Neon gas imaging of gold in the field ion microscope

An improved field ion microscope (FIM) technique has been developed for the neon gas imaging of gold specimens. The technique produces images which are stable at best image voltage at a tip temperature (T_T) of 30 K or less. The first stage of the technique consisted of the development of an end form at 55 K in the presence of a partial pressure of air (～ 2 × 10^?8 Torr gauge pressure) and neon gas (～ 3 × 10^?5 Torr gauge pressure) followed by further field evaporation at 28 K. The second stage involved neon gas imaging of the previously developed end form in a baked FIM in a background pressure of (0.5 to 3) × 10^?9 Torr. The FIM images obtained in conjunction with the field ionization characteristic curves showed that there is a working range (in the sense defined by Southon and Brandon). A detailed study was made of artifact vacancies detected in the {203}, {321}, {315}, {421}, {671} and {731} planes, and it was found that at 28 K their concentration was < 2.5 × 10^?3 at.fr. Approximately 191,000 atomic sites were examined for artifact vacancies. The artifact vacancy concentrations measured in the present study were a factor of 13 to 60 lower than those measured earlier by Schmid and Balluffi who employed a background pressure of ～ 5 × 10^?8 Torr in their FIM. Hence, the artifact vacancy concentrations detected in gold are dependent upon the background partial pressure employed in the FIM. This latter result plus the result that the images are only stable in ultra-high vacuum (UHV) conditions indicates the need for UHV conditions for the successful imaging of gold surfaces.     

Pressure dependence of small signal gain and saturation intensity of a gold-vapor laser using various buffer gases in gain medium

A pair of gold-vapor laser (627.8 nm) in an oscillator-amplifier configuration was used to investigate the small signal gain, g₀, and saturation intensity, I_s, as amplifying parameters, versus pressure at various types of buffer gas. It was shown that the small signal gain decreases and saturation intensity increases linearly with increasing the pressure. Moreover, the values of these parameters are different using various gas mixtures in gain medium. Both parameters were estimated to be more at helium buffer gas atmosphere than that of neon or their mixed ones.     

Effect of the parameters of the plasma channel produced by a high-current relativistic electron beam in dense gaseous media on the beam transportation stability

Results are presented from experimental studies of the time evolution of the plasma channel produced by a high-current electron beam (with an electron energy of E _e = 1.1 MeV, a beam current of I _b = 24 kA, and a pulse duration of t = 60 ns) in helium, nitrogen, neon, air, argon, krypton, xenon, and humid air (air: H₂O) at pressures from 1 to 760 Torr. It is shown that, in gases characterized by a small ratio of the collision frequency to the gas ionization rate u _i , the electron beam produces a broad high-conductivity plasma channel, such that R _b/R _p < 1, where R _b and R _p are the beam and channel radii, respectively. As a result, large-scale resistive hose instability is suppressed.     

Field evaporation of silicon (111) surfaces in the presence of hydrogen

The field evaporation rate of Si(111) face in H₂ imaging gas is measured by counting the removal rates on individual net planes in a field ion microscope. The rate is found to decrease with increasing temperature and to be proportional to H₂ gas pressure. The evaporation voltage increases with temperature (T<300). A model explaining this temperature dependence is based on the rate-determining formation of surface hydrides due to field-induced chemisorption of hydrogen.     

Comparative performance of closed cycle gas turbine engine with heat recovery using different gases

The unique features of the closed cycle gas turbine engine have been recently contributing towards its adoption in a multiplicity of applications such as power plants, space, and marine power supplies. Therefore, research is reactivated to serve its future developments.In this work, a performance analysis is carried out using different gases such as air, combustion gases, CO₂ and helium. Operating variables are turbine inlet temperature T₀₃, compressor pressure ratio R_c and inlet temperature T₀₁. A computer program is taioored to calculate specific work W_s and overall efficiency η_o over a wide range of operating variables. Results show that helium gives relatively higher W_s, but η_o starts to drop early after a low optimum R_c. Air and combustion gases offer nearly equal performance.     

