The kinetics of field adsorption

In an applied field of ～ 1 V/Å or higher, gas atoms and molecules near the electrode surface can be attracted to the surface and be field adsorbed there. Field adsorption can occur at a temperature much higher than that in ordinary physical adsorption. For example, field adsorption of He and Ne on metal surfaces can occur at a temperature above 100 K. The kinetic of field adsorption on spherical, cylindrical and field ion emitter surfaces are discussed. Methods for measuring the field adsorption energy, the gas supply constant, and other physical parameters using the pulsed-laser time-of-flight atom-probe are described. These parameters are essential for describing the kinetics of field adsorption. We consider field adsorption and desorption of both atomic and molecular gases. In molecular gases, field dissociation of field desorbed species can occur. A proper account of this effect has to be included in the data analysis. A case with two adsorption states at different adsorption sites is also considered. Field adsorption must play an important role in the early stages of cloud formation, and in the adsorption-desorption and catalytical reactions on the surface of interstellar particles, etc.     

Field-Induced and Surface-Catalyzed Formation of N2H Ions

Using H₂-N₂ mixed gases, high-electric-field-induced formation of ions such as N₂ ⁺ and N₂H⁺ on a surface of gold has been investigated in the pulsed-laser time-of-flight atom-probe field ion microscope. The N₂H⁺ ions are found from the lattice steps of the surface in the range (30-40) V/nm. The formation of the ions depends not only on the applied field strength, but also on the atomic structure of the substrate. By applying theoretical values of the binding energy and ionization potentials of N₂H, as calculated by the ab initio method and using measurements of the energy distributions of the N₂ ⁺ and N₂H⁺ ions, several conclusions can be drawn.     

Chemical surface reactions in the presence of high electric fields

Since the invention of the field ion microscope (FIM) by E. W. Muller¹ and the first mass-spectrometric analysis of field ions by Inghram and Gomer,² these tools for field ionization have been developed considerably, as described in several monographs and summarizing artilces.^3–8 Field ion mass spectrometry (FIMS) can analyze solid surfaces with extreme sensitivity. The experimental facilities enable ultimately the identification of single surface particles, which can be correlated to individual surface sites within the atomic scale of a surface. The atom-probe, which was developed by E. W. Müller,⁹ can now analyze surface particles with high precision in mass resolution.     

Methanation on Rh surfaces at low pressure and low temperature: A pulsed-laser imaging atom-probe study

We have successfully detected methanation on Rh surfaces at low pressure (∼10⁻⁸ Torr) and low temperature (150 K) using pulsed-laser imaging atom-probe. Thus it is possible to observe directly the reaction intermediates in a time-of-flight mass spectrum and study the atomic steps of such reaction. We find direct evidence of dissociative chemisorption of CO on Rh surfaces and show that under the conditions of our experiment, a dissociative mechanism is responsible for methanation on Rh surfaces. We have also observed an oscillation in signal intensity of CO⁺ and that of methanation products with a time period of ∼ 8 s under the conditions of that experiment.     

Evidence of post field ionization and observation of novel features in the energy distribution of field evaporated ions

Ion energy distributions in low temperature field evaporation obtained by pulsed-laser time-of-flight atom-probe, in general, show a FWHM of the spatial zone of ion formation to be 0.3–0.5 Å; post field ionization is not responsible for the ion formation. However, when ions of two or more charge states coexist a low energy tail can be found for the higher charge state ions, similar to those found for gas ions in field ionization. This tail can extend as far as 10 Å above the surface. Ions in the tail can only be produced by post field ionization. Double peak structures are found in the energy distributions of some ion species such as Mo²⁺; the origin of which is not yet understood. At a constant rate of field evaporation, as the field is gradually reduced by continued field evaporation and the laser power density appropriately increased, the charge states shift to the lower ones. For Mo, however, Mo²⁺₂ ions are formed before Mo⁺ ions can be detected. The narrowness of the energy distributions shows that they are stable. This finding has an important implication to the study of the stability or Coulomb explosion of multiple charged cluster ions, and also the theory of field evaporation. Under intense laser irradiation, higher charge state ions can reappear and a large fraction of ions may have an excess energy of a few hundred eV. These may be produced by multiphoton ionization and also by interaction with the over-heated electrons in laser-solid interactions.     

