The spatial distributions of excited secondary ions with certain energies formed under ion bombardment at various angles of the (111) face and amorphized silicon surface are obtained using the coincidence method. Calculations are carried out for comparison with the experimental results. A comparison of the spatial distributions obtained by the simulation and in experiments on the sputtering and secondary-ion emission from the (001) and (111) faces of a Ni4Mo alloy single crystal is presented. The obtained results offer a prospect for application of the coincidence method for studying the electronic properties of surfaces.

A unique experimental set-up for coincidence measurements is used for the detection of simultaneous emission of two particles, such as an ion and a photon, emitted by this ion immediately after its ejection from the solid. This set-up was also used to measure spatial and energy distributions of secondary excited ions. Quasi-resonant electron exchange between secondary ions and the surface was observed, and a correlation between the surface electronic structure and the yield of secondary ions with a particular energy was discovered. Some of the measured energy spectra of excited secondary ions showed oscillatory behaviour. The obtained results open up the prospect for the use of the coincidence technique to investigate electronic properties of surfaces. The aim of this review is to describe the results of our pioneering use of the coincidence method to study secondary ion, ion-electron and ion-photon emission.

The effect of atomic ordering on the angular distribution of the sputtering of the (001) face of a Ni4Mo single crystal irradiated with 10-keV Ar ions is studied using molecular-dynamics simulation and experimentally. It is shown that, in the case of a disordered crystal, both the Ni and Mo components are ejected in the same crystallographic direction. In the case of an ordered crystal, Mo emission is observed in the same directions, and the Ni emission deviates from them. The results are compared with data obtained for secondary ion emission from the Ni4Mo (001) face.

The (001) crystal surface of RbTiOPO4 (RTP) were bombarded by Ar+ ion during different times (2–20 min) and the resulting chemical modification on surface has been studied by X-ray photoelectron spectroscopy (XPS) and first-principles calculations. Ion bombardment can lead to great structural modification on the surface of RTP crystal. Reduction of Ti4+ ions to the lower valence state (Ti3+) and formation of oxygen vacancies occur with Ar+ ion irradiation. With the increase of irradiation time, proportion of reduced Ti3+ ions and number of oxygen vacancies increase accordingly. Fraction of oxygen in P-O-Ti bonds decreases with increasing oxygen vacancies after bombardment for 5–20 min. We think that oxygen vacancies come from breaking of P-O-Ti bonds. The results were also confirmed theoretically by first-principles calculations. Existence of oxygen vacancies is also confirmed by absorption spectra. Formation of Ti3+ state can be ascribed to the loss of surrounding oxygen caused by Ar+ ion irradiation. This work helps us to better understand the ion-beam interaction with RTP surfaces.

The dependence of secondary electron emission coefficient σ on the angle α of primary electron incidence onto single crystals of metals with different crystalline lattice has been studied for undisturbed surface and for disturbed one by sputtering. We used the single crystals of Cu (fcc), Mo (bcc), Zn (hcp) and Ni4Mo (tetragonal lattice). It was shown that the coefficient σ is smaller for the disturbed surface, than for initial one due to absorption of secondary and scattered electrons by the lateral surfaces of hills and cones which are formed as a result of sputtering. For the initial surfaces (of Cu, Mo and Ni4Mo) the maxima of σ(α) in the low-index directions of the crystal lattice arise as a result of primary and secondary electron scattering on the atoms in open channels. At the same time, for the preliminary highly oxidized single crystal surface (of Zn) the minimum of σ in the direction of open channel was observed. The last can be explained by a reduction of work function of surface, and increase in penetration depth of electrons in open channel and by a rise of electron-phonon interaction. Angular dependences of secondary electron emission for a sputtered surface have a more complicated structure with the additional maxima and minima caused by interaction of secondary and scattered electrons with a cone-shaped relief.

Secondary ion emission from sapphire single crystal has been studied experimentally and by means of computer simulation. The particular oscillations of secondary ion energy spectra and two specific maxima of O+ and Al+ ions were observed under irradiation of (0001) sapphire face by 1 and 10keV Ar+ ions. We have explained this by the interplay of the charge exchange processes between moving particles and solids. The existence of two maxima in energy spectra of O+ and Al+ secondary ions can be also connected with special features of single-crystal sputtering: the low-energy peak can be formed by random sputtering and the high-energy peak from focusing collisions. In addition some similarity was found between the positions of low-energy maximum in energy spectra of Al+ ions emitted from sapphire and the principal maxima of Al+ ions ejected from the aluminum single crystal. This indicates a possibility to explain the presence of low-energy maximum in energy spectra of secondary ions ejecting from sapphire by emission of Al+ ions from aluminum islands appearing in a number of cases on the sapphire surface due to preferential sputtering of oxygen. These different mechanisms of creating the energy spectra of ions emitted from sapphire should be taken in account.

