High resolution time-of-flight low energy ion scattering

Low energy ion scattering (LEIS) is a well known technique for quantitative composition and structure analysis. Two different detection methods can be used in a LEIS experiment, i.e. an electrostatic analyzer or a time-of-flight (TOF) spectrometer. Both techniques have specific advantages. Nevertheless, the time-of-flight technique surpasses usual electrostatic analyzers used in LEIS in terms of energy resolution. The possibility to measure ions and neutrals, and the better energy resolution permit to study neutralization or charge exchange processes in much more detail.

Here we present a TOF–LEIS setup with an energy resolution of better than 0.4% for 3 keV He ions and report experimental results for 3 keV He ions backscattered from a polycrystalline Cu target. The resulting ion spectrum shows interesting inherent features, which are analyzed. Possible causes for the appearance of these features are discussed.     

Characterization of the Ne-Al scattering potential using low energy ion scattering maps

In this study, we examine the scattering of inert-gas ions from Al(1 1 1) using low energy ion scattering (LEIS). These techniques, because of their high surface specificity, provide structural and compositional information from the first atomic layer of the surface and can be used to determine the configuration of low-Z adsorbates. Extracting structural information embedded in LEIS data presents many challenges, given the complex collision processes which ultimately contribute to the detected scattering and recoil signals. To aid in the interpretation of these data, we map scattered Ne⁺ ion signals over a wide range of crystal orientations with respect to the incident beam in order to investigate a variety of scattering geometries. The signals are also simulated using a modified version of the MARLOWE binary collision code and related to the surface structure. We make quantitative comparisons between the simulated results and the experimental data using reliability factors, and are able to show how the interatomic potential can be calibrated.     

Comparison of experimental and simulated low energy alkali ion scattering

Computer simulations performed using the MARLOWE code are compared with experimental results obtained for 1 keV Li⁺ ion scattering from W(001). Trajectory analysis using distributions of impact points, deflections and maximum penetration depths allow characterization of the contributing scattering mechanisms. Computer simulation is found to reproduce many of the observed features in the energy distributions and in the incident angle dependence, but there are also important differences. In agreement with experiment, a sharp, surface sensitive binary collision peak is found at certain angles but at others this peak may broaden, shift to lower energy and grow in intensity due to subsurface scattering mechanisms. Simulations overestimate the relative scattering intensity near critical edges and the importance of focused scattering from second layer atoms.     

Measuring correlated-electron effects with low energy alkaline earth ion scattering

A system of many electrons can display emergent phenomena beyond what would be predicted from the behavior of individual electrons. A method for triggering correlated electron behavior is to introduce a spin impurity into a metal, which causes the free electrons to respond collectively. In the present work, the unpaired valence electron on singly charged alkaline earth ions is used to provide the spin impurity during low energy ion scattering experiments. Previous calculations have suggested that electron correlations would result in an anomalous temperature dependence of the ion neutralization probability. The neutralization of magnesium (Mg) scattered from polycrystalline gold (Au) shows little dependence on temperature. Scattered strontium (Sr) does, however, show a marked temperature dependence of the neutralization that provides clear evidence of electron correlations. The results are discussed in terms of the velocity of the outgoing projectiles.     

Charged particle spectra of palladium thin films during low energy deuterium ion implantation

An effort was made to create cold fusion in thin films of palladium under low energy bombardment with high currents of deuterium ions in a vacuum chamber. Samples were monitored with a silicon surface barrier detector in order to detect any massive energetic reaction products. The films were deposited by sputtering onto smooth silicon substrates, and consisted of approximately 130 Å of palladium sandwiched by thin layers of high chrome stainless steel. These films were bombarded with deuterium ions at an energy of 1.5 keV with a current density of about 0.5 mA/cm². The sample temperature was monitored during implantation, and was subject to some measure of control using a combination of beam heating and cooling with flowing liquid nitrogen. Several counts were observed to appear in the high energy region of the spectra which did not appear during an extended background spectrum, however, the source of these counts is unclear at this time.     

Analysis of the Auger neutralization of He at Cu surfaces in low energy ion scattering

Recently, strong crystal effects in P⁺ were observed for He⁺ and Cu in the Auger neutralization regime, with differences in the ion fraction by up to a factor of three non-equivalent Cu surfaces. In this contribution, it is shown that these findings can quantitatively be described within the jellium model assuming perpendicular velocity scaling of P⁺.     

