Interferences in coherent electron emission from diatomic molecules

Recent studies of electron emission from molecular hydrogen by the impact of fast ions have shown the existence of interference effects. The interferences are manifested as oscillations in the velocity (or energy) distributions of the ejected electrons, and are analogous to the interference of light in Young’s two-slit experiment. The frequencies of the oscillatory structures depend strongly on the electron observation angle and to a lesser extent on the collision velocity. Additionally, secondary oscillations with ∼2-3 times higher frequencies attributed to scattering of the primary electron “wave” at the other atomic center are found to be superimposed on the primary oscillations. More recently, electron interference studies have focused on diatomic molecules more complex than H₂, including N₂ and O₂, for which only structures due to secondary interferences are apparently observed. Here, these various results are reviewed, outstanding questions identified, and future directions indicated.     

Density effects on the guided transmission of 3 keV Ne ions through PET nanocapillaries

Experiments for guided transmission of 3 keV Ne⁷⁺ ions through nanocapillaries in insulating PET polymers are reported. The ion guiding was studied for a two types of PET samples which consist of 200 nm capillaries with densities of and . The width of the emission profile and the fraction of transmitted ions were measured as a function of the capillary tilt angle. For the high capillary density the profile width of the transmitted ions is independent of the tilt angle in agreement previous studies. However, for the low-density sample the profile width was found to increase by a factor of 2 as the tilt angle increases from 0° to 8°. The results for the fraction of transmitted ions are used to evaluate the guiding angle, which specifies the guiding power of the material. The guiding powers were found to be equal for the two samples. The present results are discussed in terms of scaling laws, which have recently been established.     

Secondary electron yield of Au and Al2O3 surfaces from swift heavy ion impact in the 2.5-7.9 MeV/amu energy range

We report on the secondary electron yields of Au and oxidized aluminum (Al₂O₃) by impact of heavy ions with energies ranging from 7.92 MeV/amu (¹²C₆) to 2.54 MeV/amu (¹⁰⁷Ag₄₇). The obtained results, the first in this energy range using medium-heavy ions, extend the validity of proposed scaling laws obtained with lighter ions. Measurements have been performed using the SIRAD irradiation facility at the 15 MV Tandem of the INFN Laboratory of Legnaro (Italy), to evaluate the performance of ion electron emission microscopy at SIRAD.     

Time evolution of ion guiding through nanocapillaries in a PET polymer

We measured the time evolution of the guided transmission of 3 keV Ne⁷⁺ ions through nanocapillaries in insulating PET polymers. Capillaries with a diameter of 200 nm and a density of 4 × 10⁶ cm⁻² were used. The angular distribution of the transmitted ions was measured as a function of the charge deposited on the sample surface, which is a measure of time. The evolution of the angular transmission profiles was acquired for different tilt angles ranging from 0° to 5°. The transmission profiles appear as a superposition of essentially 3 localized peaks which exhibit significant changes in intensity as time varies. This observation provides evidence for the formation of temporary charge patches produced in the interior of the capillary besides the primary charge patch created in the entrance region.     

L-subshell ionization of heavy elements by S ions with energy of 0.4-3.8 MeV/amu

The L-shell X-ray production cross sections have been measured for sulphur ions in the energy range of 12.8-120 MeV for Au and Bi elements. The experimental L X-ray spectra were analyzed using the method that takes into account the multiple ionization in outer shells. The L-subshell ionization cross sections have been obtained from measured X-ray production cross sections for resolved Lα_1,2 Lγ₁ and Lγ_2,3 transitions using the L-shell fluorescence and Coster-Kronig yields modified by the multiple ionization effects in the M and N shells. The results are compared with the predictions of ECUSAR theory, which is the modified ECPSSR approach describing both direct ionization and electron-capture processes and the semiclassical approximation (SCA) calculations for direct ionization. These approaches were modified by the L-subshell coupling effects within the “coupled-subshell model” (CSM). Both modified approaches are in good agreement with the data. Remaining discrepancies are discussed in terms of the L-shell decay rates modified for the multiple ionization effects.     

K-shell absorption jump factors and jump ratios in elements between Tm (Z = 69) and Os (Z = 76) derived from new mass attenuation coefficient measurements

The K-shell absorption jump factors and jump ratios were derived from new mass attenuation coefficients measured using an energy dispersive X-ray fluorescence (EDXRF) spectrometer for Tm, Yb elements being Tm₂O₃, Yb₂O₃ compounds and pure Lu, Hf, Ta, W, Re and Os. The measurements, in the region 56-77 keV, were done in a transmission geometry utilizing the K_α1, K_α2, K_β1 and K_β2 X- rays from different secondary source targets (Yb, Ta, Os, W, Re and Ir, etc.) excited by the 123.6 keV γ-photons from an ⁵⁷Co annular source and detected by an Ultra-LEGe solid state detector with a resolution of 150 eV at 5.9 keV. Experimental results have been compared with theoretically calculated values. The measured values of Tm, Yb, Lu, Hf, Ta, W, Re and Os are reported here for the first time.     

