Dynamic dependence of the interaction potentials for grazing scattering of fast atoms from metal and insulator surfaces

For scattering of fast atoms from metal and insulator surfaces under axial channeling conditions pronounced peaks in the angular distributions of scattered projectiles are interpreted in terms of rainbow scattering. The angular position of such “rainbow peaks” are closely related to the interaction potential and its corrugation in the topmost surface region. We have scattered N and O atoms, with energies ranging from 10 to 70 keV, from clean and flat Al(0 0 1) and LiF(0 0 1) surfaces along low index axial directions in the surface plane and studied the positions of the rainbow peaks as function of the kinetic energy of the atomic projectiles normal to the surface. For the insulator surface the rainbow angle does not depend on projectile energy for constant normal energy, whereas for the metal surface we find pronounced dynamic effects. We interpret this different behaviour as arising from a projectile energy dependent contribution to the underlying interaction potentials owing to embedding the projectiles into the free electron gas in the selvedge of the surfaces, which is present for the metals but absent for insulators.     

Temporal development of sputtered atom distributions from Au(1 1 1) targets following bombardment with 100 keV/atom Au2 ions

Recently Bouneau et al. measured the angular and energy distributions of negative Au_n (n=2–7) ions emitted from gold targets following bombardment with swift gold cluster projectiles. They found that the energy distributions could be fitted with a spike-like model, and that the angular distributions were independent of the azimuthal emission angle and relatively strongly forward directed. We have used MD simulations to investigate the temporal development of energy and angular distributions of sputtered atoms from Au(1 1 1) targets following bombardment with 100 keV/atom Au₂ ions. Our results show that during the very early stages of the collision cascade the energy distribution of sputtered atoms is described well by the linear cascade model. Essentially all high energy sputtered atoms are emitted during this phase of the collision cascade. However, the energy distributions of atoms sputtered after 0.5 ps were typical of emission from a thermal spike and could be fitted well with a Sigmund–Claussen model. The polar angle distributions of sputtered atoms were strongly forward directed early in the collision cascade, but became less forward directed as the thermal spike developed.     

Synergism in sputtering of copper nanoclusters on graphite substrate at low energy Cu2 bombardment

Molecular dynamics simulation of Cu cluster sputtering by 50-200 eV/atom Cu₂ dimers and Cu single atoms has been performed. The clusters were located on a (0 0 0 1) graphite surface and consisted of 13-195 atoms. Synergy features were identified in the sputtering yield and energy distributions of sputtered particles calculated for the cases of cluster bombardment with Cu dimers and monomers at the same velocity. The reason for the nonlinear effects in surface cluster sputtering is the overlapping of collision cascades generated by each of the dimer atoms.     

Sputtering of Langmuir-Blodgett multilayers by keV C60 projectiles as seen by computer simulations

Fundamental processes induced in a thick organic system composed of long, well-organized linear molecules by an impact of 5-20 keV C₆₀ are investigated. The organic system is represented by Langmuir-Blodgett multilayers formed from bariated molecules of arachidic acid. The thickness of the system varies between 2 and 16 nm. Coarse-grained molecular dynamics computer simulations are applied to investigate the energy transfer pathways and sputtering yields as a function of the kinetic energy of the projectile and the thickness of the organic overlayer.The results indicate that an impact of keV C₆₀ projectiles leads to significant ejection of organic material. The efficiency of desorption increases with the kinetic energy of the projectile for a given layer thickness. For a constant primary kinetic energy, the sputtering yield goes through a maximum and finally saturates as the LB layer becomes thicker. Such behaviour is caused by a competition between signal enhancement due to increasing number of organic molecules and signal decrease due to lowering of the amount of the primary energy being backreflected into the organic overlayer by the receding organic/metal interface as the layer is getting thicker. When the sample thickness becomes much larger than the penetration depth of the projectile, the sputtering yield is independent of thickness. The deposited energy is channelled by an open and ordered molecular structure, which leads to abnormally long projectile penetration and ion-induced damage.     

The influence of projectile charge state on ionization probabilities of sputtered atoms

The ionization probability of atoms sputtered from a clean polycrystalline metal surface was measured for different charge states of the projectile used to bombard the sample. More specifically, a polycrystalline indium surface was irradiated with Ar⁺ and Ar⁰ beams of energies between 5 and 15 keV, and In⁺ secondary ions and neutral In atoms emitted from the surface were detected under identical experimental conditions regarding the sampled emission angle and energy. The resulting energy integrated ionization probability of sputtered In atoms is consistently found to be smaller for neutral projectiles, the difference decreasing with decreasing impact energy. The observed trends agree with those measured for kinetic electron emission, indicating that secondary ion formation is at least partly governed by kinetic substrate excitation.     

