Stability of subatomic carbon films on metal surfaces against low-energy gas ion bombardment

The earlier developed original experimental technique for measuring and analyzing the parameters of low-frequency fluctuations of the field-emission current in metal film systems is used to measure the sputtering yield Y _f of carbon films (with a coverage Θ ranging from 1 to 4) applied on Fe, Nb, Ta, and U substrates. The value of Y _f is calculated by an expression derived within a theoretical model developed. The sputtering ratios were measured for the case when the carbon films are sputtered by H⁺ and He⁺ ions with an energy E _i between 2 and 10 keV. With Θ fixed, the energy dependences of Y _f are obtained for each of the ions. In addition, for each of the ions, the Θ dependences of Y _f are found for several values of E _i. In all the cases, the measured values of Y _f far exceed those for pure carbon. With another original technique that combines field-ion microscopy (FIM) and precise measurement of current and/or luminous properties of local regions in FIM images, the energy thresholds E _th of sputtering carbon films applied on the metal surfaces are found. The energy distributions of Y _f in the near-threshold energy range for various Θ are obtained.     

Electron and ion emission from liquid metal surfaces

The application of positive or negative electric fields to small-area liquid metal surfaces leads to very high brightness DC ion or pulsed electron emission. The stabilization of a cone-shaped structure by the balance of electrostatic and surface tensions forces is described. Electron and ion emission occurs by field emission and field evaporation mechanisms     

Secondary ion emission from Si under nitrogen ion bombardment

The yield and energy distribution of positive secondary ions emitted from Si under N₂⁺ ion bombardment were measured. The obtained mass peaks correspond to three types of secondary ion species, that is, physically sputtered ions (Si⁺, Si₂⁺), chemically sputtered ions (SiN⁺ Si₂N⁺) and doubly charged ions (Si²⁺). The dependence of secondary ion emission on the primary ion energy was studied in a range of 2.0–20.0 keV. The yields of physically and chemically sputtered ions were almost independent of the primary ion energy. The yield of the doubly charged ion strongly depended on the primary ion energy. The energy distribution of secondary ions of the three types showed the same dependence on the primary ion energy. The most probable energy of the distribution increased with the primary ion energy. On the other hand, for the energy distribution curves of sputtered ions, the tail factors N in E^?N were constant and showed a m/e dependence.     

Mechanisms of oxidation-reduction processes on metal and oxide surfaces under ion bombardment

Surface oxidation occurs if metals are bombarded with low-energy (1–5 keV) ions of a chemically active gas (oxygen) in vacuum. It is ascertained that ion bombardment leads to the generation of lower, intermediate, and higher oxides. The composition and thickness of an oxidized layer depend on the metal reactivity and the dose and energy of oxygen ions. The mechanism underlying the ion-beam oxidation of metal surfaces is proposed. Surface reduction is observed if higher oxides are bombarded with low-energy (1–5 keV) ions of inert gases (argon and helium) in vacuum. It is revealed that ion bombardment not only generates intermediate and higher oxides but sometimes gives rise to surface metallization. The composition and thickness of the reduced layer are determined by the oxide type, the kind of inert gases, and the dose and energy of bombardment. The mechanism describing the ion-beam reduction of higher metal oxide surfaces is proposed.     

Low-energy ion reflection from metal surfaces

A review is given of the today available adequate information in the field of low-energy (? 30keV) ion reflection from metal surfaces. Considerable attention is paid to the so-called binary collision approach and its ranges of validity. In addition, the different existing models describing the scattering from a surface, and recent results of experimental verification of them are discussed.     

Multiatomic clusters emerging from a metal surface under ion bombardment

[Ag _n ]^? ions emerging from a silver target under bombardment with energetic krypton ions have been measured forn ranging up to 60. The relative intensities of ions with odd numbers are strikingly favoured. The numbersn=8 and 20 mark minima in the abundance of ions with even numbers. The observations are briefly discussed.     

Cluster emission under ion bombardment of metallic targets

Mass spectroscopic studies of the neutral particles sputtered by Ar⁺ ions at 8 keV from polycrystaline samples have been performed, using non-resonant laser ionization and subsequent time-of-flight mass spectroscopy. Besides sputtered atoms, also dimer and trimer contributions in the order of 10^–1 to 10^–2 and 10^–3 to 10^–4, respectively, are found in the sputtered flux. The data obtained here together with previously published data by other groups for different bombarding energies provide strong support for the validity of the recombination model.     

Electron emission from crystal surfaces of condensed aromatics under the impact of metastable rare gas atoms

Energy distributions of electrons ejected from polycrystalline surfaces of naphthacene, perylene and coronene by the impact of metastable He ^*, Ne ^* and Ar ^* atoms have been measured. Two types of peaks, which are similar to the “non-moving” and “moving” structures in photoelectron spectra, are observed in each spectrum. The non-moving structures (ca. 1 eV) for perylene and coronene are similar to those in the photoelectron spectra, whereas the relative intensities of the two non-moving structures for naphthacene (0.6 and 1.7 eV) are remarkably different from the corresponding structures in the photoelectron spectrum. The peak positions (but not necessarily their intensities and widths) for the moving structures for all the samples (> 2 eV) agree with those of the corresponding photoelectron spectra. The origin of these moving structures is ascribed to Penning ionization on the solid surface.     

