Angular distributions of reflected MeV light ions from a single crystal surface at glancing incidence

The angular distributions of ions reflected from a SnTe(001) surface are measured for a glancing angle incidence of MeV protons and He⁺ ions, with the glancing angle less than 7 mrad. The observed angular distributions are compared with the calculated distributions derived from the stochastic theory taking account of both the thermal vibration of surface atoms and the scattering by electrons near the surface. From comparison of the experimental and calculated results the surface Debye temperature is determined to be 72 ±6 K.     

Interaction of fast hydrogen ions with silicon surface at glancing angles of incidence

Angular distributions of 380 keV protons reflected from (1 1 1) surface of Si monocrystal were measured in the range of projectiles glancing angle from 0.3° up to 0.8°. It is shown that increase of glancing angle causes non-linear change of such distribution parameters as angular width of the front rise, angular width of the distribution, the maximum yield value. Registered energy spectrum of reflected particles for glancing angle of 0.5° consists of several peaks with practically constant angular intervals between them and maxima weakly reducing towards lower energy region. It is experimentally shown that the most energetic peak relates to the reflection from the very surface and the rest ones are caused by successive scattering of ions by inner silicon crystallographic planes.     

Small angle scattering of MeV protons at glancing incidence on Au and Ag targets

Energy spectra of backscattered protons from a beam of monoenergetic MeV protons incident on polycrystalline Au and Ag targets at glancing angles have been measured for incident and scattering angles less than 4.5°. The experimental energy spectra of scattered protons and the dependence of the proton yield on the scattering angle were compared with the theory of multiple small angle scattering developed by Remizovich et al., and good agreement between the experimental and theoretical results was found at small scattering angles.     

Theoretical study of swift molecular ions specularly reflected from solid surfaces under glancing angle of incidence

We develop a theoretical model to study grazing scattering of fast diatomic molecular ions from a solid surface, based on the dielectric response formalism within the specular reflection model, where the plasmon pole approximation for dielectric function is employed to describe the single-particle and the collective excitations of the electron gas at the surface. Evolution of the bound-electron densities at the constituent ions of a molecule in the course of scattering is described by an approach similar to recent implementation of the Brandt–Kitagawa model for single-ion surface grazing scattering. We solve numerically the equations of motion for the constituent ions and obtain the ion scattering trajectories in the presence of Coulomb explosion modified by the surface wake potential, for the initial molecular-axis orientations in either random directions or along the beam. Vicinage effect on the total energy loss is discussed on the basis of analyzing the position-dependent stopping powers of individual ions and the interferences in the electron excitations of the substrate.     

Study of ion emission from a germanium crystal surface under impact of fast Pb ions in channeling conditions

A thin germanium crystal has been irradiated at GANIL by Pb beams of 29 MeV/A (charge state Q_in = 56 and 72) and of 5.6 MeV/A (Q_in = 28). The induced ion emission from the sample entrance surface was studied, impact per impact, as a function of Q_in, velocity v_in and energy loss ΔE in the crystal. The Pb ions transmitted through the crystal were analyzed in charge (Q_out) and energy using the SPEG spectrometer. The emitted ionized species were detected and analyzed in mass by a time-Of-flight multianode detector (LAG). Channeling was used to select peculiar ΔE values in Ge and hence peculiar Pb ion trajectories close to the emitting entrance surface. The experiment was performed in standard vacuum. No Ge emission was found. The dominating emitted species are H⁺ and hydrocarbon ions originating from the contamination layer on top of the crystal. The mean value 〈M〉 of the number of detected species per incoming Pb ion (multiplicity) varies as (Q_in/v_in)^p, with p values in agreement with previous results. We have clearly observed an influence of the energy deposition ΔE in Ge on the emission from the top contamination layer. When selecting increasing values of ΔE, we observed a rather slow increase of 〈M〉. On the contrary, the probabilities of high multiplicity values, which are essentially connected to fragmentation after emission, strongly increase with ΔE.     

Charge state distributions of heavy ions after scattering at surface atoms

The charge state distributions (CSDs) of Li, C and F ions after a single collision with Au atoms deposited as a submonolayer coverage on Si were measured at energies below 500 keV/amu and compared to the equilibrium CSDs. The influence of the surface on the charge state of the outgoing ions is demonstrated for Li ions at 3 MeV. It is found that the CSD after a single collision is generally different from the equilibrium distribution.     

Negative ion formation during scattering of fast ions from diamond-like carbon surfaces

Fractions of negative ions after grazing scattering of fast hydrogen and oxygen atoms and ions from diamond-like carbon (DLC) surfaces are studied as function of projectile velocity. We reveal a pronounced kinematic resonance behaviour for the negative ion fractions as function of projectile velocity which provides important information on the charge transfer mechanism. The conversion of neutral atoms into negative ions during scattering from DLC surfaces is used for the detection of energetic neutral atoms in space.     

Proton sputtering from polycrystalline Al surface interacting with highly charged ions at grazing incidence angle

Sputtering processes of protons from a polycrystalline Al surface interacting with Ar^q+ (q = 3-14) ions at a grazing incidence angle (∼0.5°) were investigated. The intensity of protons (I_H) detected in coincidence with scattered Ar atoms was measured as a function of q. I_H saturated at q ? 10, although it increased rapidly with q at 3 ? q ? 8. The angular distribution of protons with low kinetic energy (?2 eV) began to deviate from the cosine distribution and assumed a rather flat equidistribution as q increased. To analyze the sputtering processes of protons at the grazing incidence angle, a modified model of the “above-surface potential sputtering model” was proposed by considering image acceleration of projectile ions.     

