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.     

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.     

Resonant mechanisms for negative ionization of secondary emitted atoms from sputtered metals

A model is presented to describe negative ionization of low energy, secondary atomic particles ejected from sputtered metal surfaces. Focus is made on the diatomic systems formed, in the collision cascade generated by the primary ion beam, between secondary emitted atoms and their nearest-neighbor substrate atoms that provide the initial impulse for ejection. Two different resonant ionization mechanisms are investigated such that a conduction electron may tunnel into the affinity orbital of the ejected atom either by direct hopping or after an intermediate transition via the affinity orbital of the substrate atom. A numerical method is outlined to calculate the negative ionization probability of secondary emitted atoms. A good agreement is found with van Der Heide’s measurements of the Cu⁻ population sputtered from a clean Cu-surface, at emission energies below 100 eV.     

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.     

Inelastic processes in ion scattering spectroscopy of solid surfaces

In ion scattering spectroscopy of solid surfaces, spectral peaks corresponding to single scattering do not always appear at a kinetic energy given by a well-known formula that is calculated for the elastic collision of a projectile ion with a free target atom. This is because (a) the target atom is actually not free but bound to surrounding atoms at the surfaces, (b) the target atom may be excited electronically, and (c) inelastic electron exchange between the projectile ion and the surface may occur. These three inelastic effects are discussed on the basis of experimental results in the low energy region. Auger and resonance electron exchange processes between the projectile ion (or atom) and the surface are also discussed.     

Sputtering of condensed noble gases by keV heavy ions

Bombardment of condensed Kr and Xe by 2–8 keV noble gas ions results in very high sputtering yields. A considerable fraction (10–30%) of the sputtered particles consists of Van der Waals clusters, with Kr₂, Kr₃, Xe₂, XeKr, XeKr₂ and ArKr having been observed. The kinetic energy distributions of the sputtered monomer-species are in agreement with a collision cascade mechanism. However, at the low energy side an excess yield is observed. This is explained by a model which takes into account the large sputtering yields and the damage of the surface during the sputtering process. The energy distributions of the dimers and trimers are satisfactorily explained by a statistical model. It is concluded that the dimer and trimer species are sputtered during an early stage of the cascade.     

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.     

Measurement of the ionization probability of the 1sσ molecular orbital in half a collision at zero impact parameter

We have measured, for the first time, the ionization probability P_1sσ of the 1sσ molecular orbital on the way into a nuclear reaction (in half a collision at zero impact parameter) in a near symmetric collision ⁵⁸ Ni+⁵⁴ Fe at 230 MeV which leads to a compound nucleus of ¹¹² Xe highly excited which decays first by sequential emission of charged particles and then by sequential emission of gamma rays. The determination of P_1sσ is based on the coincidence measurement between X-rays and γ-rays. The Doppler shift method is used to discriminate between the “atomic” and “nuclear” X-rays.     

An impulsive collision model for the vibrational and rotational excitation of sputtered molecules

An impulsive collision model describing the vibrational, rotational and translational energy of sputtered diatomic molecules is presented. A pre-existing molecule at the surface undergoes a single collision with a subsurface particle and is consequently ejected from the solid. Repulsive potential interactions and an impulsive approximation of the collision determine energy transfer to both atoms of the molecule and thus the partition over internal and kinetic motion. Model calculations of the vibrational, rotational and kinetic energy distributions yield a good agreement with recent experimental data of sputtered diatomic sulfur molecules. Almost all of the qualitative features are reproduced by the model calculations.     

Electron-impact excitation of argon: Optical emission cross sections in the range of 300-2500 nm

We present measurements of optical emission cross sections for excitation from the ground state of the Ar atom into over 185 excited atomic and ionic levels. Measurements were made at electron energies of 25, 50, and 100 eV, at a gas pressure of 5 mTorr. Due to radiation trapping of resonance levels, many of the cross sections depend on the target pressure. Detailed pressure dependence for over 50 levels is also provided. The energy dependence of the excitation cross sections for over 175 levels in the energy range of 0-250 eV are provided as fitted parameters for a standard analytical function.     

