Singly charged tungsten and tantalum ions observed during high-temperature field evaporation

High-temperature field evaporation of tungsten and tantalum emitters in the temperature range from room temperature to 2500 K is studied using a static magnetic mass spectrometer equipped with a field source of ions. At room temperature, triply charged W³⁺ and Ta³⁺ ions alone are observed in the mass spectra. However, as the emitter temperature grows, the charge of the ions decreases. At T ≈ 1000 K, doubly charged W²⁺ and Ta²⁺ ions dominate in the spectra, and singly charged W⁺ and Ta⁺ ions appear in the temperature range 1900 < T < 2500 K. The evaporation rate of the singly charged ions is one to two orders of magnitude lower than the evaporation rate of the doubly charged particles. The energy parameters of field evaporation for differently charged tungsten ions are found.     

Track-membrane-based interface for field evaporation of ions from polar solutions in the diffusion-drift regime

The possibility of realization of barrier-free field evaporation regime for ions from polar solutions in the conditions when the flux of ions from the liquid is limited by their diffusion and drift from the bulk to the surface is considered. The strength of the electric field extracting ions is estimated by simulating electric fields in an ion source with a track membrane as the interface with allowance for the sizes of channels in the membrane and their density. It is shown that when time-dependent electric fields are used, the regime of barrier-free field evaporation can be realized with an appropriate choice of geometrical parameters of the membrane.     

Stopping of slow ions

The 50th anniversary of the appearance of a series of seminal papers by O.B. Firsov is taken as an occasion to have a look at some central aspects of the stopping of slow ions in matter. A brief characterization of Firsov’s model of electronic energy loss is given in comparison with alternative views. The main part of the paper is devoted to central problems such as scaling behavior, Z ₁ and Z ₂ structure, deviations from velocity proportionality, threshold behavior, metal-insulator and gas-solid differences, and correlation between nuclear and electronic stopping.     

The temperature jump and slow evaporation in molecular gases

We set up model transport equations that describe the behavior of molecular (diatomic and polyatomic) gases with a molecule collision rate proportional to the molecular velocity. In deriving these equations we allow for the internal (rotational) degrees of freedom, while the vibrational degrees of freedom are assumed “frozen.” We also set up an exact equation for the problem of the temperature jump with allowance for slow evaporation from the liquid surface into the saturated vapor atmosphere. Finally, we derive explicit formulas for calculating the coefficients of the temperature jump and gas-density jump above a flat surface and do the necessary numerical calculations. Zh. éksp. Teor. Fiz. 114, 956–971 (September 1998)     

On the crystalline field of rare-earth ions in the cubic intermetallic compound: YSb

Resolved fine structure spliting of Gd in YSb yields a fourth order crystalline field parameter b₄ = (29±3)G. A correlation between b₄ and the crystalline field experienced by other rare-earth ions in the same host is demonstrated.     

Ionization of solvent molecules by the field evaporation of ions from glycerol and ethylene glycol solutions

Using an electromembrane source, mass-spectrometric investigations into the field evaporation of ions from KI solutions in a water-glycerol mixture and in ethylene glycol are performed. The concentration of negatively charged solvent ions (deprotonated molecules) on the emitting surface of the liquid is determined. It is shown that, under the conditions of intense field evaporation of ions, the surface concentration of deprotonated glycerol and ethylene glycol molecules is several orders of magnitude higher than their equilibrium concentration in the absence of an electric field. The high concentration of solvent ions is associated with an increase in the autoprotolysis constant in a strong electric field.     

High-temperature field evaporation of rhenium

High-temperature field emission of Re, Pt, Ta, and W is studied by field-emission methods. Metal ions are found to evaporate mainly from the tops of thermal-field microprotrusions produced by high electric fields and temperatures on the emitter surface. For fi eld intensities of up to F=1–2 V/Å and temperatures of 1500–2000 K, the ion currents i are recorded from the entire emitter surface. They range from several tenths of nanoamperes to several nanoamperes. The activation energies of field evaporation determined from the Arrhenius plots logi=f(1/T) are found to be appreciably lower than those calculated within the charge exchange model for known parameters of the process and the metals evaporated. Reasons for such a difference in the activation energies and mechanisms of ion evaporation at high F and T are discussed.     

Excitation of magnesium ions in slow electron-ion collisions

The method of intersecting modulated beams was used to measure the absolute excitation functions of 3s-3p and 3p-4s transitions of MgII in collisions of electrons with magnesium ions at energies between the threshold of excitation and 100 eV.     

Charged particle evaporation from compound nuclei formed by bombardment of58Ni with 70 MeV16O ions II

The experimental data have been compared with the result of extensive compound nucleus calculations based on the statistical theory. The shapes and the absolute cross sections for the high energy parts of the proton- and -spectra are reproduced with a level density parametera 0·11 A MeV^–1 and a moment of inertiaJ1·3J _rig. The same parameters fairly well reproduce the angular distributions corresponding to the high energy parts of the spectra whereas deviations between theory and experiment are observed for the lower energies where particles emitted by higher order processes contribute. The observed total cross section for emission of an -particle is 500 mb or 1·7 times calculated value. For protons the corresponding numbers are 2313 mb and 3·0. The increase in cross section is ascribed to the process in which several particles are evaporated. Various methods are discussed for determination of the average orbital angular momentum of the emitted particle or the average spin of the daughter nucleus.On leave fromFaculty of Matematics and Physics, Charles University, Prague, Czechoslovakia.     