Post collision stark-interaction after excitation of fast ions by a solid surface at grazing incidence

The anisotropic distribution of the angular momenta of atoms or ions excited by the ion beam surface interaction at grazing incidence is modified by post collision Stark-interaction due to the electric field at the surface. By studying the effect of the Stark-mixing on the anisotropy in dependence of the time the atom spent in the electric field conclusions can be drawn with respect to the nature of the field and the excitation process.     

Grazing scattering of 1-2 MeV HeH ions from KCl(0 0 1): Effect of surface track potential

Three dimensional (3D) distributions (energy E, scattering angle θ and azimuth angle φ) of the fragment protons dissociated from HeH⁺ during grazing angle scattering from a KCl(0 0 1) are measured using a magnetic spectrometer in order to study the effect of the surface track potential. The distributions of the fragment protons scattered from a SnTe(0 0 1) are also measured as a reference. Although the observed distributions for KCl(0 0 1) and SnTe(0 0 1) are basically the same, there is small differences, especially in the scattering angle distribution. While the fragment protons are scattered at the specular angle from SnTe(0 0 1), the protons are scattered at slightly larger angles from KCl(0 0 1). The observed angular shift is more pronounced for the trailing protons than the leading protons. It is also found that the angular shift increases with decreasing ion energy. The observed angular shift can be qualitatively explained by the surface track potential induced by the partner He ions using a simple model of the surface track potential.     

Position-dependent charge-exchange and energy-loss processes for MeV He ions near a crystal surface

Inelastic interaction of fast ions at glancing angles of incidence on a low index crystal surface is described by the probabilities, which depend on the distance of the ion from the surface; the charge state and energy distributions of the scattered ions are found to result from position-dependent interactions along their trajectories in vacuum. A stochastic equation to describe the charge exchange and energy loss of specularly reflected ions is proposed. Taking account of ion scattering at surface steps, the theory is applied to derive the position-dependent probabilities of electron loss and electron capture of MeV ions from the observed charge state and energy distributions of the scattered ions at glancing angle incidence on the (100) surface of SnTe.     

Elastic scattering cross-section of proton from helium at the laboratory angle of 165°

The differential He(p,p)He reaction cross-section was determined at a laboratory angle of 165° for proton energies between 1.6 and 3.6 MeV. A new method for preparation of helium-titanium (Ti) targets by direct current (DC) magnetron sputtering was employed for the first time. The currently measured cross-section data are compared with the available literature values. They show a similarly trend in the higher energy range but with a maximum difference of up to 12% in the absolute value. The uncertainty in the present cross-section data is about 7.3%.     

Nucleus-nucleus bremsstrahlung from scattering angle selected Cl-Sn collisions

Continuum X-rays above the K lines of Sn were measured at 90° and 0° angle relative to the incident beam direction in coincidence with Cl projectiles scattered at very large laboratory angles between 8° and 164°. By varying the isotope mass of the projectile ions (³⁵Cl and ³⁷Cl) and of the target atoms (¹¹²Sn and ¹¹⁸Sn) the radiation at scattering angles > 10° could be clearly identified as bremsstrahlung from the elementary scattering process of the two nuclei. The intensity of the radiation emitted at 0° and 90° relative to the beam direction can be reasonably well described by a quantum mechanical calculation of nucleus-nucleus bremsstrahlung by Reinhardt et al. [7] which includes the interference between dipole and quadrupole radiation amplitude.     

小角X射线散射方法测定结晶聚合物的平均过渡层厚度

一般用一维电子密度相关函数来计算结晶聚合物的过渡层厚度。由于结晶聚合物中的过渡层是弥散的，修正的Porod定律是解决这种问题的有效方法之一。本文将用相关函数法和Po-rod定律法求出的结晶聚合物的过渡层厚度进行了比较，它们的计算结果是一致的。     

Skipping motion of 30 keV H ions on single crystal surface of KCl

The angular distributions and the energy spectra of scattered H⁺ ions are studied at glancing angle incidence of 30 keV H⁺ ions on the (001) surface of KCl single crystal. Discrete energy loss spectra of the scattered ions are observed. The observed exit angle distributions of the ions which have lost anomalous large amounts of energy at various angles of incidence do not depend on the angle of incidence. From the observed results, it is concluded that skipping motion occurs on the surface of KCl.     

X-ray yields from high-energy heavy ions channeled through a crystal: their crystal thickness and projectile dependences

X-rays emitted from Ar¹⁷⁺, Fe²⁴⁺ and Kr³⁵⁺ ions of about 400 MeV/u transmitting through a thin Si crystal of about 20 μm thickness have been measured in a planar channeling condition and compared with those in a random incident condition. We have found that the X-ray yield from Ar¹⁷⁺ ions is larger for the channeling condition than for the random incidence, while those from Fe²⁴⁺ and Kr³⁵⁺ ions are rather smaller. Such tendencies are explained by considering the projectile dependences of excitation and ionization probabilities together with X-ray emission rates. A crude simulation has qualitatively reproduced these experimental results. When the crystal thickness is small, the X-ray yield is smaller in the channeling condition than in the random incident condition, because excitation is depressed. However, for thicker crystals, the X-ray yield is larger, since the survived population of projectile-bound electrons is larger due to small ionization probabilities under the channeling condition. This inversion occurs at a specific crystal thickness depending on projectile species. Whether the thickness of the used crystal is smaller or larger than the inversion thickness determines enhancement or depression of the X-ray yield in the channeling condition.