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We have developed a method for interpreting and classifying mechanisms from molecular-dynamics simulations. The reason for this effort is that molecular-dynamics simulations contain a wealth of information regarding the microscopic details of atomic motions. It is at best tedious, however, to extract all of the essential information in very complex processes. In particular, we are interested in particle ejection due to the keV-particle bombardment. Our method isolates the motion of the last two collisions, which we depict in a single frame showing the time sequence of the positions of the three atoms involved. The remainder of the atoms are shown at their initial positions. These atoms give the perspective of the entire system but small or irrelevant displacements do not distract from the key motions. This method is utilized to elucidate the structure-sensitive mechanisms of keV-particle bombardment of the I001I face of diamond-lattice crystals. We have observed that two mechanisms are responsible for the major features of the angular distributions, namely, the 6& and the 53 mechanisms. The two mechanisrns are characterized by the difference in the number of layers of the atoms involved in the final momentum transfer.

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The structure of As-terminated Al/GaAs͕001͖-(2ϫ4) has been determined in atomic detail using angleresolved secondary-ion-mass spectrometry. We find an abrupt interface is formed at room temperature by deposition of 0.3 ML of Al onto GaAs͕001͖-(2ϫ4) prepared in situ by molecular-beam epitaxy. The Al atoms are found to adsorb in the troughs between two As 2 dimers, in ordered sites 0.79Ϯ0.10 Å above the surface plane. These dimers maintain their 2.73-Å spacing after Al deposition. The structure is determined from angular distributions of Al ϩ and Ga ϩ ions desorbed by keV ions and computer simulations of the ionbombardment event.