Collision cascades induced by aluminum clusters impacting on gold thin films

The collision cascades of 0.2keV/atom aluminum clusters impacting on gold thin films were investigated. The energy spectra and the number density as a function of time were calculated by molecular dynamics simulation. The results were compared with monatomic bombardments for the same energy per atom. Aluminum atoms with energies larger than their initial energy were found. The highest energy of gold recoils is about 5 times that allowed in an isolated two-body collision. Craters were found on the target surfaces. Multiple successive collisions and collisions between moving atoms are considered to be important in producing such high energy atoms. The dependence of the energy transfer on the mass ratio and the scattering geometry of two colliding atoms are also discussed.     

Collision cascade temperature

Interaction of a projectile with a solid has been considered in detail. It has been found that any collision cascade generated by a projectile can be characterized by the average kinetic energy of cascade atoms that represents an “instantaneous temperature” of the cascade during its very short lifetime (10⁻¹² s). We refer to this value as the “dynamic temperature” in order to emphasize the fact that cascade atoms are in a dynamic equilibrium and have a definite energy distribution. The dynamic temperature defines the electron distribution in the cascade area and, hence, the ionization probability of sputtered atoms. The energy distribution of cascade atoms and, as a consequence, the dynamic temperature can be found experimentally by measuring the energy distribution of sputtered atoms. The calculated dynamic temperature has been found to be in good agreement with the experimental data on ion formation in the case of cesium and oxygen ion sputtering of silicon. Based on the developed model we suggest an experimental technique for a radical improvement of the existing cascade sputtering models.     

Sputtering and Ion-Source Technology

Sputtering is dependent on a number of projectile and target parameters. It is shown that the dependence of the sputtering yield on projectile energy, angle of incidence and atomic number is well understood. Also, the dependence on the bulk properties of the target is described reasonably well by theory, while the dependence on the actual surface topography of the target is difficult to quantify. Positive-ion sources mainly depend on the number of atoms sputtered per incoming ion (sputtering yield), while also energy- and angular-distributions of the sputtered material are of primary importance for negative-ion sources. These distributions are reasonably well known and allow a direct calculation of the emittance of some negative-ion sources.     

Molecular simulations of cluster ejection by CO2 cluster impact on carbon-based surfaces

Single (CO₂)_N (N = 1-20) cluster impact on three different carbon-based surfaces of fullerite (1 1 1), graphite and diamond (1 0 0) has been investigated by MD simulations with the cluster collision energy from 5 to 14 keV/cluster as a first step toward the general modeling of the reactive sputtering by cluster impact of a solid surface. A crater permanently remained on the fullerite and graphite surfaces while it was quickly replenished with fluidized carbon material on the diamond surface. In spite of the smaller crater size as well as the crater recovery resulting in the reduction of the surface area, the sputtering yields were the highest on diamond. The effective energy deposition near the surface contributes to the temperature rise and consequent sputtering seemed highly reduced due to the collision cascades especially on the fullerite target.     

Internal energy distribution of sputtered sulfur molecules

Bombarding targets of CS₂ and sulfur with keV Ar⁺ ions induces among other species the sputtering of S₂ molecules. We measured the internal energy (vibration and rotation) of these sputtered molecules with a laser induced fluorescence technique. The results show a Boltzmann behaviour for both the vibrational and rotational distributions. A vibrational temperature T_vib = 1500 K and a rotational temperature T_rot = 300 K is obtained for both target materials. The results are compared with different ejection mechanisms, where a molecule on the surface receives one (single collision) or two (double collision) momentum transfers from the solid. Two classical models are able to explain part of the experimentally observed internal energy distribution: a Monte Carlo calculation of the double collision model, and a single collision model assuming a sudden momentum transfer to the molecule as a whole, which yields an analytical expression (via a relation between the internal and kinetic energy). If we assume that some additional rotational cooling, due to time dependent relaxation effects, occurs during the ejection from the surface, the experimental results can be reasonably interpreted within the proposed sputtering models.     

Sputtering mechanisms near the threshold energy

Threshold energies for sputtering cannot be calculated directly but have to be evaluated from the energy dependence of the sputtering yields. This paper investigates trajectories of projectile and recoils near the threshold energy for sputtering, where the collision cascade becomes increasingly simple. Statistics of the different collision events show which processes dominate the sputtering close to the threshold energy for selfbombardment of different light and heavy targets. The differential cross-sections for scattering and recoil production explain qualitatively the probability for the various processes.     

