Energy dependence of angular distributions of sputtered particles from the Ag{111} surface

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Abstract
The angular distributions of Ag atoms ejected from a Ag{111} surface due to 2-keV Ar bombardment have been calculated from molecular-dynamics simulations of the particle impact event. For atoms ejected with kinetic energies of above 15 eV, the distributions are found to be sensitive to variations in the ejection energy. The direction along which the maximum sputter intensity is detected alternates between two different 〈211〉 azimuths as the kinetic energy of the ejected atom is increased. A time-exposure representation of the surface-collision cascade is introduced for examining the collision mechanisms that cause atoms to eject along specific crystallographic directions. It shows that, in addition to the presence of the open spacings between the first-layer surface atoms, there are subtle collision effects that determine the preferred direction of ejection. Atomic ejection along those 〈211〉 azimuths which have the non-close-packed row of atoms extending to the bulk is mainly induced by the up-down-, the head-on-, and the sideswipe-collision processes. The up-down-collision process may result in ejection of atoms at a distance of more than 20 Å away from the point of the primary impact. Further, the low-energy atoms are usually emitted from the surface due to sideswipe collisions by the moving surface atoms. The dependence of the preferred direction of ejection on the collision time of the ejected atom is also discussed.