Fully differential cross section for single ionization of helium by 1 KeV electrons in the eikonal approximation

We report new results for fully differential cross sections for the single ionization of helium by 1 KeV electron impact at the ejection energy of 10 eV. Investigations have been made for both the scattering and perpendicular planes. The present calculation is based on the eikonal approximation due to Glauber. Here we have also incorporated the effect of classical post collision interaction (PCI) in the Glauber approximation (GA). A comparison is made of the present calculation with the results of other theoretical methods and the recent experiment [M. Dürr, C. Dimopoulou, B. Najjari, A. Dorn, K. Bartschat, I. Bray, D.V. Fursa, Z. Chen, D.H. Madison, J. Ullrich, Phys. Rev. A 77 (2008) 032717]. The Glauber results are in good agreement with the experiment in the scattering plane, but strong discrepancies are observed in the plane perpendicular to it. The effect of PCI is not substantial in the present kinematics.     

Determination of K shell absorption jump factors and jump ratios in the elements between Tm(Z = 69) and Os(Z = 76) by measuring K shell fluorescence parameters

The K shell absorption jump factors and jump ratios have been measured in the elements between Tm (Z = 69) and Os(Z = 76) without having any mass attenuation coefficient at the upper and lower energy branch of the K absorption edge. The jump factors and jump ratios for these elements have been determined by measuring K shell fluorescence parameters such as the total atomic absorption cross-sections, the K_α X-ray production cross-sections, the intensity ratio of the K_β and K_α X-rays and the K shell fluorescence yields. We have performed the measurements for the calculations of these values in attenuation and direct excitation experimental geometry. The K X-ray photons are excited in the target using 123.6 keV gamma-rays from a strong ⁵⁷Co source, and detected with an Ultra-LEGe solid state detector with a resolution 0.15 keV at 5.9 keV. The measured values have been compared with theoretical and others’ experimental values. The results have been plotted versus atomic number.     

K X-ray emission induced by C and O ion impact on selected lanthanoids

Measurements of K-shell X-ray production cross sections by ¹²C⁴⁺ (beam energies between 12 MeV and 14 MeV), and ¹⁶O⁵⁺ ions (with energies between 12.5 MeV and 15 MeV) are presented. The target elements were selected lanthanoids (Ce, Gd, Dy, Ho and Er). The resulting measurements are evaluated through comparisons with the eECPSShsR-UA theory, the MECPSSR model and the adiabatic perturbation (also known as direct molecular orbital, MO) theory, using a scaling based on the reduced velocity parameter . Consideration is given to multiple ionization effects and electron capture contribution to K-shell ionization. An evaluation with previously published values is also given. It is shown that the behavior of the ratios of experimental to theoretical cross sections is different for both ions. The models do not seem to be accurate to predict the X-ray production cross sections for ¹²C⁴⁺ ions, while the MECPSSR theory predicts much better the experimental data for ¹⁶O⁵⁺ than the eECPSShsR-UA.     

Measurement of K-L radiative vacancy transfer probabilities in selected rare earth elements bombarded with 3-4 MeV protons

K-shell X-ray intensity ratios for selected rare earth elements were measured following irradiation with proton beams having energies between 3 and 4 MeV. Using the X-ray intensity ratios, the radiative vacancy transfer probabilities from the shell K to the L sub-shells were determined. The experimental data were compared to theoretical predictions for X-ray line intensities and radiative vacancy transfer probabilities. The results showed a good agreement between theory and experiment.     

Molecular dynamics simulations of C2, C2H, C2H2, C2H3, C2H4, C2H5, and C2H6 bombardment of diamond (1 1 1) surfaces

The sticking and erosion of C₂H_x molecules (where x=0-6), at 300 and 2100 K onto hydrogenated diamond (1 1 1) surfaces was investigated by means of molecular dynamics simulations. We employed both quantum-mechanical and empirical force models. Generally, the sticking probability is observed to somewhat increase when the radical temperature increases and strongly decrease with increasing number of H atoms in the molecule.     

Stopping force of 0.5-3.5 MeV/u Cl ions in polymers

The stopping force of Cl ions in LR-115 and Kapton polymers, at energies 0.5-3.5 MeV/u, has been measured. These measurements have been performed using 15UD Pelletron accelerator facility at Inter University Accelerator Centre, New Delhi, India. The measured stopping force values have been compared with the corresponding computed values based on the semi-empirical formulations viz. Ziegler et al., Paul and Schinner, Hubert et al., Diwan et al., and standard data tables viz. Northcliffe and Schilling, ICRU-73 report. The aim of the present study is to measure stopping force around Bragg’s peak and to check the compatibility of the widely used semi-empirical formulations and standard data tables through comparison with the experimentally measured values.     