An examination of field evaporation theory

Contrary to opinions expressed by earlier investigators, thorough examination of the existing models for field evaporation indicates that they are capable of predicting changes in the relative abundance of ionic species as temperature is changed and evaporation rate is held constant, without assuming atomic tunneling or slowness of electronic transition. However, this need not imply that atomic tunneling and slowness of electronic transition do not exist. Furthermore, the inclusion of energy level shifts in field evaporation theory results in the prediction of higher charge states than would be otherwise expected. This is in agreement with the observations of various investigators on copper and other metals. The examination further reveals that very significant differences in the predicted variation of evaporation rate with field exist between the image-hump and intersection models. This suggests a means by which one can attempt to determine experimentally the “correct” model to apply in a given situation. Furthermore, rigorous interpretation of the intersection model in conjunction with the experimental data of Tsong and Müller can result in significantly different values for the distance of the equilibrium position of the surface atom from the metal surface and the polarizability of the surface atom, than obtained using linear approximations.     

Charged particle evaporation from compound nuclei formed by bombardment of58Ni with 70 MeV16O ions part I

Highly excited compound nuclei of⁷⁴Kr have been formed by bombardment of⁵⁸Ni with 70 MeV¹⁶O ions. The spectra of protons and -particles resulting from the decay of the compound nucleus (and several daughter nuclei) have been recorded at laboratory angles from 30° to 155°. The CM differential cross sections corresponding to definite energies for the particles emitted were calculated from the data. The charged particle spectra and the angular distributions show that evaporation processes are dominant. A study of spectral shapes and angular distributions has to some extent made it possible to distinguish primary particles from those evaporated after the emission of one or several particles.On leave fromFaculty of Mathematics and Physics, Charles University, Prague, Czechoslovakia.     

Reciprocity in the electronic stopping of slow ions in matter

The principle of reciprocity, i.e., the invariance of the inelastic excitation in ion-atom collisions against interchange of projectile and target, has been applied to the electronic stopping cross section of low-velocity ions and tested empirically on ion-target combinations supported by a more or less adequate amount of experimental data. Reciprocity is well obeyed (within ~10%) for many systems studied, and deviations exceeding ~20% are exceptional. Systematic deviations such as gas-solid or metal-insulator differences have been looked for but not identified on the present basis. A direct consequence of reciprocity is the equivalence of Z₁ with Z₂ structure for random slowing down. This feature is reasonably well supported empirically for ion-target combinations involving carbon, nitrogen, aluminium and argon. Reciprocity may be utilized as a criterion to reject questionable experimental data. In cases where a certain stopping cross section has not been or cannot be measured, the stopping cross section for the inverted system may be available and serve as a first estimate. It is suggested to build in reciprocity as a fundamental requirement into empirical interpolation schemes directed at the stopping of low-velocity ions. Examination of the SRIM and MSTAR codes reveals cases where reciprocity is obeyed accurately, but deviations of up to a factor of two are common. In case of heavy ions such as gold, electronic stopping cross sections predicted by SRIM are asserted to be almost an order of magnitude too high.     

Interaction of slow highly charged ions with surfaces

We show how a classical approach can be used to model electronic processes taking place when slow highly charged ions collide with surfaces. In particular we derive expressions for the distance of the first charge flow and for the image energy gain of an ion of chargeq outside a surface described by a workfunctionW and a static dielectric constant ?.     

Anisotropy of low-temperature field evaporation of metals

A metal-surface microtopography formed by low-temperature field evaporation is analyzed by the methods of field-ion microscopy and computer modeling. It is shown that the anisotropy of the field evaporation and the regional distribution of the brightness of the ion-microscope image correspond to the configuration of the first Brillouin zones. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 7, 516–520 (10 April 1997)     

Three-body recombination of atomic ions with slow electrons

Using the time-dependent wave-packets method, we have quantum mechanically investigated the three-body recombination process for electron energies varying from 10 to 0.01 eV. The classical "bottleneck" prediction on the probable population was confirmed by our S-wave quantum calculations for electron kinetic energies (K(E)) as low as 0.1 eV. But for K(E)<0.1 eV, the quantum three-body recombination tends to populate lower n levels than the classical theory predicted. We also find that, in the same n level, the recombination tends to populate higher angular-momentum states with K(E) decreasing. These results may shed light on the intrinsic dynamics of both ultracold plasmas and frozen Rydberg gases.     

Electron multiplicity in slow collisions of Ar ions with C

Electron capture by Ar⁸⁺ in collisions with C₆₀ fullerene has been investigated using coincident measurements of the number n of ejected electrons, the mass and charge of multicharged Cr+ ₆₀ recoil ions and their fragments Ci+ m and the final charge state of outgoing projectiles Ar(8-s)+ (). The number of captured electrons r is the sum of the numbers of stabilized and emitted electrons: r = n + s. The ratio n / s decreases by a factor three with s increasing from 1 to 7 showing that the multiply excited states populated by capture of a large number of electrons are rather stable against auto-ionisation. Each kinetic energy spectrum of Ar⁺ and Ar²⁺ projectiles is composed of two peaks which we attribute to collisions “inside” and “outside” the C₆₀ cage. The measured energy shift of the projectile keV is consistent with the corresponding energy loss keV in a carbon foil with an equivalent thickness. Inside collisions are characterized by a strong dissociation of recoil ions into light monocharged fragments and by a high multiplicity of ejected electrons. Received: 25 March 1998 / Received in final form and Accepted: 9 June 1998     

Stopping of slow ions in electron gas with magnetic interaction

The electronic stopping and straggling of low velocity point charge in the electron gas subjected to magnetic ordering is analysed within the linear response theory. The electrons were described by the Lindhard dielectric function ?(k,ω)$\mathrm{?}(\mathbf{k},\omega )$. The magnetic interactions were included in the model along the Stoner treatment of itinerant electrons. The effect caused by phase transition was shown to be negligible comparing to the experimental result.