The use of low energy alkali ion scattering as a probe of surface structure

An overview is given of the use of low energy ion scattering as a probe of surface structure with emphasis on work done using alkali ions. Various schemes for extracting structural information from the ion energy and angle distributions are discussed in terms of advantages and disadvantages of each. The scattering potential, which is the primary nonstructural parameter needed for analysis, is discussed in terms of recent experimental results. The structures of clean and reconstructed surfaces are discussed, with examples of measurements of layer relaxations on the Mo (111) surface and missing row reconstructions on the Au (110) and Pt (110) surfaces. Studies of adsorbate covered surfaces are presented with respect to location of the adsorbate and its effect on the structure of the underlying substrate. Finally, examples are given which demonstrate the sensitivity of ion scattering to surface defects and disordering on reconstructed Au (110) and Pt (110) surfaces and unreconstructed Mo (111) surfaces, and to ordering of adsorbates on Mo (001).     

Nuclear information obtainable from low energy neutron spectroscopy

The results of measurements of thermal-neutron cross sections, level distributions in the resonance region, ratios, and determinations of the effective nuclear radius from potential:.scattering of slow neutrons are considered.A discrepancy in the results of measurements of the thermal-neutron fission cross secticn in U²³⁵ is noted. The distribution of levels exhibits a deficiency in small spacings as compared with a random distribution, i. e., there seems to be a repulsion of levels. The dependence of on atomic number A is nonmonotonic, in accordance with the complex-potential model of the nucleus; however there is a considerable discrepancy between the experimental data and theory for atomic numbers in the neighborhood of 100. The effective radius determined from potential scattering of slow neutrons indicates a nonmonotonic dependence on A, in agreement with the model of a semi-transparent nucleus with smeared-out edges.     

Computer studies of the scattering of low energy hydrogen ions from polycrystalline solids

Reflection of 50 eV to 10 keV H atoms from polycrystalline Cu, Nb and Au targets has been calculated using the binary collision cascade program MARLOWE. The fractions of particles and energy reflected (backscattered) increase with increasing atomic number of the target and decrease with increasing incident energy. The results indicate that the effects of polycrystallinity are modest, reducing the amorphous reflection coefficients by about 25%. The calculations agree quite well with the experimental data for Cu and Au, but are about a factor of two larger than is observed for Nb.     

Low energy ion scattering from the Au(110) surface - structural results

The computer code MARLOWE is used to study the scattering of low energy K⁺ ions from the reconstructed Au(110). A detailed trajectory analysis allows conclusions about the surface structure, i.e. the contraction of the surface.     

在线低能气体离子源

材料中氦和氢积累可引起材料性能的恶化甚至失效。为研究材料内氦和氢的存在形式、氦与氢及缺陷的相互作用、气泡的形成和演变过程以及各种因素的影响,建立一套离子束能量最高20keV的潘宁型气体离子源引出和聚焦系统,与200kV透射电镜联机,在离子注入现场原位观察氦和氢不同注入浓度下材料内部的微观结构及变化过程。对离子源进行氦离子的起弧、引出和聚焦测试。离子源在15–60mA放电电流范围内稳定地工作。在5×10–3Pa和1.5×10–2Pa工作气压下,放电电压约380V和320V。低气压下引出离子束流比高气压下大,且引出束流随放电电流和吸极电压的增加而增加。等径三圆筒透镜有显著聚焦作用,在距透镜出口150cm处,离子束流密度提高一个量级以上。能量10keV左右的氦离子获得束流密度约200nA·cm–2的离子束,可满足多种材料进行在线离子注入和原位电镜观测的需要。     

Monte Carlo simulation of X-ray spectra from low energy electrons using optical data

An approach using optical data to simulate both the bremsstrahlung continuum and characteristic K and L X-ray lines generated by low energy electrons (cfr. electron microscopy) in solids is discussed in this paper. The necessary analytical expressions together with the data to calculate the relevant cross sections for elastic and inelastic interactions at these energies along with variance reduction techniques are given. The results of the Monte Carlo simulation are compared to experimental data measured with a JEOL 6300 electron microscope.     