Stopping of 0.3-1.2 MeV/u protons and alpha particles in Si

The stopping cross sections ε(E) of silicon for protons and alpha particles have been measured over the velocity range 0.3-1.2 MeV/u from a Si//SiO₂//Si (SIMOX) target using the Rutherford backscattering spectrometry (RBS) with special emphasis put on experimental aspects. A detection geometry coupling simultaneously two solid-state Si detectors placed at 165° and 150° relative to each side of the incident beam direction was used to measure the energies of the scattered ions and determine their energy losses within the stopping medium. In this way, the basic energy parameter, E_x, at the Si/SiO₂ interface for a given incident energy E₀ is the same for ions backscattered in the two directions off both the Si and O target elements, and systematic uncertainties in the ε(E) data mainly originating from the target thickness are significantly minimized. A powerful computer code has been elaborated for extracting the relevant ε(E) experimental data and the associated overall uncertainty that amounts to less than 3%. The measured ε(E) data sets were found to be in fair agreement with Paul’s compilation and with values calculated by the SRIM 06 computer code. In the case of ⁴He⁺ ions, experimental data for the γ effective charge parameter have been deduced by scaling the measured stopping cross sections to those of protons crossing the same target with the same velocity, and compared to the predictions of the SRIM 06 computer code. It is found that the γ-parameter values generated by the latter code slightly deviate from experiment over the velocity region around the stopping cross section maximum where strong charge exchanges usually occur.     

Ion induced high energy electron emission from copper

We present measurements of secondary electron emission from Cu induced by low energy bombardment (1-5 keV) of noble gas (He⁺, Ne⁺ and Ar⁺) and Li⁺ ions. We identify different potential and kinetic mechanisms and find the presence of high energetic secondary electrons for a couple of ion-target combinations. In order to understand the presence of these fast electrons we need to consider the Fermi shuttle mechanism and the different ion neutralization efficiencies.     

Be AMS measurements at low energies (E < 1 MeV)

The longlived isotope ¹⁰Be is of great importance in earth sciences for dating applications, reconstruction of the solar activity or in climate research. Routine AMS measurements with BeO samples are performed on accelerators with a terminal voltage above 2 MV. Applying the degrader foil technique for boron suppression, first tests with BeO samples on the 0.6 MV ETH/PSI machine were limited by background to a ¹⁰Be/⁹Be ratio of 10⁻¹³. The background was identified as ⁹Be which reaches the detector by scattering processes. By applying an additional magnetic mass filter to the high energy mass spectrometer the background was effectively removed. A ¹⁰Be/⁹Be background ratio of <5 × 10⁻¹⁵ was achieved. The overall efficiency (detected ¹⁰Be compared to BeO⁻ injected into the accelerator) was 7-8%.     

Energy loss straggling in carbon foils by 1-2 MeV/atom lithium and carbon cluster-ions

Energy loss straggling of lithium and carbon cluster-ions with 1-2 MeV/atom energies incident upon thin carbon foil were measured to investigate the difference of energy loss straggling between a single ion and cluster-ions. The results show that the enhancement of the energy loss straggling is observed at lower energies for two different cluster-ions in a 5 μg/cm² carbon foil. For a thicker sample, i.e. a 15 μg/cm² carbon foil, the ratio of cluster-ions to the one of a single atom at equivalent energies lies slightly close with the unity. As increasing the number of the cluster-ions, no enhancement of energy loss straggling was observed in this study.     

Irradiation effects in an HfO2/MgO/HfO2 tri-layer structure induced by 10 MeV Au ions

In this report, we present radiation damage effects in a thin film, tri-layer structure, HfO₂/MgO/HfO₂. Irradiations were performed with 10 MeV Au ions in a recently developed medium energy ion irradiation facility at Los Alamos National Laboratory, which is described in this paper. Energy deposition by 10 MeV Au ions corresponds to a mixed regime, wherein electronic and nuclear stopping contribute to radiation damage. In this study, we investigated modifications of both surface and bulk properties in order to assess the structural stability of our oxide tri-layers under the severe irradiation conditions employed here. The most dramatic structural changes were observed to occur on the surfaces of the tri-layer samples. Surface features consisted of large craters and spires. The dimensions of these craters and spires exceed those of the individual ion tracks by almost three orders of magnitude. As for the bulk tri-layer structure, our conclusions are that this structure is stable in terms of: (i) resistance to amorphization; (ii) resistance to compositional mixing and (iii) resistance to pronounced nucleation and growth of extended defects. The main effect observed in the tri-layer structure was the transformation of the first HfO₂ layer from a monoclinic to either a tetragonal or cubic form of HfO₂.     

Information about TS-1 and TS-2 double ionization mechanisms for positron and electron impact ionization of argon

Ejected electron angular distributions are measured for single and double ionization of argon by 500 eV positron and electron impact. Double to single ionization ratios show marked differences as a function of projectile charge. Combinations of the positron and electron data and a simple double ionization model are used to obtain differential information about the TS-1 and TS-2 mechanisms. Our analysis suggests that both mechanisms contribute roughly equally to the emission of two electrons and that interference between the two mechanisms significantly alters the ejected electron spectra.