Direct observation of surface atomic reconstruction by pulsed-laser heating

Surface atomic reconstruction of Pt and Ir surfaces, induced by pulsed-laser heating, can be directly observed in the field ion microscope. The (1 × 1) Pt and Ir (110) planes prepared by low temperature field evaporation can be reconstructed to the (1 × 2) structure with missing rows of atoms. The Ir (001) can also be reconstructed, but the structure is very complicated. Details of it have not yet been worked out.     

Surface studies with an imaging atom-probe

The imaging atom-probe (TAP) is an atom-probe field-ion microscope in which the conventional time-of-flight mass spectrometer is replaced by a nanosecond-gated channel-plate image intensifier which is used to record the arrival positions of ions of preselected mass-to-charge ratio which have been field-evaporated from the surface of a field-ion emitter. The evaporated metal ions have the same radial projection that is seen in the normal field-ion image and the recorded image displays the points of origin of the ions with a spatial resolution of 1 nm or better. The field of view covers a selected area of about 0.3 steradian at the curved specimen surface. The mass resolution, limited by the channel-plate gating time, is of order $\text{Δm}\text{m}\text{=}\text{1}\text{15}\text{to}\text{1}\text{25}$. The wide field of view allows the rapid determination of the crystallographic distribution of differently-charged ions from those elements which evaporate as a mixture of charge species, and shows that this distribution is frequently a sensitive function both of the local surface geometry and of the presence or absence of field-adsorbed helium or neon at the specimen surface. The wide field of view of the IAP gives it a considerable advantage over the conventional atom-probe in the detection of segregation at grain boundaries.     

ToF atom-probe fim investigation of surface segregation in dilute alloys

A time-of-flight atom-probe field ion microscope was successfully employed for the first time to investigate surface segregation in dilute alloys. We were able to achieve a single atomic layer resolution and obtained the absolute concentrations of alloy species of each surface layer. Cr atoms are found to segregate to the surface of Stainless Steel 410 whereas no segregation of the minority species was found for Pt-8% W and Pt-5% Ru. The first layer concentration of Cr in the {110} plane of Stainless Steel 410 at 500°C was found to be 38.5 ± 12.5% and 63.4 ± 10.2% respectively for samples with near surface layers Cr average concentration of 6.3 ± 2.1% and 11.9 ± 2.5%. The heat of segregation of Cr to the {110} plane of Stainless 410 was found to be 3.43 and 3.92 kcal/mole respectively from the two sets of data. The data also gives the difference in surface tensions between iron and chromium at this plane to be 269 and 282 erg/cm² respectively. Segregation studies on the {012} plane as well as on a grain boundary of Stainless Steel 410 were also done. In some cases, though the first surface layer is enriched with Cr in Stainless Steel 410, the near surface layers show a depletion of Cr. In Pt-8% W and Pt-5% Ru, our concentration depth profiles with a single atomic layer resolution show no segregation or depletion of the minority species either for the top layers or for the near surface layers.     

Energy spectrum of field ionization at a single atomic site

Energy deficit spectra of field ions coming from above a single atomic site are measured by using an atom-probe FIM modified with a Möllenstedt energy analyzer. This device offers a resolution of 5 × 10^?5 and is inherently more efficient and less noisy than a retarder. The energy spectra made up of 10 to 100 ions/sec are displayed on the screen of an assembly of two microchannel plates and are photographically recorded within a few seconds. Jason peaks for H₂⁺ and Ne⁺ are confirmed, and are also found for He⁺. High-order multiple peaks appear when ions are taken from the flat, closely packed net planes of W and Ir field ion emitters. The results are in quantitative agreement with a resonance model similar to one by Alferieff and Duke and by Jason. Noble gas ions are also observed from the forbidden zone near the surface, and interpreted as apex-adsorbed atoms ionized by or after excitation by the electron shower coming from farther-out field ionization of other gas atoms. Energy from excited metastable apex-adsorbed atoms may account for artifact vacancies observed particularly when field evaporation is performed in neon.     

A study of local density of states of atom clusters on the W(110) surface

The kinetic energy distributions of field desorbed He ions from tungsten clusters of one to five atoms on a W(110) surface are measured using a high resolution pulsed-laser time-of-flight atom probe. The He field ion energy distribution from the single W adatom shows an extra peak-like feature centered at 2.7 eV above the Fermi level. It has a full width at half maximum (fwhm) of 2.3 eV. The data from two tungsten adatoms separated by two lattice constants have nearly the same feature with the extra peak located at 2.5 eV above the Fermi level. These peaks arise from resonance tunneling with the adatom local density of states (LDOS). The He ion energy distribution of a tungsten dimer has an extra peak centered at 1.5 eV above the Fermi level. The fwhm is about 4 eV. The spectra from four and five tungsten adatom clusters show only one peak each, similar to that from a flat plane.     