Influence of the projectile charge state on the ionization probability of sputtered particles

We present a computational study of the effect of the projectile charge state on secondary ion formation in sputtering. A molecular dynamics simulation of an atomic collision cascade is combined with a kinetic excitation model including electronic friction and electron promotion in close atomic collisions. The model is extended to account for potential excitation following the bombardment with a highly charged ion (HCI). The spatial spreading of the excitation generated in the cascade is treated in an diffusive approach. The excitation energy density profile obtained this way is parametrized via an effective electron temperature, which is then used to calculate the ionization probability of each sputtered atom in terms of a simple charge exchange model. The results obtained for the impact of a 5 keV Ag atom onto a solid silver surface show that the average ionization probability increases from 4.7×10^-4 for a neutral projectile to 5.4×10^-4 for a highly charged projectile ion with a total ionization energy of 576 eV.     

Mechanisms of Transient Radiation Effects

Transient radiation effects are defined to be manifestations of electrons excited in materials, i. e., ionization effects. Manifestations of particular interest are the emission of secondary electrons from surfaces, conduction in ionized gases, semiconductors and insulators, and the emission and absorption of optical radiation by excess electrons. In each case the processes can be described as a sequence leading from the primary interaction of the incident nuclear radiation with an atom in the target material through the slowing down of the electrons which are detached in this interaction to the eventual recapture or trapping of the free electrons. Most of the phenomena can be described in terms of a mean lifetime for the electrons and a mobility. Assuming reasonable values for these quantities, calculations can be performed to estimate the order of magnitude of transient effects in typical materials and electronic components.     

Charge transfer and excitation in high-energy ion-atom collisions

Coincidence measurements of charge transfer and simultaneous projectile electron excitation provide insight into correlated two-electron processes in energetic ion-atom collisions. Projectile excitation and electron capture can occur simultaneously in a collision of a highly charged ion with a target atom; this process is called resonant transfer and excitation (RTE). The intermediate excited state which is thus formed can subsequently decay by photon emission or by Auger-electron emission. Results are shown for RTE in both the K-shell of Ca ions and the L-shell of Nb ions, for simultaneous projectile electron loss and excitation, and for the effect of RTE on electron capture.     

The possible role of anisotropy in kinetic electronic excitation of solids by particle bombardment

The kinetic excitation of a solid surface by impact of energetic particles is investigated by means of internal electron emission across a buried metal-insulator-metal (MIM) tunnel junction. By bombarding the top metal surface of such a device with keV noble gas ions, internal emission yields were determined as a function of projectile impact energy and angle of incidence with respect to the surface normal. In order to understand the observed impact angle dependence, we apply a modified formalism originally published to describe external electron emission. As a result, we find that the measured data can be explained by assuming the spatial distribution of excited electrons propagating towards the buried oxide interface to be strongly influenced by the projectile impact angle. A simple ballistic model assuming excited electrons generated by direct collisions with the projectile to preferably propagate along the direction of the original projectile motion, while electrons excited by scattering from moving recoils propagate isotropically, appears to describe the observed experimental data quite well.     

Numerical analysis of highly efficient laser-based method of radioactive Cs isotope separation utilizing light-induced drift in D1 and D2 transitions in rare gases

We investigated the feasibility of separating radioactive Cs isotopes using the effects of light-induced drift (LID). LID is the massive flow of particles caused by the difference between the collision frequencies of the buffer particles in the optically connected ground and excited states. We numerically evaluated the LID velocities of Cs-133, Cs-135, and Cs-137 in rare gases based on the currently available experimental technologies. The calculations were performed utilizing the steady-state analytical model and assuming that all of the collisions were strong. The velocity-dependent collision cross-sections were estimated using the WKB approximation with the recently developed ab initio atom–atom interaction potentials. A maximum velocity of more than 10 m/s was obtained when He was utilized as the buffer gas, and the expected maximum annual throughput was approximately 600 g, which is sufficient for practical purposes. We also examined the negative effects that were neglected in the employed model.     

