The theoretical and experimental study of the ionization processes during the low energy ion sputtering

The process by which atoms are ionized as they are sputtered from a metal surface has been analyzed both theoretically and experimentally. In the theoretical part the expressions for ionization coefficient R⁺ of atoms having the ionization energy much larger than the metal work function have been derived using a molecular orbital method. The effect of the level crossing was estimated in an approximate way. In the experimental part the SIMS experiments on clean Ni and Al surfaces and on Ni surface covered with a submonolayer of adsorbed K, Na and Al are reported. It has been found and it is for the first time reported that the energy distribution of ions sputtered from a submonolayer of adatoms is independent of energy (200–2500 eV) and mass (Ar⁺ Xe⁺ of incident ions and depends only upon the adsorption energy of the adatom. The energy distribution of ions sputtered from bulk samples has been found dependent on the primary ion energy. The measurement of the absolute value of R⁺ has shown that there is a strong correlation between the number of the adatom valence d-electrons and the value of R⁺, the value of R⁺ being smaller for atoms with more d-electrons. These experimental data have been compared with the theoretical expressions and the important role of the mechanism which takes into account the bending of the adatom energy level has been assessed.     

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.     

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.     

Ionization probability of sputtered particles as a function of their energy: Part II. Positive Si ions

In this paper we represent the experimental ionization probability of sputtered silicon atoms as a function of their energy, which has been obtained for positive Si⁺ ions sputtered from silicon by O₂⁺ ion beam. To explain the experimental data, we have considered ionization of an outgoing atom at a critical distance from the surface, which occurs due to the electron transition between this atom and the surface, and suggested the formation of a local surface charge with the polarity opposite to that of the outgoing ion that has just been formed. Then we have considered the interaction between those two charges, outgoing ion, and surface charge as a process of the particle passage through a spherical potential barrier; as a result, we have obtained the theoretical energy distribution of secondary ions. Together with the well-known Sigmund-Thompson energy distribution of sputtered atoms, the obtained ion energy distribution allowed us to derive the equation for the secondary ion yield versus the sputtered particle energy. Both equations derived have exhibited a quite good correlation with our experimental results and also with a large number of published experimental data.     

Multiphoton fragmentation and ionization of CF2HCl molecules and clusters by UV radiation

The results of experimental studies of multiphoton ionization of CF₂HCl molecules and clusters by UV laser radiation in the wavelength range 217–236 nm are reported. In the case of molecules, the main reaction products are CF₂H⁺ and CF⁺ ions as well as atomic chlorine. It is found that the spectra of the products of ionization of free molecules and molecules condensed into clusters differ qualitatively: multiphoton ionization of clusters does not yield CF₂H⁺ ions. The dependences of the ion yield on the intensity of laser radiation and its wavelength are measured. The effect of a constant electric field and the radiation spectral width on the multiphoton ionization process is demonstrated. The shape of the velocity distributions is determined for a number of products. A strong anisotropy is detected in the reaction of formation of CF₂H⁺ ions. Possible mechanisms for these processes are discussed.     

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.     

Energy dependence of the ionization probability of sputtered Cu and Ni

Despite its great sensitivity, the usefulness of secondary ion mass spectrometry (SIMS) for many applications has been limited by an inadequate understanding of the probability of sputtering an atom in an ionized state. To determine this ionization probability for clean Cu and Ni surfaces, I have measured the energy distribution of sputtered neutrals and ions by quadrupole mass filtering and retarding potential analysis using potential modulation differentiation. Analysis of sputtered neutrals was accomplished by electron impact ionization. Because the neutrals outnumber the ions by at least two orders of magnitude, the ratio of sputtered ions to neutrals is an accurate measure of the ionization probability. For energies below 20 eV the dependence of the ionization probability on energy goes as P(E) α Eⁿ, where n = 0.65 for clean Cu. The absorption of oxygen on the Cu surface increases the total ion yield while causing a reduction in the value of the exponent n. Similar results are found for nickel, where n = 0.54 for the clean surface.     

Resonance ionization spectroscopy of bismuth at the IGISOL facility

A programme of research has commenced at the IGISOL facility, Jyväskylä, combining the technique of resonance ionization spectroscopy with the development of the highly selective laser ion source trap (LIST). The first element of interest is bismuth, which contains three isomers of multi-quasiparticle states in near-spherical nuclei, namely ²⁰⁷Bi (21/2⁺, 182 μs), ²⁰⁴Bi (10^?, 13 ms) and ²⁰⁴Bi (17⁺, 1.07 ms). A measurement of the optical isomer shift provides a direct comparison of the mean?square charge radii between the isomer and the nuclear ground state. Due to the short isomer lifetimes the spectroscopy will be done either within the ion guide or in a sextupole ion beam guide (SPIG), located after the ion guide and used in the development of the LIST. A mixed dye-Ti:Sapphire laser ionization scheme has been successfully tested for bismuth and first off-line results have been obtained.     