A sensitive method of visualization by sputtering of deformed areas buried under a surface is developed. The effect of varying the lattice constant and binding-energy of surface atoms of binary disordered alloys on the sputtering of areas deformed by a concentrated load was examined using molecular dynamics. The calculation was carried out for Cu87Sn13, Ni97Fe3 and Fe97Ni3 alloys, which are used for manufacturing of coins and items with marking signs. Experimentally and by MD simulations of energy and angular distributions of sputtered particles, we have shown that the best conditions for detection of deformed areas, buried under a surface, occur in the case of irradiation by inert gas ions of sufficiently large mass (Ar+, Kr+) with energy of 7–10 keV, when the sputtering yield is close to its maximum value. We suggest using an inclined ion beam, near the maximum of the angular dependence of the sputtering coefficient. This leads to reduction of the time necessary for ion irradiation, and to an increase in clarity of the obtained image due to a cumulative effect in the narrow near-surface layers of the target. Using the recommended parameters of sputtering has allowed us to reveal a destroyed image on a bronze coin and images of completely ground off marking numbers on steel objects.

Kinetic characteristics of sapphire (α-Al2O3) single crystal irradiated by keV electrons are studied. We compare untreated sapphire with surfaces bombarded with Ar+ ions and/or coated with a thin Al film. The electron emission yield and the charging characteristics for the untreated sapphire surface and for that bombarded by Ar+ ions differ significantly. This difference is less when the sapphire sample is coated with a thin film of Al. We propose an explanation for the observed effects.

The difference between the forms and positions of the maximum of the energy spectrum of secondary 63Cu+ ions emitted from the (100) face of a Cu single crystal in close-packed <110> directions and between them at different target temperatures is revealed. For a sample at room temperature, it is established that the most probable ion energy E max and the width h of the energy spectra at half-height increase with the polar emission angle, θ, measured from the normal to the surface. The opposite tendency, namely, a decrease in E max with increasing θ, is obtained for ions emitted from a single crystal heated to several hundred degrees Celsius. Based on modern models of secondary ion emission, the observed regularities, which must be taken into account in the theoretical description of the process and in the practical application of secondary ions in mass spectroscopy, are discussed.

We have studied the spatial distribution of particles sputtered from the base (0001) plane of a sapphire single crystal with trigonal crystalline lattice (α-Al2O3) that can be considered a superposition of two hexagonal close packed (hcp) structures–the ideal sublattice of oxygen and a somewhat deformed sublattice of aluminum. It is established that the particles sputtered from the base plane of sapphire are predominantly deposited along the sides of an irregular hexagon with spots at its vertices. The patterns of spots have been also studied for sputtering of particles from the (0001) face of a zinc single crystal with the hcp lattice. The spots of sputtered Zn atoms are arranged at the vertices of concentric equilateral hexagons. In both cases, the observed anisotropy of sputtering is related to focused collisions (direct and assisted focusing) and the channeling process. The chemical composition of spots has been determined in various regions of sputtered sapphire deposition. The results are discussed in comparison to analogous earlier data for secondary ion emission from an α-Al2O3 single crystal.

Secondary ion emission from silicon and graphite single crystals bombarded by argon ions with energies E 0 varied from 1 to 10 keV at various angles of incidence α has been studied. The evolution of the energy spectra of C+ and Si+ secondary ions has been traced in which the positions of maxima (E max) shift toward higher secondary-ion energies E 1 with increasing polar emission angle θ (measured from the normal to the sample surface). The opposite trend has been observed for ions emitted from single crystals heated to several hundred degrees Centigrade; the E max values initially remain unchanged and then shift toward lower energies E 1 with increasing angle θ. It is established that the magnitude and position of a peak in the energy spectrum of secondary C+ ions is virtually independent of E 0, angle α, and the surface relief of the sample (in the E 0 and α intervals studied). Unusual oscillating energy distributions are discussed, which have been observed for secondary ions emitted from silicon (111) and layered graphite (0001) faces. Numerical simulations of secondary ion sputtering and charge exchange have been performed. A comparison of the measured and calculated data for graphite crystals has shown that C+ ions are formed as a result of charge exchange between secondary ions and bombarding Ar+ ions, which takes place both outside and inside the target. This substantially differs from the ion sputtering process in metals and must be taken into account when analyzing secondary ion emission mechanisms and in practical applications of secondary-ion mass spectrometry.

