Ion-surface Collisions Research Articles

Ab initio molecular dynamics study of ion-surface interactions

Ion-surface collisions have been investigated theoretically using ab initio molecular dynamics within density functional theory. The temporal evolution of the position of the bombarding ion, as well as its nearest neighbors, was studied for initial kinetic energies of 0 and 3.5 eV (0 and 5 km/s, respectively). Also investigated was the ion-surface interaction prior to collision and the following energy transfer, as indicated by changes in ion velocity. At 3.5 eV collision energy, the calculation results suggest the formation of local structural disorder within the simulation time frame studied. These results are of fundamental importance for an increased understanding of the ion-surface interaction during a collision event, with resulting changes in atomic level structure.

Temperature-dependent scattering of hyperthermal energyK+ions

Extensive experimental and theoretical results are presented for the scattering of a beam of K + ions incident on aC u(001) surface along the k100l azimuth with incident energies of 50 to 154 eV. Energy-resolved scattered intensity spectra reveal three distinct peaks whose widths and intensities vary with surface temperature. Using the results of a classical trajectory simulation, four distinct ionic trajectory types are assigned to these peaks. Using a classical mechanical theory that contains the correct equilibrium thermodynamics of the Cu crystal, general features of the K + energy-resolved spectra are calculated for each trajectory type and compared with the experimental results. For the case of single ion-surface atom collisions, the dependence of the peak intensities and widths on the incident energy and surface temperature is well explained by the classical theory.

Simulations of electron transfer in grazing incidence ion–surface collisions

During the interaction of a low energy ion beam with a metallic surface in grazing incidence, electron transfer plays an important role in the final state of the scattered beam. In this work we compare two approaches—rate equations resolution and a Monte Carlo numerical code named ETISC1D—to describe the beam evolution and to compute the final populations of the various atomic and ionic species as well as the corresponding angular distributions. Results of both methods are compared to experimental data obtained by Hecht et al. for 2.3 keV He +(1s) and He*(1s2s, 3S 1) beams impinging on an Al(1 1 1) surface under 0.79° of incidence. The limitations of the rate equations method and the advantages of Monte Carlo simulations are pointed out. In particular, we show that although the rate equation approach can give a fast and rather accurate evaluation of the populations, it is unable to provide correct calculations of more sensitive quantities like angular distributions of scattered species as obtained with the ETISC1D code.

Grazing incidence collisions of fast protons with insulators: electron emission around the convoy peak

A general model to calculate electron spectra in grazing incidence ion-surface collisions is presented. It is based on the use of atomic form factors for projectile ionization and incoherent scattering functions for the target. The model is applicable for arbitrary systems whose electrons can be described by localized atomic orbitals, like ionic crystals. For 100 keV protons on LiF surfaces we find that the projectile electron contribution to the convoy peak is larger than the contribution from target ionization. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Erratum: Model calculation for energy loss in ion-surface collisions [Phys. Rev. A67, 062901 (2003)

Fil: Miraglia, Jorge Esteban. 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

Studies of an [formula omitted] surface by low-energy ion scattering

A single-crystalline Fe 3 O 4 (1 1 1) surface has been investigated by means of a low-energy ion spectroscopy (LEIS) technique with a Ne + and Ar + ion beam in the temperature range of 85– 300 K . The LEIS results were compared to those on the (0 0 1) surface. The ion-surface collision on the triangular-shaped surface channels differs from that on the squared-shaped one by a less enhancement of the Ne +–Fe peak-intensity at 90 K , an existence of additional peak around 150 K attributed to positive-charged recoil-oxygen ions from the double Ne–O collisions, a larger enhancement of the Fe recoil signal and a specific characteristic of the azimuthal-angle distribution under Ar + ion bombardments. The large influence of the surface semi-channels on the ion emission yield was explained as due to the screening action of the neighbouring Fe ions.

Grazing Ion-Surface Collisions

Electron emission after grazing ion-surface collisions is studied for high impact velocities. We have focused on glancing angles of electron emission where the dominant mechanism is the ionization from atomic bound states. To describe this process, we introduce a quantum model called field distorted-wave (FDW) approximation, which takes into account the effect of the surface interaction on the electronic transition.The FDW model is applied to analyze electron distributions produced by impact of protons on Al and LiF surfaces, which are metal and insulator materials respectively. In the case of metals, we also evaluate the contibution coming from the valence band by employing the binary collisional formalism. Calculated electron emission yields are in reasonable agreement with the available experimental data. We find that the maximum of the convoy electron distribution is accelerated for Al and decelerated for LiF, with respect to its position in ion-atom collisions, in quantitative accordance with experiments.

