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⁺.     

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.     

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.     

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.     

Effect of energy loss in the simulation of slow ion scattering by a solid surface

As previously shown [B. Arezki, Y. Boudouma, P. Benoit-Cattin, A.C. Chami, C. Benazeth, K. Khalal, M. Boudjema, J. Phys.: Condens. Mat. 10 (1998) 741, K. Khalal-Kouache, A.C. Chami, M. Boudjema, P. Benoit-Cattin, C. Benazeth, Y. Boudouma, Nucl. Instr. and Meth. B 183 (2001) 279], energy spectra of scattered particles from amorphous solid surface can be deduced from angular distributions of the total path length of the projectiles. In preceding papers [K. Khalal-Kouache, A.C. Chami, M. Boudjema, P. Benoit-Cattin, C. Benazeth, Y. Boudouma, Nucl. Instr. and Meth. B 183 (2001) 279, K. Khalal-Kouache, A. Mekhtiche, A.C. Chami, M. Boudjema, Surf. Coat. Technol. 201 (2007) 8420], a model based on the transport theory was used to simulate the scattering of slow ions by a solid target. Angular distributions of the total path length were computed and compared, in absolute value, to those simulated using a Monte Carlo code. A good agreement was obtained except for the highest values of the total path length L. In the transport theory, the variation of the scattering cross section versus energy during the slowing down of the particle was not taken into account. The purpose of this contribution is to take into account this effect within the model. The obtained results are found to be in agreement with Monte Carlo computation for angular distributions of the total path length and for energy spectra of the scattered ions.     

低能离子溅射引起表面损伤的研究

本文应用背散射沟道技术研究了低能离子轰击不同晶向表面所引起的晶格损伤与离子能量、剂量的关系,讨论了损伤形成的动力学过程。     

High negative ion yield from light molecule scattering

Molecular oxygen and hydrogen ions were scattered at grazing incidence from several diamond-like carbon (DLC) surfaces in the energy range from 190 eV to 2400 eV. Most surfaces were hydrogen terminated. For incident positive oxygen ions, scattered negative ion fractions of up to 33% were recorded, and for incident positive hydrogen ions, negative ion fractions of more than 5% were measured. These values are among the highest ever reported, especially for oxygen. They have been matched only by results of scattering experiments using a hydrogen terminated surface of natural diamond.     

Determination by Raman scattering of the in depth damage profile in high energy ion implantation

Ion implanted GaAs samples have been chemically etched to form a small angle beveled surface. First order Raman spectroscopy has been performed along the bevel of as-implanted samples bombarded with 2.3 MeV Be and 10.9 MeV Se ions. The phonon confinement interpretation [H. Richter et al., Solid State Commun. 39 (1981) 625] has been employed to fit the spectra and obtain the damage distributions. Comparisons with X-ray diffraction (XRD) and RBS-channeling (RBS) measurements show good agreement with regard to the basic shape and depth of the profiles. The Be-implanted sample shows a damaged layer at a depth of 2.5 to 4 μm while the Se implant shows a much wider damaged layer at a depth between 1 and 3.5 μm.     

Irradiation effect on PET surface using low energy argon ion beam

A study of various physical properties of a PET film irradiated with an Ar beam is reported. SEM images and UV–VIS and FTIR spectra were obtained for a number of ionic fluxes and three irradiation times. Small changes in the energy gap of degradated samples were found, and the SEM images indicate that the optimum homogeneity and roughness are reached after 30 min of irradiation. These results may well be of practical interest. A modified saddle field ion source was used as a preparation tool of the surface of polyethylene terephthalate PET polymer substrate to be ready for coating or thin film deposition. Argon ion beam was used for this purpose, where the scanning electron microscope (SEM) shows that, the best sample is the one which was irradiated to 30 min, where this sample is more homogenous and roughness than other irradiated samples. Also the (UV–VIS) spectrum tells us that, there is small change on energy gap and this is meaning that, the change on electric properties is small also. In this case the sample is more homogenous and of higher roughness than other irradiated samples.     

Influence of surface morphology on the low energy hydrogen ion erosion yields of beryllium

It has been found that the surfaces of both vacuum-deposited and hot-pressed beryllium develop closely spaced, high angle cones under extended low energy (1000 eV) hydrogen ion bombardment. This needlelike eroded surface morphology results in a lowering of the 1000 eV hydrogen ion erosion yield by about 30–40% compared to the uneroded material. Measured erosion yields for both hot-pressed and vacuum-deposited beryllium are: Be (virgin, 1000 eV) = ~0.04 atoms/ ion; Be (eroded, 1000 eV) = 0.027 atoms/ion; Be (virgin, 250 eV) = ~0.011 atoms/ion; Be (eroded, 250 eV) = 0.0091 atoms/ion.     

Annular gas ionization detector for low energy heavy ion backscattering spectrometry

A gas ionization chamber for use in backscattering spectrometry has been built. It has the shape of a hollow cylinder and can be placed in-line with the incident ion beam. The entrance window for detected particles is composed of a circular array of silicon nitride membranes. A low noise preamplifier with cooled FET is used for charge amplification. The detector resolution has been measured for a variety of ions in the mass range from He to Si and for energies between 0.5 and 8 MeV. The energy resolution of the ionization chamber surpasses the one of a state-of-the-art silicon charged particle detector for all ions heavier than Li. For Si ions the improvement in resolution is more than a factor of 2. The device does not suffer from any radiation damage. For He particles around 1 MeV the resolution is between 13 and 16 keV (FWHM). Therefore the new detector is not only well suited for heavy ion backscattering spectrometry but can also be applied for standard He RBS, allowing the use of a single detector for all types of projectiles in a wide energy range.