The field ionization characteristics of individual atomic planes

A detailed study of the field ionization characteristics of nine different planes of tungsten in the (001)-(011)-(111) standard triangle has been performed as function of tip temperature (T_T) between 11°K and 86°K, and local radius of curvature (rT). The measured helium ion current-voltage characteristic curve for each plane was found to consist of two distinct regimes. The first regime was linear on a log-log plot, and the slope varied from 27 to 41. A modified version of Gomer's model for the very low field ion current fitted the regime I data reasonably well. The second regime of each characteristic curve was quite complicated and exhibited several maxima and minima whose positions were functions of both T_T and crystallographic plane. A qualitative explanation for the behavior of the ion current in regime II was given in terms of a patch field model consisting of three dominant spatial regions on the surface of the field ion microscope specimen. In addition, the explanation also considered the role played by a lateral supply of gas atoms on the specimen's surface, and a slowly increasing field dependent radial supply function of gas atoms. It was also found that the probability of ionization was a strong function of both T_T, and crystallographic plane. An expression was derived for the temperature dependence of this effect which fitted the data for atomically smooth planes [e.g., the (011) plane]. Finally, the ion current from individual planes was proportional to a power of rT (at constant electric field) which varied between 2.3 and 2.9. This result was at variance with the existing theories of the supply function, and indicated that the shank of the specimen was a significant source of imaging gas atoms.     

Emission characteristics of a discharge iodine vapor plasma in the spectral region of the main absorption maximum of DNA molecules

Radiation of glow and capacitive discharges in inert gas-iodine vapor mixtures is studied in the spectral range 150–210 nm, which coincides with the main absorption maximum of the DNA molecules. Iodine atomic spectral lines at 150.7, 161.8, 170.2, 183.0, and 206.2 nm are observed in the spectra. The emission intensity of the iodine spectral lines is optimized by varying the glow discharge current, capacitive discharge frequency, as well as pressure and composition of the gas mixtures. The glow and capacitive discharges are ignited in cylindrical quartz tubes with interelectrode gaps of 10 and 6 cm. Helium and neon are found to be the most effective buffer gases. The optimum partial pressures of the light inert gases and iodine vapor in the glow discharge are within 0.4–0.6 kPa and 100–150 Pa, respectively. In the capacitive discharge in He(Ne)-I₂ mixtures, the optimum partial helium, neon, and iodine vapor pressures are within 0.8–2.0 kPa, 0.5–1.0 kPa, and ≤ 60 Pa, respectively. It is demonstrated that pulsed bactericidal radiation sources with light pulse lengths of 400–500 ns and continuous radiation sources emitting within the spectral range 150–207 nm can be designed on the basis of low-density iodine vapor plasma.     

Xe2Cl fluorescence and absorption in self-sustained discharge XeCl lasers

Fluorescence at 490 nm from the triatomic excimer Xe₂Cl^* has been investigated to determine the 308 nm absorption due to this species in an x-ray preionized, self-sustained gas discharge XeCl laser. The dependence of Xe₂Cl^* density on laser intensity (at 308 nm), buffer gas and Xe and HCl partial pressures has been determined for discharges with a peak electrical power deposition of 2.5 GWl^–1. Xe₂Cl^* absorption is estimated to reach 0.6% cm^–1 under non-lasing conditions but decreases to a non-saturable 0.2% cm^–1 for intracavity laser intensity>1 MW cm^–2. XeCl^* and Xe₂Cl^* fluorescence intensities were found to be a similar for both helium and neon buffer gases but laser output was a factor of two greater with a neon buffer.     

Role of neon in a decaying high-purity helium plasma

The evolution of the electron and atomic and molecular metastable densities and the radiation of the decaying plasma of helium with a 10^–5-fraction of neon additive is experimentally studied. A model of elementary processes in He–Ne plasma is constructed, which describes the formation and destruction of HeNe⁺ and Ne₂ ⁺ molecular ions and their contribution to the formation of the afterglow spectrum by the electronion recombination. The various criteria influence of neon on the parameters of the decaying plasma are studied. The possibility of determining the amount of neon in helium by measuring the relative intensities of helium molecular bands and neon spectral lines in the afterglow is considered.     