The change in the structure and composition of deformed regions hidden beneath the surface of samples are studied experimentally upon their identification by the method of successive processes of polishing and sputtering. Two present-day steel coins coated with different binary alloys, a 1-ruble coin (coated with the alloy Ni1%Fe) and a 50-kopeck coin (coated with the tompak alloy Cu10%Zn), and also a 5-kopeck coin minted in 1990 consisting of brass L60 are used as the samples. It is found that a change in the surface structure (different for the studied coins) and an increase in the light-component concentration take place in the region of increased deformation by pressing due to its diffusion to the side of greater deformation. The obtained results can be used for improving the means of determining hidden deformed regions using sputtering and stage-like analysis of the surface composition.

Secondary ion emission from an ion-bombarded binary compound in the ferro- and paramagnetic states has been studied using experimental methods and molecular dynamics simulations. The experiments were performed with a widely used NiPd binary compound, which was bombarded by obliquely incident 10-keV Ar ions. It is established that the intensity of Ni+ and Pd+ ion emission from a polycrystalline NiPd sample decreases significantly when it passes from the ferromagnetic to paramagnetic state. This effect is explained by a change in the surface binding energy and density of surface states at the Fermi level and by a cumulative process related to sputtering. The energies and directions of emission of secondary particles from poly- and single-crystalline NiPd samples in the ferromagnetic state have been jointly studied. It is established that the maximum of the polar angular distribution of secondary particles deviates toward the normal to the irradiated surface with a decrease in their energy. A difference in the azimuthal distribution of emitted Ni and Pd particles has been observed during the bombardment of a (001)NiPd crystal face and explained by specific features of correlated collisions. The best agreement of experimental data with the results of molecular dynamics simulations is obtained if the calculations are performed with allowance for an experimentally established modified composition of the three uppermost surface layers [39].

The influence of changes in lattice constant and binding energy of surface atoms of binary alloys on sputtering of deformed areas has been examined by a molecular-dynamic method. As a target we chose materials, which are used for coin manufacturing: tin bronze Cu87Sn and Ni–Fe alloys with different component concentration. Energy and spatial distributions of sputtered atoms before and after deformation are studied for mono- and polycrystals by considering segregation and heating of targets. The results obtained can be used for improving procedures for detecting deformed areas hidden under the surface of different products.

Sputtering and secondary ion emission (SIE) from semiconductors with different values of the band gap have been studied experimentally and by means of computer simulation. The particular oscillations of secondary ion energy distribution for graphite, sapphire, and silicon single crystals were observed under irradiation by 1 and 10 keV Ar+ ions. This result cannot be interpreted using existing theories of SIE. We have explained them by the interplay of the charge-exchange processes between emitted and incident particles. Comparing the evidence from our experiments and computer simulation, we have drawn the conclusion that secondary ions emerge as a result of charge exchange with Ar+ ions not only when exiting the target, but also in the bulk. This is very different from what happens in metals, and needs to be taken into account when describing the processes in SIE, as well as in practical applications in mass spectrometry of secondary ions. (c) 2013 Elsevier Ltd. All rights reserved.

The secondary emission of carbon atoms from the (0001) plane of graphite nanocrystallites bombarded with argon ions with energies of 1 and 10 keV and the incidence angle α = 45° is investigated. The unusual oscillating energy distributions of secondary C+ ions with main maxima E max in the range of 40–60 eV and peaks corresponding to the energies E 1 ≈ 20, 30, 70, 80, and 100 eV have been revealed. The C+ ion yield decreases, the energy spectrum increases, and the maximum E max shifts to larger energies E 1 with increasing emission angle (with respect to the normal to the surface). The secondary-ion emission from the (0001) face of graphite is numerically simulated with allowance for the charge exchange of secondary ions to obtain a qualitative explanation of the observed results.