Mechanism of negative ion emission from surfaces of ferroelectrics

Experimental studies of the mechanism of negative ion and cluster ion emission from surfaces of ferroelectrics are described. The emission was produced by negative voltage pulses with the amplitude of about 400 V, with a rapid rise-time (below 10 ns) and a slow decay‐time (several μs). The pulses were applied between the back side of the ferroelectric sample and the metal tip touching the front emitting side. The surface of the ferroelectrics could be cleaned in situ by 1 keV Ar⁺ bombardment. The morphologic changes around the tip were observed with an atomic force microscope (AFM). Mostly negative ions and cluster ions were emitted and studied in our experiments. Positive ions were detected with much lower probability and are produced by an entirely different microscopic process than negative ions. Masses as well as energies of emitted ions were measured with a time-of-flight (TOF) spectrometer and compared with available spontaneous desorption (SD) spectra and Cs‐SIMS spectra in order to clarify the mechanism of the emission. The trajectory of ions emitted from the sample was studied by computer simulation. The conclusion of these studies is that the negative ion emission is caused by the Coulomb explosion of a polarization cloud rapidly formed at the front edge of the pulse. The explosion takes place in the vicinity of the tip-sample contact at distances several tens of μm from the contact where the stabilizing effect of the positively charged tip is already small.     

Photon studies of ion induced Ta continuum emission

Photon emission from Ta metal bombarded with 55 keV Ar⁺ is investigated under both ultra high vacuum and oxygen rich environments. It is shown that oxygen is necessary to produce broad band continuum emission and that Ta oxide molecules are most probably the origin of this emission.     

Interaction of rare gas atoms with metal surfaces: A pseudopotential approach

The pseudopotential formulation of the helium-metal surface interaction due to Harris and Liebsch has been generalized to treat the heavier rare gases. The atom-surface repulsion is calculated to first-order in the pseudopotential and is combined with a damped van der Waals interaction to produce the total physisorption potential. Reasonable results are obtained for all the rare gases considered, with a systematically increasing potential-well depth through the rare gas sequence. A single surface-atom model is also considered and its limitations discussed.     

Direct comparison of ion scattering and secondary ion emission as tools for analysis of metal surfaces

In this work an experiment is described which allows in situ comparison of SIMS (Secondary Ion Mass Spectrometry) and ISS (Ion Scattering Spectrometry). Measurements on Cu and stainless stell surfaces show that in some respects qualitative agreement between both methods exists. In both cases quantitative surface composition analysis is hampered by the lack of knowledge of secondary ion yields. Especially in case of SIMS the adsorbed species like oxygen have a great influence on the ion yield.     

Auger electron emission by ion bombardment of light metals

The energy distribution of secondary electrons ejected from a metallic target upon ion bombardment (Ar⁺, 10 keV) is studied with a 180° magnetic analyser, particularly in the vicinity of the Auger peaks that appear in the electron spectra of the light metals (Li, Be; Na, Mg, Al, Si). A qualitative explanation based on the correlation diagrams which described the evolution of the electronic levels during the collision of two atoms inside the target is proposed to interpret the variation of the Auger peak intensity with the atomic number of the target.     

Inelastic processes in the scattering of metastable rare gas atoms at metal surfaces

Electron spectra of various metastable rare gas atoms systematically measured on a Pt(111) surface with Rb coverages ranging from submonolayers (3%) to multilayers are presented. The different decay channels of the excited particles are discussed in terms of resonant electron exchange processes between the substrate and the projectile in relation to the work function. It is shown that below a certain value of the work function a highly excited negative rare gas atom is formed which can undergo different de-excitation processes. A careful discussion of the branching ratios into the decay channels offers a natural explanation of the variations in the electron spectra induced by alkali metal adsorption. Additionally, an attempt is made to extract information about the alkali metal chemisorption state from the observed electron spectra.     

Pressure induced friction collapse of rare gas boundary layers sliding over metal surfaces

In this Letter we show that friction of anticorrugating systems can be dramatically decreased by applying an external load. The counterintuitive behavior that deviates from the macroscopic Amonton law is dictated by quantum mechanical effects that induce a transformation from anticorrugation to corrugation in the near-surface region. We describe the load-driven modifications occurring in the potential energy surface of different rare gas-metal adsorbate systems, namely, Ar, Kr, Xe on Cu(111), and Xe on Ag(111), and we calculate the consequent friction drop for the commensurate Xe/Cu system by means of combined ab initio and classical molecular dynamics simulations.