Calculation of ion-surface collisions for a wide range of scattering geometries

A theoretical calculation that accounts for a fairly complete description of the resonant charge-exchange process occuring in the H + scattering by metal surfaces is presented. Realistic trajectories defined by the binarycollision model are considered. The interaction with nuclei and electrons of the all surface atoms that the projectile can see along its trajectory is calculated within a mean-field approximation, and in this form the contributions of the short-range interaction terms to the energy level shift are well contemplated. The long-range contributions and the motion of the projectile respect to the surface reference frame are also taken into account to define the level shift. All these ingredients are incorporated into a quantum mechanical description of the time evolution. The negative ion fractions calculated in this form show an excellent agreement with the experimental data for three different incoming energies and for a wide range of exit angles.

Reactive Ion Scattering Spectrometry of Mixed Methyl- and Hydroxy-Terminated Alkanethiolate Self-Assembled Monolayers

Reactive ion scattering spectrometry (RISS) utilizing low-energy (<100 eV) ion-surface collisions was characterized by using model systems composed of mixed methyl- and hydroxy-terminated self-asse...

Model calculation for energy loss in ion-surface collisions

The so-called local plasma approximation is generalized to deal with projectiles colliding with surfaces of amorphous solids and with a specific crystalline structure (plannar channeling). Energy loss of protons grazingly colliding with aluminum, SnTe alloy, and LiF surfaces is investigated. The calculations agree quite well with previous theoretical results and explain the experimental findings of energy loss for aluminum and SnTe alloy, but they fall short to explain the data for LiF surfaces.

Ion surface collisions of CH 3+, CH 4+, CH 5+ and CD 4+

Surface-induced dissociation of the hydrocarbon ions CH 3 +, CH 4 +, CH 5 + and CD 4 + upon impact on a hydrocarbon covered stainless steel surface has been investigated in the collision energy range from about 0 up to 50 eV. In this energy regime, typical of many laboratory and technical plasmas, the product ion mass spectra showed in general increasing fragmentation of the projectile with increasing collision energy. Besides ions resulting from the dissociation of the projectile ions, the product ion mass spectra contained also ions being either produced by pick-up reactions or by chemical sputtering from the hydrocarbon layer on the target surface.

Effects of electron–hole excitations in ion–surface collisions

We investigate effects related to electron–hole pair production and atomic level shift in atom scattering at surfaces by using a recently proposed exactly soluble model. We show that electron–hole pair production weakens Stückelberg oscillations and enhances loss of memory of the initial atomic charge state for narrow bands because of the diffusion of an electron or hole captured by the band. Wide band materials tend to display memory loss at lower velocities than do narrow band materials. Allowing the atomic energy level to shift above the Fermi energy tends to reduce memory loss.

Shattering of Peptide ions on self-assembled monolayer surfaces.

Time- and collision energy-resolved surface-induced dissociation (SID) of des-Arg(1)- and des-Arg(9)-bradykinin on a fluorinated self-assembled monolayer (SAM) surface was studied by use of a novel Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially equipped to perform SID experiments. Time-resolved fragmentation efficiency curves (TFECs) were modeled by an RRKM-based approach developed in our laboratory that utilizes a very flexible analytical expression for the internal energy deposition function capable of reproducing both single- and multiple-collision activation in the gas phase and excitation by collisions with a surface. Both experimental observations and modeling establish a very sharp transition in the dynamics of ion-surface interaction: the shattering transition. The experimental signature for this transition is the appearance of prompt (time-independent) fragmentation, which becomes dominant at high collision energies. Shattering opens a variety of dissociation pathways that are not accessible to slow collisional and thermal ion activation. This results in much better sequence coverage for the singly protonated peptides than dissociation patterns obtained with any of the slow activation methods. Modeling demonstrated that, for short reaction delays, dissociation of these peptides is solely determined by shattering. Internal energies required for shattering transition are approximately the same for des-Arg(1) and des-Arg(9)-bradykinin, resulting in the overlap of fragmentation efficiency curves obtained at short reaction delays. At longer delay times, parent ions depletion is mainly determined by a slow decay rate and fragmentation efficiency curves for des-Arg(1) and des-Arg(9)-bradykinin diverge. Dissociation thresholds of 1.17 and 1.09 eV and activation entropies of -22.2 and -23.3 cal/(mol K) were obtained for des-Arg(1) and des-Arg(9)-bradykinin from RRKM modeling of time-resolved data. Dissociation parameters for des-Arg(1)-bradykinin are in good agreement with parameters derived from thermal experiments. However, there is a significant discrepancy between the thermal data and dissociation parameters for des-Arg(9)-bradykinin obtained in this study. The difference is attributed to the differences in conformations that undergo thermal activation and activation by ion-surface collisions prior to dissociation.