Interaction of energetic clusters (Au3, Au400 and C60) with organic material and adsorbed gold nanoparticles

Using molecular dynamics simulations (MD), this contribution compares the interaction of three energetic clusters (Au₃, Au₄₀₀ and C₆₀) with a hybrid surface of crystalline polyethylene (PE) covered by a layer of gold nanoparticles. This model system mimics the situation encountered in metal-assisted secondary ion mass spectrometry. The chosen impact points are representative of the PE surface, the metal particles and the frontier between the metal and the polymer. The simulations show the differences between the impact over the Au nanoparticle and the polymer surface, in terms of projectile penetration, crater formation and sputtering yield of PE and gold species. For C₆₀ and Au₃ projectiles, a simple correlation is found between the quantity of energy deposited in the top polymeric layers and the quantity of sputtered polymer material, including all the impact points. The results obtained with Au₄₀₀ do not fit on this line, indicating that other physical parameters are prevalent. The mechanistic view of the interaction provided by the MD helps explain the differences. In short, while C₆₀ and Au₃ quickly break apart, creating energetic recoils and severing many bonds in the surface, Au₄₀₀, with the largest total momentum by far (∼10 times larger than the others) and the lowest energy per atom (25 eV), tends to act and implant in the solid as a single entity, pushing the polymeric material downwards and breaking few bonds in the surface.     

MD simulation of cluster formation during sputtering

The cluster ejection due to cluster impact on a solid surface is studied through molecular dynamics (MD) simulations. Simulations are performed for Cu cluster impacts on the Cu(1 1 1) surface for cluster energy 100 eV/atom, and for clusters of 6, 13, 28 and 55 atoms. Interatomic interactions are described by the AMLJ–EAM potential. The vibration energy spectrum is independent of the incident cluster size and energy. This comes from the fact that sputtered clusters become stable through the successive fragmentation of nascent large sputtered clusters. The vibration energy spectra for large sputtered clusters have a peak, whose energy corresponds to the melting temperature of Cu. The exponent of the power-law fit of the abundance distribution and the total sputtering yield for the cluster impacts are higher than that for the monatomic ion impacts with the same total energy, where the exponent δ is given by Y_n∝n^δ and Y_n is the yield of sputtered n-atom cluster. The exponent δ follows a unified function of the total sputtering yield, which is a monotonic increase function, and it is nearly equal to δ −3 for larger yield.     

Sputtering of graphite in pulsed and continuous arc and spark discharges

The environment that leads to the sputtering of graphite electrodes and formation of carbonaceous discharge has been studied with emission spectroscopy. Population level densities, excitation & vibrational temperatures and electron densities have been obtained from a set of three ion sources. The sources operate in continuous and pulsed discharge modes. The sputtered species include monatomic, diatomic and higher carbon clusters. The main sputtered species are excited and ionized C₁ (CI, CII, respectively) and C₂. In the continuous arc discharge the vibrational temperature derived from the Swan band of C₂ is ∼10,000 K, whereas, in the pulsed arc the excitation temperature of Neon is ∼11,000 K. The spark discharge yields an average excitation temperature of CI and NI ∼ 5500 K.     

MD study on temperature dependence of sputtering yield

Temperature dependence of sputtering yield is studied through molecular dynamics (MD) simulation that is performed for Ag sputtered by 12.6 keV Ar impacting at normal incidence. The target temperature is considered from 300 to 1235 K. It is found that the target temperature has little effect on the monomer yield because it comes from the energetic collision cascade. On the other hand, the sputtered cluster yield increases with the target temperature. It seems that the sputtered cluster is produced due to the thermal spike near the surface and the thermal spike is strongly influenced by the target temperature.