Mechanism of adatom formation on Ag(1 1 0) studied by combined quasi-elastic He atom scattering and low energy ion scattering

Two complementary techniques, i.e. low-energy ion scattering in the neutral impact collision ion scattering spectroscopy (NICISS) mode and energy resolved He atom scattering (HAS) are combined to study the adatom formation mechanism on Ag(1 1 0). The NICISS spectra have been collected between 300 and 900 K along the 0 0 1, and azimuthal directions while HAS measurements of the diffuse elastic peak has been performed along the directions between 190 and 800 K. Both techniques are sensitive to surface disorder and HAS data have been corrected by using the NICISS results in particular on the anharmonicity estimated through a two atoms scattering model. In fact, above 500 K surface anharmonicity is clearly detected and accompanies the proliferation of thermal induced defects. Without this anharmonic correction the adatom formation energy E_a is too low and similar to the surface diffusion barrier. Instead, including the anharmonicity the estimate, E_a = (0.38 ± 0.03) eV, is in excellent agreement with the predictions of molecular dynamics simulations.     

Polymer processing by a low energy ion accelerator

Ion implantation is a process in which ions are accelerated toward a substrate at energies high enough to bury them just below the surface substrate in order to modify the surface characteristics. Laser-produced plasma is a very suitable and low cost technique in the production of ion sources. In this work, a laser ion source is developed by a UV pulsed laser of about 10⁸ W/cm² power density, employing a C target and a post ion acceleration of 40 kV to increase the ion energy. In this work, we implanted C ions on ultra-high-molecular-weight-polyethylene (UHMWPE) and low-density polyethylene (LDPE). We present the preliminary results of surface property modifications for both samples. In particular, we have studied the modifications of the surface micro-hardness of the polymers by applying the “scratch test” method as well as the hydrophilicity modifications by the contact angle measurements.     

低能离子注入对脱氧胸苷的辐照损伤

用紫外分光光度法测定了脱氧胸苷（ｄＴＲ）在２０ｋｅＶ Ｎ＾＋离子束辐照下残余活性Ｓ和碱基Ｔ释放量的剂量效应。发现Ｓ与样品的质量有关，且受损ｄＴＲ分子中约有５０％可产生碱基Ｔ。而强碱、强酸的后处理使这一比例显著下降，平均分别只有２３％和４１％，与γ辐射时的情况相反。     

低能离子探测器的设计及测试

自行设计制作并测试了用于电子-离子碰撞电离截面测量实验的低能离子探测器。该探测器由Al_2O_3转换板、加速电极、单通道电子倍增器三部分组成。离子沿70°方向轰击转换板,产生数目与离子能量成正比的二次电子,电子由单通道倍增器测量。采用合适的电子学线路和甄别阈,可以有效地把离子计数与本底分开。对10keV Ar~+在入射束流小于10~4s~(-1)时,探测效率接近100％。本探测器可用于低能重离子的计数测量。     

Low energy positron scattering from helium

A new experiment has been developed for high resolution studies of positron scattering from atoms and molecules. Based on the Surko trap technology, a pulsed positron beam has been used to obtain preliminary measurements of low energy, differential elastic scattering cross sections from helium. The operation of the beamline is described and preliminary absolute cross section values for scattering energies of 5, 10 and 15 eV are presented and compared with contemporary theoretical calculations.     

Collision of low energy Na^＋ ion with C60

The collison-induced reaction of Na^ ion with neutral C60 vapour was investigated at the energy range of 10 to 150eV.The deposited films were studied via laser desorption time-of-flight mass spectroscopy(TOFMS).The fragment products of C60 with even number of carbon atoms,such as Cn^ (n=58,56,54),and adduct products,such as Cn^ (n=62,64,66,68) were observed in the positively charged TOF mass spectra.The endohedral fullerene ion of Na @C60^ was detected when collision energies are above 20eV.Besides,no negative endohedral fullerene was detected,the products appearing in positively charged mass spectra were also observed in the negatively charged mass spectra.In addidtion,a series of products with odd number of carbon atoms,such as Cn^-(n=53,55,57,59,61,63,65,67),also appeared.but the intensities of their signals were not as high as that of even numbered.Finally,it is interesting to find that the deposited films are insoluable in toluene,benzene or water.Their Fourier transform infrared (FTIR),spectra seem like those photopolymerized ones.Some of the reaction schemes are proposed to explain the experimental results.     

Calculations for very low energy scattering of positrons by molecular hydrogen

We give a progress report on ongoing calculations of phase shifts for very low energy elastic scattering of positrons by molecular hydrogen, using the generalised Kohn variational method. Further, provisional calculations of Z_eff for molecular hydrogen at low energies are presented and discussed. The preliminary nature of the work is emphasised throughout.     

An ion beam chopping method for low energy time-of-flight measurements

Chopping an ion beam by electrically sweeping it over an aperture produces a degradation of the beam quality. After a short review of the problem, a simple alternative method is presented and experimental results are shown.