Electronic excitation in sputtered atoms and the oxygen effect

Bombardment of surfaces by ions gives rise to a variety of inelastic collision events leading to the ejection of excited atoms and ions. Such excited sputtered particles have been studied since more than 80 years through their optical emission, when they decay in front of the target to the electronic ground state, having lifetimes of 10⁻⁹ to 10⁻⁷ s, typically. Information on the energy distribution of such excited states can be obtained by two different techniques: light vs distance measurements (LvD) and by studying line profile broadening in light emission due to the Doppler effect. Only recently it has become possible to study in addition metastable excited atoms using laser induced fluorescence spectroscopy (LIF). Relative sputtering yields and energy distributions have been measured for such metastable states and two types can be distinguished. States with a very low excitation energy (0–0.3 eV), being sublevels of the electronic ground state, were found to have yields and energy distributions comparable to the electronic ground state, while metastable states at higher excitation energies (above 1 eV) seem to behave similar to short lived excited states, typically observed in secondary photon emission (BLE) with excitation energies in the range of 2–6 eV. This behaviour is also clearly visible with respect to oxygen surface coverage or increased near surface oxygen concentration where, similar to secondary ion emission, drastic changes in the yield by orders of magnitude have been found for excited atoms as well as for ions. In addition, under the same conditions a strong decrease in the sputtering yield of neutral ground state atoms has been observed for a number of metals. LIF results for highly excited metastable states are compared with recent results obtained by studying line profile broadening in light emission for Ca, Al and Cr targets. Different mechanisms that have been proposed to account for the observations will be discussed.     

Evidence for the bulk nature of self-assembled monolayer surface of fluorinated alkyl thiol from hyperthermal energy ion-surface collisions

Kinetic energy distributions of fragment ions from hyperthermal energy collisions of ions with a self-assembled monolayer surface of fluorinated alkyl thiol on gold substrate and a LiF surface (vapor deposited on titanium substrate) have been measured as a function of scattering angle and fragment ion mass. These distributions for the thiol surface show two energetically and angularly distinct pathways for the dissociation of ions while those from LiF show only one peak. A plot of the velocities of the fragment ions as a Newton diagram for these two processes clearly demonstrates that one process is due to the collision of ions with a fraction of the molecular chain of the monolayer surface molecule with scattering over a broader angular range while the other process is due to collision of the projectile ions with the monolayer surface acting as a bulk surface with fragment ions scattering close to the surface parallel and resembles the Newton diagram from the LiF surface.     

Precipitate dissolution by high energy collision cascades

The evolution of the microstructure during irradiation is now widely recognized due to: (1) radiation altered kinetic phenomena; (2) collisional processes by energetic cascades. In this paper, we investigate the specific nature of the collisional interaction between energetic cascades and precipitates. A new Monte Carlo-based computer program, TRIPOS, has been developed for the TRansport of Ions in POlyatomic Solids. The computer code utilizes standard nuclear and electronic energy loss formulas, and compares well with experimental data on particle reflection, penetration and sputtering. One of the unique features of the code is its applicability to problems involving multispecie media in multilayers of 3-dimensional configurations. The interaction of neutron-initiated high energy collision cascades is demonstrated to result in the partial dissolution of precipitates. However, the maximum precipitate size that may be completely destroyed by a high energy collision cascade is only a small fraction of the cascade size. Matrix atom implantation inside precipitates as well as preferential sputtering of light atoms from the surface of precipitates into the matrix is demonstrated to lead to changes in precipitate stoichiometry.     

Cusp-electrons used for velocity measurements of heavy ion projectiles

A new method for precise velocity measurements (10⁻⁴) for heavy ions is presented. Electrons at projectile velocity have been measured at an observation angle of zero degree for 1.4 MeV/A U³³⁺ traversing thin gas and carbon solid targets. The velocity distribution of the electrons corresponds to the energy and angular distributions of the projectiles which is clearly demonstrated by energy loss measurements in thin carbon foils.     

