Further aspects of enhanced ion scattering near 180 degrees.
The role of rainbow scattering in enhanced ion scattering near 180{degree} was explored by comparing calculated enhancements for protons and antiprotons. Since the rainbow effect does not occur for negative particles and since the enhancement was much smaller for them than for positive particles, this comparison demonstrated in a more direct way than previously done the importance of rainbow scattering in producing the enhancement. A procedure for calculating a similarity factor between the ingoing and outgoing portions of a trajectory was developed, and its use showed that trajectories returning near 180{degree} differ in nature from random-return trajectories. A discussion is given of the possible relationship of the enhancement to phenomena such as the correlation effect in self-diffusion by the vacancy mechanism in solids and surface-structure studies by impact-collision ion-scattering spectrometry.
New aspects of enhanced ion scattering near 180 degrees.
A new approach to the 180/sup 0/ backscattering phenomenon has been taken by looking at it as a flux peaking effect. A simple model calculation based on the flux peaking point of view shows considerable agreement with experimental results. Because flux peaking in the present circumstances suggests the existence of more than one possible path in reaching a target atom, plots were made of the inward and outward portions of any ion trajectory that emerged from the target within a very small angle (0.025/sup 0/) of the exact backward direction. In these plots we have observed there to be a sizeable contribution to the effect by interchangeable return paths that exist. As a consequence, the usual explanation of the phenomenon as reversibility back along the original ingoing path accounts for only part of the enhancement in a typical case. Also, it appears that all trajectories returning very near 180/sup 0/ are transformed in nature from those that occur at angles well away from the backward direction. Our results demonstrating the existence of interchangeable return paths provide part of the explanation of why the two-atom model predicts too small an effect. Another part of the explanation appears to lie in themore » fact that multiple small-angle collisions with target atoms provide even more focusing or path tolerance than a collision with just a single target atom. The present work has been on random solids but the same mechanism should also act in scattering from either the bulk or the first few atomic layers of a crystalline solid.« less
Near surface yield enhancement in narrow acceptance 180° elastic scattering for He ions in non-crystalline and poly-crystalline solids
Abstract Observations of an unusual near surface ( 0 ⩽ x A ) yield enhancement in the energy distribution of elastically scattered He ions from amorphous and polycrystalline materials is reported. The integrated excess yield for small ( θ 1 2 =0.2° ) versus large ( θ 1 2 =4.5° ) acceptance angle at 180° scattering is typically 20% for He energies in the range 0.2 ⩽ E ⩽2.5 MeV . Maximum peak to normal yield ratios have been observed as high as 60–70%. Materials in which the effect has been observed include amorphous germanium, the glassy metal alloy Nb 40 Ni 60 , fine grain polycrystalline Au, Pt, Mo, and Cu and also in single crystals of Si and Au continuously rotated during data collection to simulate a random lattice. The excess yields are largest for materials of atomic number above Z =40 and decreases rapidly for elements below this value. The angular width and depth dependence of this effect suggest that it is sensitive to multiple scattering and also implies that the particle, upon scattering, must retrace its inward path within a cone of ≲0.2° from depths comparable to or less than a few hundred A.