Field ionization of ozone

Experimental results on the field ionization of ozone at an iridium surface using a magnetic sector atom-probe FIM show that at the lowest field of ~1.5 V/Å only 25% of all ions produced are O⁺₃, while O⁺₂ and a small percentage of O⁺ indicate field induced dissociation of ozone and recombination of atomic oxygen into O₂. At high fields and free space ionization, O⁺₂ is predominant, while the atomic oxygen as a dissociation product escapes without being ionized and cannot be detected. Using pure oxygen as an imaging gas in the FIM, O⁺₂ ions do not dissociate under the same conditions.     

Atom-probe field-ion microscopy of GaAs and GaP

The atom-probe field-ion microscope (atom-probe FIM) was applied for the first time to GaAs and GaP which belong to the III–V compound semiconductors. The general character of the pulsed field-evaporation of GaAs and GaP was quite similar. Ga field-evaporated predominantly in the form of singly charged ions. As and P also field-evaporated mainly as singly charged ions, but their abundances were small compared with Ga⁺. It appears that As (or P) atoms can field-evaporate more easily in the form of AsO⁺ (or PO⁺) in the presence of oxygen on the surface. In all experiments, GaAsⁿ⁺ and GaPⁿ⁺ were rarely detected. After chemical etching the surfaces were covered with oxide films and various oxide ions were detected. The abundance of oxide ions dramatically decreased after field evaporation in hydrogen. No distinct difference between the 〈111〉 orientations of these materials which have zinc-blende structure could be observed. Most of the experimental results obtained were explained in terms of the existing theory of field evaporation. It was concluded that field penetration effects have a considerable influence on the field evaporation processes of these materials as well as on the field ionization processes.     

Field-ion atom probe analysis

The atom-probe, introduced by Müller in 1967, is a combination of a field-ion microscope with a time-of-flight mass spectrometer. A specimen is prepared and imaged by field-ion microscopy in the normal manner, and the image of any feature of interest may be constrained to fall over an aperture in the imaging system. By applying a high-voltage pulse to the specimen the feature can be field-desorbed as a positive ion or ions. The desorbed ion travels down a flight-tube to a detector and from the measured flight-time its mass-to-charge ratio may be calculated. In this manner individual atoms on a specimen surface may be both imaged and identified. Using the process of field-evaporation, features in the bulk of the specimen may be brought to the surface and analysed. The atom-probe work at Cambridge has been directed both at the development of the instrumentation associated with this technique of microanalysis and at its application to small-scale segregation problems of metallurgical importance. An electronic timing device and automatic data-handling system has been built so that large numbers of ions may be analysed. The apparatus will be described and some of the experimental results obtained to date will be presented. These experiments include the segregation of carbon to grain-boundaries in iron, the clustering of titanium atoms in a nickel-titanium alloy and the analysis of small precipitates in steels. The processes occurring at the specimen surface during field-evaporation have also been studied.     

Effects of inverse bremsstrahlung in laser-induced plasmas from a graphite surface

The kinetic energy of electrons emitted due to laser interaction with a graphite surface was studied with a time-of-flight spectrometer. In addition the yields of carbon atomic and molecular ions were measured as a function of laser pulse energy. Pulse energy thresholds for ion emission are observed to correlate with the observed maximum electron energies. Furthermore, the data suggest that ionic carbon clusters can be dissociated by energetic electrons or photons created in the plasma. We believe that initially photoemitted electrons are accelerated by inverse bremsstrahlung to the energies required for electron impact ionization and dissociation     

Direct observation of individual atoms on metals

The ability to observe the structure of surfaces and to monitor the processes occurring on them with atomic resolution is crucial to a better understanding of surface properties. The field ion microscope provides this capability routinely, and with the atom probe the chemical identity of single atoms can be ascertained as well. Recent work with these techniques, aimed at a better understanding of atomic events on metal surfaces, will be reviewed with special emphasis on the following topics: (1) Observation of atomic binding sites. (2) Interfacial structure. (3) Chemical analysis on the atomic level. (4) Determination of binding energies. (5) Atomic events in surface diffusion.     

