(001) surface prepared by a chemical treatment and confirmed that the topmost layer comprised a TiO 2 layer and the SrO fraction was estimated to be less than 5%.

Molecular dynamics simulations of inner shell electronic energy losses in cluster-surface collisions

Previous molecular dynamics simulations have predicted that in energetic cluster-surface collisions atoms can be core-excited leading to Auger transitions outside the target [5]. However, in these previous simulations the inelastic energy losses accompanying core-excitations were not taken into account. In our simulations a simple critical distance of approach to detect inner-shell excitations is used. Energy is removed from atoms in a single time step by moving the atoms apart from each other, taking away potential energy. Molecular dynamics simulations are carried out for 63-atom Al clusters and composite Al 38Au 25 clusters incident on Au(100) targets and Au(111) targets. Each event is simulated with and without inelastic losses. Incident cluster energies ranged from 30 to 100 keV. Our results indicate a threshold for significant inelastic loss effects at approximately 70 keV/cluster, i.e., about 1 keV/atom. Above the threshold energy region, sputtered atom and ejected-cluster atom energy distributions were affected substantially.

Inelastic energy loss and inner-shell electron promotion in low energy Ne ions scattered from Fe and Ni surfaces

Inelastic energy loss and inner-shell electron promotion of Ne + ions scattered from Fe(110) and Ni(100) surfaces have been studied systematically in the energy range from 1 to 7 keV at large scattering angles. The inelastic energy loss shows a threshold-type greater increase with incident energy in both scattering systems. Similar threshold-type behaviour is also observed in the scattered Ne + ion yield, the scattered Ne ++ ion yield and the peak fwhm. The threshold incident energy for such phenomena is found to increase with the atomic number Z of the target. It is suggested that excitations of the Ne 2s electrons occur during the close collision of the Ne with Fe or Ni atoms, resulting in a sudden increase in the inelastic energy loss. The excitation probability is found to depend on the atom position of the target relative to the swapping points of the Ne inner-shell energy levels. The ionization of the doubly excited Ne∗∗ particles alters the charge exchange behaviour of the Ne projectiles on their outgoing trajectories, leading to a remarkable increase in the scattered ion yield. The results show that inner-shell electron excitations could be significant in low energy ion surface scattering and may play an important role in the charge exchange process.

Simulation of ballistic effects during scattering under glancing angles of incidence from crystal surfaces

A direct comparison of the results of molecular dynamics (MD) and sequential binary collision (BCA) simulations of the scattering of 3 keV light (He) and heavy (Xe) particles from a Cu(111) surface along the [11̄0] direction under glancing incidence conditions is presented. Inelastic energy losses are neglected. Clear differences between the MD and BCA results are seen in the differential scattering distributions. The origin of these differences arises from approximations in the way “simultaneous” collisions between the projectile and several nearby surface atoms are treated in BCA-type simulations.

Plasmon dispersion and broadening in A3C60 (A=K, Rb).

Inelastic electron energy loss measurements of the 0.5-eV charge carrier plasmon in ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ are presented, showing that the plasmon has a very large broadening but a negligible dispersion. The response function is calculated in a self-consistent, conserving approximation, showing that the plasmon broadening can be explained in terms of a decay in an electron-hole pair dressed by phonons. The plasmon dispersion can be understood within the random-phase approximation. Local-field effects tend to give a large negative dispersion, which is compensated by interband effects. \textcopyright{} 1996 The American Physical Society.

Elastic interaction of MeV/amu heavy multicharged ions with solids

Heavy multicharged ion (MCI) moving in solid interacts with nuclei and electrons of the matter atoms. At projectile velocity exceeding Bohr velocity V > V0 the main process is inelastic interaction, i.e. excitation and ionization of bound electrons. Therefore inelastic energy loss of the MCI in the substance is more than the elastic energy loss by order of magnitude. Elastic interaction being the result of electrostatic repulsion of the MCI nucleus and the target atom nucleus is the key mechanism in the point defect production in the whole velocity range. There is a well-known method of elastic interaction cross section calculation for velocity range V < V0. The nuclei of neutral atoms are considered to be screened by bound electrons in the frames of Thomas-Fermi model. The screening becomes less effective in the V > V0 velocity range because the MCI has lost part of its electrons and target atom nucleus is screened only by electrons, orbital velocities which are higher than the collision velocity. Elastic interaction cross section is obtained taking into account the velocity dependence of MCI charge and in accordance with screening length which is also a function of velocity V. Attenuation of the screening effect leads to the marked rise of defect production cross section. Elastic energy loss increase is hardly distinguished from the well-known results.