A study of the surface self-diffusion of tungsten induced by ion impact

Ion impact induced surface self-diffusion is studied with a field electron microscope using a new phenomenon, the matter transport of this diffusion caused by capillary forces. Studied is the blunting of the edges of a tungsten crystal by helium ion impact. The order of the coefficient D_i describing this diffusion is determined. D_i is about 10⁶ smaller than estimated by Dranova and Mikailovski. The influence of the ion impact surface roughening is discussed.     

Field ion microscopy of W and PtTi at about 1000°C

Bright new images including the image of atoms have been observed when a positive high voltage (～ 15 kV) was applied to a specimen heated to ～ 1000°C in a field ion microscope. A channel plate multiplier was especially effective to this effect. Since those images were observed even in a high vacuum of 10^?7 Torr, where imaging gas hardly exists, and the brightness of the images does not depend on the residual gas pressure, they can not be the usual images caused by gas ions emitted from the tip of the specimen. A tentative interpretation is that they are caused by metal gas ions which are vaporized from the heating loop of tungsten and ionized at the tip of the specimen.     

Formation of high-energy electron beams in gases of different densities

An investigation was made of the formation of beams of fast electrons in different gases at pressures of from 0.01 to 100 kPa. Plots were made of the dependences of the electron beam currents I_e on the gas pressure p for different electric field strengths E. The dependences I_e=f(p) for air were found to intersect the similar dependences for other gases (helium, nitrogen, neon, and argon) at a pressure of p=10.6 kPa and for E=2.3·10⁵ V·cm^–1. This fact is explained by the influence of the oxygen ions and atoms on the electron beam formation process. Another experimental result, the appearance of a minimum in the dependences I_e=f(p) for all gases, is explained by defocusing of the electron beam, the appearance of a reverse current, and magnetic neutralization of the beam. Electron beams were obtained having a pulse duration of 15–20 nsec and a current of 10⁵-10⁶ A/m² per unit cathode area.Institute of Atmospheric Optics, Siberian Branch, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 67–70, June, 1993.     

Laser-based dynamic evaporation and surface shaping of fused silica with assist gases: a path to rimless laser machining

Evaporation and ablation are fundamental processes which drive laser-material processing performance. In applications where surface shape is important, control of the temperature field and the resulting spatially varying material response must be considered. For that purpose, assist gases are useful in, first, lowering treatment temperatures and, second, in changing interfacial and bulk chemistry to limit capillary-driven flow. Additionally, laser-matter coupling is influenced by pulse length as it determines the heat affected zone. Using infrared imaging of CO₂ laser-heated fused silica and surface profile measurements, we derive temperature and time dependent pitting rates along with shapes for a range of gases that include hydrogen, nitrogen, air, and helium. In the range of 1,500–4,500 K, evaporation, flow, and densification are shown to contribute to the pit shape. Analysis reveals a strong and complex dependence of rim formation on heating time and gas chemistry, mostly by lowering treatment temperature. Under dynamic heating, chemicapillarity appears to help in lowering rim height, in spite of the reactants mass transport limitations. Results on this gas-assisted approach suggest the possibility for sub-nanometer “rimless” laser-based machining.     

Dependence of the amplifying parameters on buffer gases in copper-vapor lasers

We use a pair of copper-vapor lasers in the oscillator–amplifier configuration for investigating the small-signal gain and the intensity saturation as the amplifying parameters, versus the pressure of various types of buffer gases. We show that the small-signal gain increases and the intensity saturation decreases with increase in the air pressure. Moreover, the values of these parameters are different for various gases used in the amplifier media. We show that both parameters are greater at the atmosphere of neon as a buffer gas than that of a mixture of helium and neon or air.     