Simulations of the azimuthal distribution of low-energy H atoms scattered off Ag( [formula omitted]) at grazing incidence: DFT many-body versus model pair potentials

We compare the azimuthal distribution of H atoms after scattering off Ag(1 1 0) obtained by molecular dynamics with different H–Ag(1 1 0) potential energy surfaces (PES) and experimental results. We use grazing incident H atoms and low energies (up to 4 keV). Density functional theory (DFT) calculations are performed for the static case of an H atom in front of an Ag(1 1 0) surface. The surface is represented by an 8-atom slab, and the H atoms form 1×1 and 2×2 supercells. The generalized gradient approximation is used. Classical trajectories are evaluated on the obtained PES, and the azimuthal distribution of the scattered atoms is calculated. Good agreement with experiment is obtained which gives us some confidence in the correct description of the system at low energies by the static DFT calculations. These results are also compared with pair-potential calculations. The accuracy of trajectories may be important for the correct evaluation of charge transfer, energy loss and straggling during ion–surface collisions.

A beam scattering instrument for the dynamics studies of surface-induced dissociation processes

A beam scattering instrument has been constructed to study the dynamics of ion-surface collisions, especially surface-induced dissociation of polyatomic ions. The instrument uses a bench top double focusing mass spectrometer as the source of a well-defined ion beam for transmission into a collision chamber. The primary ions are decelerated to desired ion energy (as low as 5 eV) by a series of tube lenses and collide the surface at 45°. Thus, the ion energy for surface-induced dissociations can be varied from ∼5 eV to ⩾1 keV. The secondary ions are energy and mass analyzed by a cylindrical energy analyzer and a quadrupole mass filter, respectively, and detected by a channel electron multiplier operating in pulse counting mode. The detector assembly can be rotated with respect to the collision center between the specular angle and surface parallel to measure intensity and energy distributions of scattered primary and fragment ions as a function of scattering angle. The newly constructed instrument has been tested for its performance and used to study the dissociation of ethanol molecular ions colliding with a self-assembled monolayer surface of fluorinated thiol on gold (111) crystal and first results are presented here.

A new approach to Auger and quasi-resonant processes in ion–surface collisions

We present a theoretical model of charge exchange between an ion and a solid surface which includes resonant transfer and Auger-coupling processes. The Auger processes are treated through an equation of motion approach which results in an Auger self-energy identical to that obtained through the second-order Keldysh–Green functions formalism. The model which we report involves a Lorentzian-shaped level width for the resonant processes and a wide band form for the Auger processes. It permits an exact solution of the problem in terms of well-known functions. The roles played by the characteristic parameters of the problem, which are related to the atom velocity, energy level positions, interaction potential and band structure are illustrated in some simple cases.

Dynamics of hyperthermal energy ion–surface collisions: dissociative and non-dissociative scattering of ethanol cations from a self-assembled monolayer surface of fluorinated alkyl thiol on Au (1 1 1)

Dissociation and inelastic scattering of ethanol molecular ions from self-assembled monolayer (SAM) surface of fluorinated alkyl thiol on an Au (111) monocrystal have been studied at 28.9 and 52.9eV collision energies. A single dynamics mechanism for quasi-inelastic scattering was found at both energies. Ions recoil nearly parallel to the surface with very small kinetic energy losses of the order of ≤2eV. Dissociation dynamics features for the main dissociation channel, loss of methyl radical, are dramatically different from that of inelastically scattered primary ions and are different at the two collision energies studied. At 28.9eV two energetically and angularly resolved features are observed, one corresponding to the loss of very large amounts (nearly all) of ion’s translational energy and the other appearing to gain energy (superelastic scattering). This dynamics feature is interpreted as delayed dissociation of ions transmitted through the energy analyzer as molecular ions. This implies a lifetime of such excited ions of more than 5μs. The same dynamics features are observed at 52.9eV ion energy except that a second inelastic process begins to compete with the nearly fully inelastic process. Moreover, at this energy the delayed ion dissociation mechanism is the dominant mechanism. The hypothesis that collision of ethanol cations with the SAM surface initially involves collision of the ion with a single end group of the SAM polymer chain provides a useful rationale for the observed dynamics. Support for this hypothesis is provided by Newton diagrams, which summarize momentum conservation relationships in terms of a common center-of-mass, CMeff, which provides a basis for describing scattering mechanisms for this system. Observation of three energetically distinct scattering processes suggests uniquely different ion–surface interactions contributing to surface-induced dissociation of ethanol ions. Preliminary experiments with Ar+ scattered from the same surface exhibit very similar dynamics features to that observed for ethanol cations. Finally, we note that intensities of scattered primary or fragment ions never approach the specular angle at the energies investigated here.