Pulsed laser atom-probe study of the dissociation of CO on molybdenum

The thermal decomposition of molecular CO on molybdenum surfaces has been studied using the pulsed laser atom-probe (PLAP). Field emitter tips, cleaned by vacuum field evaporation, were dosed with CO at low temperatures (<60 K) and heated to 180–350 K under fieldfree conditions. The fraction of undissociated CO⁺ ions compared to all carbon- and oxygen-containing species appearing in subsequently recorded PLAP mass spectra was taken as a measure of the extent of surface CO dissociation. Molecular (virgin) CO was found to convert to dissociated (β) CO at temperatures between 200—300 K. The analyzed surface area included both flat single crystal planes and stepped regions. Assuming first-order kinetics, the dissociation occurred with an activation energy of 0.7 ± 0.1 eV. The dissociation on the flat (110) plane, measured using the same technique but with the rest of the surface masked, occurred with a 0.12 eV higher activation energy.     

Formation of N3+ in pulsed-laser stimulated field desorption of nitrogen from metal surfaces

In pulsed-laser stimulated field desorption of nitrogen from metal surfaces, a plenty of N₃⁺ can be detected. Here we present the result of a study of the field dependence and emitter material specificity of N₃⁺ formation in pulsed-laser stimulated field desorption. These ions are formed by field ionization of N₃ molecules which are thermally desorbed from their field adsorption states by laser pulse heating of the surface.     

Specific-charge time-of-fligt separation of ions in RF fields with a quadratic potential distribution

The mechanism of specific-charge time-of-flight separation of ions in a radiofrequency (rf) field with a quadratic potential distribution is considered. A relation connecting the time of flight of charged particles with the parameters of the analyzer and the mass of ions being analyzed is derived. The focusing property of the rf field for ions with different energies, initial coordinates, and injection angles and phases is established. The results of computer simulation are used for constructing the instrument function of the time-of-flight mass analyzer.     

Secondary ion mass spectrometry for quantitative surface and in-depth analysis of materials

Secondary ion mass spectrometry (SIMS) is a technique based on the sputtering of material surfaces under primary ion bombardment. A fraction of the sputtered ions which largely originate from the top one or two atomic layers of the solid is extracted and passed into a mass spectrometer where they are separated according to their mass-to-charge ratios and subsequently detected. Because the sputter-yields of the individual species, coupled with their ionization probabilities, can be quite high and the mass spectrometers can be built with high efficiencies, the SIMS technique can provide an extremely high degree of surface sensitivity. Using a particular mode like static SIMS where a primary ion current is as low as 10^?11 amp, the erosion rate of the surface can be kept as low as 1 Å per hour and one can obtain the chemical information of the uppermost atomic layer of the target. The other mode like dynamic SIMS where the primary ion current is much higher can be employed for depth profiling of any chemical species within the target matrix, providing a very sensitive tool (～ 1 ppm down to ppb) for quantitative characterization of surfaces, thin films, superlattices, etc. The presence of molecular ions amongst the sputtered species makes this method particularly valuable in the study of molecular surfaces and molecular adsorbates. The range of peak-intensities in a typical SIMS spectrum spans about seven to eight orders of magnitude, showing its enormously high dynamic range; an advantage in addition to high sensitivity and high depth-resolution. Furthermore, the high sensitivity of SIMS to a very small amount of material implies that this technique is adaptable to microscopy, offering its imaging possibilities. By using this possibility in static SIMS or dynamic SIMS mode of analysis, one can obtain a two-dimensional (2D) surface mapping or a three-dimensional (3D) reconstruction of the elemental distribution, respectively within the target matrix. Secondary ion yields for elements can differ from matrix to matrix. These sensitivity variations pose serious limitations in quantifying SIMS data. Various methods like calibration curve approach, implantation standard method, use of relative sensitivity factor, etc. are presently employed for making quantitative SIMS analysis. The formation of secondary ions by ion bombardment of solids is relatively a complex process and theoretical research in this direction continues in understanding this process in general. The present paper briefly reviews the perspective of this subject in the field of materials analysis.     

Layer-by-layer analysis of charge state distribution of field evaporated ions from the W(011) plane

A charge state distribution of the field evaporated ions is layer-by-layer analyzed in a W(011) plane by using an atom-probe field ion microscope. A specimen temperature is varied from ～ 20 to ～ 250 K. The results indicate that at any temperature, the surface tungsten atoms field evaporate as triply and quadruply ions from the (011) plane, and the quadruply charged ions are always detected during the final collapse of the plane. They are also discussed on the basis of the post-ionization model.