Electron Scattering from Neon Via Effective Range Theory

Elastic cross-sections for electron scattering on neon from 0 energy up to 16 eV are analyzed by an analytical approach to the modified effective range theory (MERT). It is shown that energy and angular variations of elastic differential, integral and momentum transfer cross-sections can be accurately parameterized by six MERT coefficients up to the energy threshold for the first Feshbach resonance. MERT parameters are determined empirically by numerical comparison with large collection of available experimental data of elastic total (integral) cross-sections. The present analysis is validated against numerous electron beams and swarm experiments. The comparison of derived MERT parameters with those found for other noble gases, helium, argon and krypton, is done. The derived scattering length (for the s-partial wave) in neon, 0.227 a ₀, agrees well with recent theories; it is small but, differently from Ar and Kr, still positive. Analogue parameters for the p-wave and the d-wave are negative and positive respectively for all the four gases compared.     

The influence of surface coordination on field evaporation processes in tungsten

The sequence of evaporation of atoms on (111), (332), (433), and (411) surfaces of tungsten has been observed in the field ion microscope for the evaporation of successive layers. Atoms with many different types of surface coordination were observed to evaporate. The data were processed to give the relative probabilities of evaporation (ρ) for each type of atom using a statistical procedure adapted to give probability limits for the ρ values. The results showed that for (111) surfaces the ρ values were the same with helium or neon as image gas and that in general differences in ρ values between differently coordinated atoms at 78 K were observed to be greater than at 20 K. At both these temperatures however the atoms with high surface coordination generally had lower ρ values. With (111), (332) and (433) surfaces the atomic arrangement allowed prediction of surface diffusion paths and with atoms in (111) surfaces an easy diffusion path was essential before evaporation could occur. Amongst atoms which had a diffusion path available, ρ values were relatively lower when the path had a saddle point protruding from the surface and which allowed the atom to be subjected to a higher field. On (411) surfaces diffusion paths are less predictable and the correlation with ρ was less.     

On the Re-function of the A-X system of No+

The band intensity factors, and r-centroids are reported, for bands of the A-X system of NO⁺, a stable molecular ion of astrophysical and aeronomical interest. The need for further laboratory band intensity measurements, is indicated, in order to resolve the existing ambiguity of the R_e-function of the system.     

Field evaporation experiments with a magnetic sector atom-probe FIM

A magnetic sector atom-probe FIM has been successfully operated for dc field evaporation of tip materials such as Rh, W, Ir, Mo and Ti. A limited number of evaporated metal ions were clearly identified forming a line spectrum. Field evaporation of Rh in the presence of ³He and ⁴He gases showed that the formation of the helium compound (RhHe)²⁺ is quite sensitive to He gas pressure; no helium compound were observed below 5 × 10^?7 Torr and all ions detected as helium compound above 5 × 10^?5 Torr at 78 K.     

Measurement of enhancement in field ionization of helium and neon by self field adsorption

A direct measurement of field ionization enhancement by self field adsorption has been carried out for helium and neon gases. The result shows a large enhancement factor for both helium and neon, in disagreement with present theories of field ionization with field adsorption.     

Characteristics of a high-frequency barrier discharge in mixtures of mercury diiodide vapor with gases

The spectral and electric characteristics of atmospheric-pressure high-frequency barrier discharge plasma based on mixtures of mercury diiodide with neon and admixtures of argon, xenon, and nitrogen are analyzed. A repetition rate of sinusoidal voltage pulses of about 100 kHz is used both to produce the gas discharge plasma and to excite the components of the working mixture. The radiation of the discharge in the range 200–900 nm is analyzed with a high resolution. It is found that, in the range 400–900 nm, the system of bands of excimer molecules HgI(B → X) emits 85% of the barrier discharge radiation. It is established that the radiation intensity of HgI(B → X) molecules is maximal in the mixture HgI₂/Xe/Ne = 0.6/10/90 kPa. In this mixture, UV radiation of molecules XeI(B → X) and XeI(B → A) is observed. The regular features of the spectral and electric characteristics of the gas discharge plasma are discussed. An atmospheric-pressure high-frequency barrier discharge in mixtures of mercury diiodide with gases is of interest for use in a selective (Δλ = 438–446 nm) excilamp with a cylindrical working aperture.