Molecular secondary ion formation under cluster bombardment: A fundamental review

A brief review is given regarding the application of cluster ion beams as desorption probes in molecular SIMS. The general observation is that the efficiency of secondary ion formation, particularly that of complex molecular species, is significantly enhanced if polyatomic projectiles are employed instead of atomic species. Apart from the sensitivity increase, cluster bombardment also appears to allow for molecular depth profiling studies without the accompanying damage accumulation normally associated with atomic projectiles. A few fundamental aspects are addressed in an attempt to highlight the physics behind these observations. It appears that much of the benefit associated with cluster bombardment is connected to the fact that these projectiles give access to very high sputter yields which are not accessible with atomic primary ions.     

Estimation of ion scattering by atomic chains on single crystal surfaces

Estimation of ion movement close to atomic chains taking into account the elastic and inelastic energy losses and vacancy type imperfections was performed on the basis of a model of binary interactions of ions with the atoms of a crystal.

An analysis of results shows that at scattering by atomic chains a phenomenon which makes a substantial contribution to scattering by the single crystal surface the inelastic energy losses exceed the single scattering ones by a factor of 5–6.     

Analysis of cardiac tissue by gold cluster ion bombardment

Specific molecules in cardiac tissue of spontaneously hypertensive rats are studied by using time-of-flight secondary ion mass spectrometry (TOF-SIMS). The investigation determines phospholipids, cholesterol, fatty acids and their fragments in the cardiac tissue, with special focus on cardiolipin. Cardiolipin is a unique phospholipid typical for cardiomyocyte mitochondrial membrane and its decrease is involved in pathologic conditions. In the positive polarity, the fragments of phosphatydilcholine are observed in the mass region of 700-850 u. Peaks over mass 1400 u correspond to intact and cationized molecules of cardiolipin. In animal tissue, cardiolipin contains of almost exclusively 18 carbon fatty acids, mostly linoleic acid. Linoleic acid at 279 u, other fatty acids, and phosphatidylglycerol fragments, as precursors of cardiolipin synthesis, are identified in the negative polarity. These data demonstrate that SIMS technique along with Au₃⁺ cluster primary ion beam is a good tool for detection of higher mass biomolecules providing approximately 10 times higher yield in comparison with Au⁺.     

Low energy ion impact phenomena on single crystal surfaces

The dynamics of a solid bombarded by a 600 eV Ar⁺ ion have been studied classically by computer simulation. The model uses a crystallite of about 250 atoms described by pair potentials derived from elastic constants and which reproduce the surface binding energy of the solid. The relative calculated yield of secondary atom emission from the three low index faces of Cu follow the previously determined experimental order (111) > (100) > (110). We find major differences in the sputtering mechanisms for these faces. On (110), the impacted atom is ejected most frequently, while on (111) and (100) it almost never leaves the solid. We report the energy distribution of the sputtered particles for each face. The simulation successfully predicts the shape of the curve including the low energy maximum which is observed experimentally near 2 eV. In addition our model shows that many low energy atoms attempt to leave the crystal but are subsequently trapped to the solid at large distances from their original sites. This mechanism of radiation enhanced diffusion inevitably occurs in conjunction with sputtering or any other heavy secondary particle emission or scattering process.     

Molecular dynamics simulation of defect formation in an aluminum crystal under low-energy ion bombardment

Atomic collision cascades initiated by Ar and Xe ions (with energies of 25, 40, and 50 eV) normally incident on the Al(100) crystal surface at a crystal temperature of 300 K have been simulated by the molecular dynamics technique. The formation of vacancies and radiation-adsorbed and interstitial atoms in a cascade is discussed. It is demonstrated that the numbers of surface and bulk vacancies formed in cascades under bombardment of the Al(100) surface by Xe ions reach two maxima within 0.2–0.3 and 0.7–1.0 ps after the cascade initiation, whereas the number of vacancies generated under bombardment by Ar ions reaches one maximum within 0.2–0.3 ps after the cascade initiation.     

Spontaneous pattern formation induced by ion bombardment of binary compounds

A theory is developed that explains the genesis of the strikingly regular hexagonal arrays of nanoscale mounds that can form when a flat surface of a binary compound is subjected to normal-incidence ion bombardment. We find that the species with the higher sputter yield is concentrated at the peaks of the nanodots and that hysteretic switching between the flat and the hexagonally ordered state can occur as the sample temperature is varied. Surface ripples are predicted to emerge for a certain range of the parameters.     