Destruction of CO ice and formation of new molecules by irradiation with 28 keV O ions

The effect of solar wind on cometary ice was studied by using oxygen ions with energy near to that corresponding to their maximum abundance in space for bombarding CO ice. This gas was condensed on a CsI substrate at 14 K and irradiated by 28 keV ¹⁸O⁶⁺ ions up to a final fluence of 1.3 × 10¹⁶ cm⁻². We have used a methodology in which the sputtering yields, the destruction rate of CO, and the rate of formation of new molecular species are determined by Fourier transform infrared spectroscopy (FTIR). In the current experiment, the condensation of a thin water ice film has prevented the CO sputtering. Quantities such as the dissociation yield, Y_d (the number of ice molecules destroyed or dissociated per projectile impact), and the formation yield, Y_f (the number of daughter molecules of a given species formed per projectile) are found to be more appropriate and useful than using an integrated or average cross section, since the projectiles are slowing down in the ice from their initial energy until zero velocity (implantation).     

Emission of large molecules from methane by ion bombardment

Condensed layers ot methane at 20 K have been bombarded by 6–8 keV Ar⁺, He⁺ and H₂⁺ ions. Mass spectra and Kinetic energy distributions of neutral species sputtered from these layers have been measured. We have found sputtered species with masses up to 72 amu and thus with at least 5 carbon atoms. In addition to this an involatile residue was formed. Analysis by pyrolysis mass spectrometry showed this residue to contain species with masses up to at least 170 amu which therefore contain at least 12 carbon atoms. The kinetic energy distributions of sputtered methane molecules lie between those of a Maxwell-Boltzmann distribution and a collision cascade. Higher values are reached for Ar⁺ than for the light ions. From these observations we conclude: for both light and heavy ions radicals are formed, which combine to new molecules. These exothermic reactions produce heat which causes desorption. The high energy tail for bombardment with argon ions shows that part of the sputtering is caused by momentum transfer.     

Photon emission produced by particle-surface collisions

Visible, ultraviolet and infrared optical emission results from low-energy (20 eV–10 keV) particle-surface collisions. Several distinct kinds of collision induced optical radiation are discussed which provide fundamental information on particle-solid collision processes. Line radiation arises from excited states of sputtered surface constituents and backscattered beam particles. This radiation uniquely identifies the quantum state of sputtered or reflected particles, provides a method for identifying neutral atoms sputtered from the surface and serves as the basis for a sensitive surface analysis technique. Broadband radiation from the bulk of the solid is attributed to the transfer of projectile energy to the electrons in the solid. Continuum emission observed well in front of transition metal targets is believed to arise from excited atom clusters (diatomic, triatomic etc.) ejected from the solid in the sputtering process. Application of sputtered atom optical radiation for surface and depth profile analysis is demonstrated for the case of submonolayer quantities of chromium on silicon and aluminium implanted in SiO₂.     

Sputtering of thin benzene films by large noble gas clusters

Molecular dynamics computer simulations have been employed to investigate the sputtering process of a benzene (C₆H₆) monolayer deposited on Ag{1 1 1} induced by an impact of slow clusters composed of large number of noble gas atoms. The sputtering yield, surface modifications, and the kinetic energy distributions of ejected species have been analyzed as a function of the cluster size and the binding energy of benzene to the Ag substrate. It is shown that high- and low-energy components can be identified in the kinetic energy distributions of ejected molecules. The mechanistic analysis of calculated trajectories reveals that high-energy molecules are emitted by direct interaction with projectile atoms that are backreflected from the metal substrate. Most of the molecules are ejected by this process. Low-energy molecules are predominantly emitted by a recovering action of the substrate deformed by the impact of a massive cluster. The increase of the binding energy leads to attenuation of both high- and low-energy ejection channels. However, low-energy ejection is particularly sensitive to the variation of this parameter. The area of the molecular overlayer sputtered by the projectile impact is large and increases with the cluster size and the kinetic energy of the projectile. Also the size and the shape of this area are sensitive to the changes of the binding energy. The radius of the sputtered region decreases, and its shape changes from almost circular to a ring-like zone when the binding energy is increased. Some predictions about the perspectives of the application of large clusters in the organic secondary ion mass spectrometry are discussed.     

Proton sputtering from polycrystalline Al surface interacting with highly charged ions at grazing incidence angle

Sputtering processes of protons from a polycrystalline Al surface interacting with Ar^q+ (q = 3-14) ions at a grazing incidence angle (∼0.5°) were investigated. The intensity of protons (I_H) detected in coincidence with scattered Ar atoms was measured as a function of q. I_H saturated at q ? 10, although it increased rapidly with q at 3 ? q ? 8. The angular distribution of protons with low kinetic energy (?2 eV) began to deviate from the cosine distribution and assumed a rather flat equidistribution as q increased. To analyze the sputtering processes of protons at the grazing incidence angle, a modified model of the “above-surface potential sputtering model” was proposed by considering image acceleration of projectile ions.