The influence of thermal vibration on focused collision sequences

Small Au crystals have been bombarded with 43 kev A+ ions and sputtered deposit patterns obtained which show preferential ejection to occur in directions close to the ⟨110⟩ crystal axes. The deposits were analysed to give the mean squared angular deviation from the ⟨110⟩ axis and the effect on this of crystal temperature was studied over the range 80 to 1170°K. The (deviation)2 increased almost linearly with temperature up to 800°K, but above this temperature the increase was more rapid. Consideration is given to several mechanisms which might cause scattering of the focused collision sequences responsible for ejection, and it is concluded that the most important is the lateral displacement of atoms from the focusing line by thermal motion. A theory of the effect is developed which predicts angular deviations in agreement with experiment, provided that the larger vibration amplitudes thought to occur on surface atoms are used. The effect of thermal scattering on the attenuation of focused collision sequences is also considered and range–energy curves calculated. Some implications in the fields of radiation damage and sputtering are considered. It is shown that such scattering renders mass transport by ⟨110⟩ sequences very inefficient. Furthermore, that both the rate of formation of defects by radiation, and their spatial distribution, should be affected by temperature. As a consequence of the shortening of the sequences’ range, a slow decrease in sputtering ratio with temperature is predicted.