Slow ion scattering by crystal surfaces and nanostructures

This review is dedicated to considering issues of low-energy ion scattering by solid surfaces. In addition to general issues, original results obtained by researchers at the department of physical electronics (Faculty of Physics, Moscow State University) are considered. They have initiated new directions in the study of slow ion scattering. Attention is focused on experimental data and the results of computer simulation. The section dedicated to discussing the charge exchange between the scattered particles and the surface, and nanostructures is an important part of this review.     

Randomisation of crystal structure within energetic collision cascades revealed by the sputtering of Au single crystals during ion bombardment

The ratio of random to preferential atomic ejection which occurs from the surface of a gold single crystal during ion bombardment has been found to be very sensitive to the ion mass. For instance, in the case of 50 keV Ne⁺ ion bombardment preferential ejection dominates, whereas in the case of 50 keV Au⁺ ion bombardment preferential ejection is only a small fraction of the total. These results are interpreted in terms of a dynamic randomisation of the crystal lattice which occurs during the creation of energetic atomic collision cascades. The disorder occurs so rapidly that a substantial fraction of atoms sputtered from the surface are ejected from an essentially random structure. It is thought that such gross disorder is only a transient phenomenon, and leaves no significant permanent effects, except perhaps for a few clustered defects.     

Fundamental studies of the cluster ion bombardment of water ice

The fundamental sputtering properties of water ice are of interest for molecular depth profiling of biological samples in their native environment. We report on a method of studying amorphous water ice films of precise thicknesses in which pure water vapor is condensed onto a pre-cooled, silver-coated quartz crystal microbalance (QCM). This scheme allows for the determination of water ice sputter yields for any primary projectile as well as providing a means for studying escape depths of atoms and molecules beneath the deposited water ice layer. Specifically, we find a removal of approximately 2500 water molecule equivalents/20 keV C₆₀⁺ projectile with an underlying silver ion escape depth of 7.0 Å.     

Photon emission from rare gas ion bombardment of metal surfaces

Metal surfaces (Mg, Cu, Zr, Mo) are bombarded with He⁺, Ne⁺ and Ar⁺ in the energy range of 400 eV to 8 keV. Radiation from scattered projectiles and sputtered target particles is observed between 200 and 700 nm. It is shown that most of the radiating particles originate from surface collisions. Auger neutralization, resonance tunneling and direct electron transitions are the important electronic processes involved.     

Evolution of pattern formation under ion bombardment of substrate

Processes of self-organization described by a nonlinear evolution equation of sixth order are considered on substrate surfaces after ion beam bombardment. Five families of exact solutions are found for the nonlinear evolution equation. A numerical method enabling one to solve the boundary value problem is presented. Simulation of the pattern formation and classification of their profiles are given. The influence of the third, fifth order dispersion terms and a nonlinear diffusion term on processes of the pattern formation is discussed.     

Simulation of fractal cluster growth on a substrate under ion bombardment

Ion-stimulated growth of a fractal cluster on the substrate is simulated in regimes that are intermediate between diffusion- and reaction-limited aggregations. It is shown that the fractal dimension of a growing cluster and the type of formed epitaxial structure are due to the sequence of ion-stimulated processes with different fractal dimensions. The algorithm for revealing the ion stimulation mechanism of processes based on fractal laws is proposed.     

Copper monolayer formation on platinum single crystal surfaces: Optical and electrochemical studies

The formation of a Cu monolayer on Pt(100), (110) and (111) was investigated by optical and electrochemical techniques. The adsorption isotherms as obtained by cyclic current-potential curves clearly show that the monolayer is deposited in various steps at underpotentials. Differential reflectance spectroscopy at normal incidence was used to detect structural changes of the adsorbate as the coverage increases. For Cu on Pt(110) a pronounced anisotropy in $\text{ΔR}\text{R}$was observed as the electric field vector of the linearly polarized light was rotated. From these measurements it was deduced that Cu in the submonolayer range is deposited onto Pt(110) in rows along the [11̄0] direction of the substrate. No such anisotropy was found for Cu on Pt(100) and Pt(111) and for surface oxide formation on all three low index faces of Pt. The spectral dependence of the normalized reflectance change,$\text{ΔR}\text{R}$, for the Cu monolayer on Pt(hkl) is shown and discussed.     

Sodium ion conduction in single crystal vermiculite

The diffusion of sodium in single crystals of Llano vermiculite has been studied using conductivity measurements. The crystals were maintained in the bilayer water state by using aqueous contacts. The diffusion coefficient at 25°C was found to be 1.9×10⁻⁸ cm²/s, with an enthalpy of motion of 11.7 kcals/mole. These values are in good agreement with sodium tracer and proton NMR studies and indicate that the sodium ions probably diffuse with their hydration spheres. The conductivity in the range 10 to 90°C is less than that in sodium beta alumina and much less than that of either surface clay cations or of aqueous sodium chloride solutions.