High-temperature field evaporation and its connection with surface ionization

High-temperature field evaporation of metals and alloys and its connection with surface ionization are considered. The main parameters of the evaporation process (dependence of the evaporation rate on the emitter temperature and on the electric field at the emitter surface, the charge of the ions being evaporated and its temperature dependence, kinetic parameters of the evaporation process, as well as the state of the emitter surface under simultaneous action of high fields and temperatures) are analyzed. The similarity and the difference between field evaporation at high temperatures and surface ionization in a strong electric field are determined.     

Multiphoton ionization of CF<Subscript>3</Subscript>I clusters by ultraviolet laser radiation

The results of the investigation of the multiphoton ionization of (CF₃I) _n clusters by ultraviolet laser radiation are reported. The yields of the I₂⁺ and I⁺ ions, which are the products of the multiphoton ionization, have been measured as functions of the intensity of the ultraviolet radiation at the wavelengths of 308 and 232.5 nm. The degree of multiphoton ionization has been determined and appears to depend on the wavelength of radiation. The velocity distributions of the products have been measured in detail for various wavelengths and various polarizations of radiation. The anisotropy parameters of the velocity distributions of the produced ions and their kinetic energy have been determined. After analysis of the data, a mechanism of the multiphoton ultraviolet ionization of the clusters under investigation has been proposed. This mechanism depends on the used wavelengths.     

Isotope selective loading of an ion trap using resonance-enhanced two-photon ionization

We have demonstrated that resonance-enhanced two-photon ionization of atomic beams provides an effective tool for isotope selective loading of ions into a linear Paul trap. Using a tunable, narrow-bandwidth, continuous wave (cw) laser system for the ionization process, we have succeeded in producing Mg⁺ and Ca⁺ ions at rates controlled by the atomic beam flux, the laser intensity, and the laser frequency detuning from resonance. We have observed that with a proper choice of control parameters, it is rather easy to load a specific number of ions into a string. This observation has direct applications in quantum optics and quantum computation experiments. Furthermore, resonant photo-ionization loading facilitates the formation of large isotope-pure Coulomb crystals. Received: 21 December 1999 / Published online: 11 May 2000     

Electron impact double ionization of Ba and Ba+

Electron impact double ionization cross-sections for Ba and Ba⁺ have been calculated in the binary encounter approximation. Hartree-Fock velocity distribution has been used for the first ejected electron and a hydrogenic velocity distribution for the second. For Ba⁺ the focusing effects of the target ion on the incident electron have been incorporated in the calculations. Contributions from ionization-autoionization to the ionization cross-sections, as observed in experiments, have been included in the present work. The calculated results show structures as observed in recent experiments.     

Effect of the ionization energy loss density of high-energy bismuth,krypton, and xenon ions on the development of hydrogen blisters in silicon

Radiation-induced athermal hydrogen removal from single-crystal silicon subjected to irradiation by high-energy heavy Bi⁺ (E = 710 MeV), Kr⁺ (E = 85 and 250 MeV), and Xe⁺ (130 MeV) ions is detected experimentally. The decrease in the hydrogen concentration depends on the specific ionization energy losses of high-energy heavy ions. At high specific ionization losses of Bi⁺ ions with E = 710 MeV (22.5 keV/nm), the hydrogen concentration decreases to a level at which blisters cannot be observed in an optical or electron microscope (which is likely to be 1 at % hydrogen at the peak of the calculated hydrogen concentration profile). At medium specific ionization losses of Xe⁺ ions with E = 130 MeV (12.5 keV/nm) and Kr⁺ ions with E = 250 and 85 MeV (9.5 and 8.5 keV/nm, respectively), the hydrogen concentration decreases to a level that does not affect blister formation but determines the blister failure (flaking) conditions.     

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.     

Double ionization of Sc<Superscript>+</Superscript>ions by electron impact

Electron impact double ionization cross-sections of Sc⁺ions have been calculated in the binary encounter approximation (BEA). Accurate expression of σ_ΔE(cross-section for energy transfer ΔE) and Hartree-Fock velocity distributions for the target electrons have been used throughout the calculations. Direct double ionization from ejection of 3d and 4s electrons has been investigated in the modified double binary encounter model incorporating the focusing action of the target ion on the incident electron. The identification of the 3p shell whose ionization provides a major contribution to double ionization through ionization-autoionization is an interesting aspect of the present investigation. The theoretical results show satisfactory agreement with the experimental observations.     