Neutralization of methyl cation via chemical reactions in low-energy ion-surface collisions with fluorocarbon and hydrocarbon self-assembled monolayer films

Low-energy ion-surface collisions of methyl cation at hydrocarbon and fluorocarbon self-assembled monolayer (SAM) surfaces produce extensive neutralization of CH 3 +. These experimental observations are reported together with the results obtained for ion-surface collisions with the molecular ions of benzene, styrene, 3-fluorobenzonitrile, 1,3,5-triazine, and ammonia on the same surfaces. For comparison, low-energy gas-phase collisions of CD 3 + and 3-fluorobenzonitrile molecular ions with neutral n-butane reagent gas were conducted in a triple quadrupole (QQQ) instrument. Relevant MP2 6-31G*//MP2 6-31G* ab initio and thermochemical calculations provide further insight in the neutralization mechanisms of methyl cation. The data suggest that neutralization of methyl cation with hydrocarbon and fluorocarbon SAMs occurs by concerted chemical reactions, i.e., that neutralization of the projectile occurs not only by a direct electron transfer from the surface but also by formation of a neutral molecule. The calculations indicate that the following products can be formed by exothermic processes and without appreciable activation energy: CH 4 (formal hydride ion addition) and C 2H 6 (formal methyl anion addition) from a hydrocarbon surface and CH 3F (formal fluoride addition) from a fluorocarbon surface. The results also demonstrate that, in some cases, simple thermochemical calculations cannot be used to predict the energy profiles because relatively large activation energies can be associated with exothermic reactions, as was found for the formation of CH 3CF 3 (formal addition of trifluoromethyl anion).

Low-Energy Ion−Surface Collisions Characterize Alkyl- and Fluoroalkyl-Terminated Self-Assembled Monolayers on Gold

Self-assembled monolayers (SAMs) formed from alkyl-terminated, CF 3 -terminated, CF3CF2-terminated, and C 1 0 F 2 1 -terminated alkanethiolates on gold substrates were examined by low-energy (eV) ion-surface collisions to determine the impact of varying the degree of fluorination of the thiolate tail group. The fluorine-terminated SAMs are compared and contrasted to previously examined CF 3 -terminated Langmuir-Blodgett films. Polyatomic (M + . for benzene and pyrazine) and atomic (Mo + . and Cr + . ) ions were collided with the different SAM films at various collision energies (20-70 eV). Data indicate that substitution of CH 3 with CF 3 as the terminal group has a substantial influence on the ion-surface interactions, including energy transfer (fragmentation), electron transfer (neutralization), and atom/group transfer (reaction). However, slight penetration into a depth of the film is apparent and illustrated with the formation of certain ion-surface reaction products and theintensities ofthose products. This penetration has a noticeable effect on low-energy ion-surface collisions. Also, results for a series of CF 3 (CH 2 ) n S-Au films (where n = 12, 13, 14, 15) illustrate that ion-surface reactions vary with the orientation of the fluorine atoms that are present in the terminal group. The results from collisions of low-energy ions with these relatively well-defined surfaces allow further characterization of the collision events that have the potential to become valuable tools in surface analysis.

Surface-topography dependence of line shapes in electron spectra due to decay of autoionizing states produced in inelastic ion-surface collisions

The line shapes of peaks in electron spectra due to decay of autoionizing states of Ne, formed in collisions of Ne with Al solid targets, were studied as a function of surface roughness. Strong variations in these line shapes were observed. These were modeled by taking into account the level shifts of the excited and ionic states of the atom in front of the surface and various resonant and Auger capture and loss processes. It is shown that rather complex line shapes may be expected. The electron spectrum generally possesses a peak at energies in the vicinity of that corresponding to autoionization of a free atom, as well as a high-energy tail and a secondary maximum. The intensity of the secondary maximum depends on how close to the surface most excited atoms decay and hence depends on surface flatness.