Ionization and dissociative ionization of methane molecules

A technique for mass-spectrometric investigation of the yield of positive ions produced by direct and electron-impact dissociative ionization of methane molecules is described, and respective experimental data are presented. Doubly charged C ₂ ⁺ , CH ₃ ²⁺ , and CH ₄ ²⁺ ions, as well as singly charged D ₂ ⁺ , CD ₃ ⁺ , and CD ₄ ⁺ ions, are detected in the mass spectrum of a methane molecule at electron energy U _e = 90 eV for the first time. From ionization efficiency curves, the ionization energy of the parent molecule and the appearance energy of fragment ions are determined. The ionization energy of the CH₄ molecule is found to be 12.62 ± 0.20 eV. Electron-molecular reactions that may take place when a low-energy electron beam interacts with a methane molecule are analyzed. The ionization process and the formation of methane molecule fragments are studied.     

Application of the surface ionization for the detection of secondary particles in the secondary-ion mass spectrometry (SIMS)

A method for postionization in the course of ion sputtering that is based on surface ionization of the sputtered particles is developed. The estimations show that the method allows a significant increase in the sensitivity of the secondary-ion mass spectrometry for several elements. Nonadditive increase in the sputtering coefficient of indium is experimentally studied using the surface-ionization method of postionization when the number of atoms in projectile clusters Bi _m ⁺ (m = 1–7) increases at energies 2–10 keV. Such a scheme for the detection of neutral particles can be used in alternative methods for the surface analysis, in particular, laser evaporation of surface and electron-stimulated desorption.     

Caesium/xenon dual beam depth profiling: Velocity of the sputtered atom and ionization probability

In this work, a caesium/xenon co-sputtering gun was used to perform depth profiles of a RhSi layer with varying caesium beam concentration. The positive ion yields were monitored with respect to the varying work function of the solid and the intensities of the ions were plotted with respect to the caesium surface concentration. As expected by the tunneling model, all the M⁺ signals decrease exponentially with the increasing caesium beam concentration. Moreover, the heaviest ion yields decrease faster than the lighter ion ones. This phenomenon can be explained by the different velocities of the departing atoms, which has an important impact on the ionization processes. We then studied the variations of the MCs⁺ yields with respect to the caesium surface concentration and with respect to the nature of the departing atom. Finally, we applied models based on the tunneling model in order to fit our results.     

Influence of the surface electric field in ion-beam-surface interaction at grazing incidence

We have measured the polarization of light emitted after ion surface scattering at small angle of incidence. The measurements are carried out with H⁺-, H ₂ ⁺ - and He⁺-ions under UHV-conditions with mono- and polycrystalline targets. We explain the typical variation of the polarization as “post collision Stark interaction” (PCSI) in the surface electric field, which can force transitions between nearly degenerate terms. The electric field is composed of two different contributions, a strong but short range surface field which is “seen” by atomsand ions and a long range but weak field due to the image charge which is “seen” to first orderonly by ions. The influence of the electric field on H-Balmer radiation is negligible at typical survival distances r_s≧0.35nm. But in contrast to H-atoms He⁺-ions feel the additional influence of the image field leading to a strong alteration of the polarization of the emitted light. The polarization of the Balmer-radiation stemming from Coulomb exploding H ₂ ⁺ -beams is observed to be modified by the electric field of the “spectator proton”.     

Field-induced ionization and Coulomb explosion of nitrogen

Femtosecond-laser field-induced ionization and Coulomb explosion of diatomic nitrogen were systematically investigated using time-of-flight mass and photoelectron spectrometry. Both linearly and circularly polarized femtosecond laser pulses were used at intensities varying from 5×10¹³ to 2×10¹⁵ W/cm². Strong N₂ ⁺, N₂ ²⁺, N⁺, N²⁺ and N³⁺ ion signals were observed for horizontally polarized pulses. Moreover, signals from the atomic ions exhibited a double-peak structure. Suppression of ionization was observed for circularly polarized pulses, while for vertically polarized pulses, only N₂ ⁺ and N₂ ²⁺ ions were observed. The angular distributions of the ions were measured under zero-field conditions in the ionization zone. The atomic ions N⁺, N²⁺ and N³⁺ exhibited highly anisotropic distributions, with maxima along the laser polarization vector and zeroes normal to the laser polarization vector. In contrast to the atomic ions, N₂ ⁺ exhibited a strong isotropic angular distribution. These observations indicate that dynamic alignment is responsible for the observed anisotropic angular distribution of the atomic ions. The kinetic energy spectrum of the photoelectrons is featureless and broad, extending above the ponderomotive potential of the laser pulse. The angular distribution is markedly anisotropic, with a maximum along the laser polarization vector. These observations further support the notion that the field-ionization mechanism is dominant under our experimental conditions. Received: 29 January 2002 / Revised version: 15 March 2002 / Published online: 12 July 2002