We report on experiments in which picosecond ultrasonic techniques are used to investigate the modification of interfacial bonding that results from ion implantation. The bonding is studied through measurements of the acoustic reflection coefficient at the interface. This method is nondestructive and can be used to create a map of the variation of the bonding over the area of the interface.

A velocity spectrum of neutral sputtered particles as well as a low resolution mass spectrum of sputtered molecular ions i" has been measured for 4.74 MeV 19F+2 incident on UF4 . The r velocity spectrum is dramatically different from spectra taken with low energy (keV) bombarding ions, and is shown to be consistent with a hot plasma of atoms in thermal equilibrium inside the target. We propose a "thermalized ion explosion" model for high energy sputtering which is also expected to describe track formation in dielectric materials. The model is shown to be consistent with the observed total sputtering yield and the dependence of the yield on the primary ionization rate of the incident ion. -76-G-03-1305]. ONE OF THE BAND AID PREPRINT SERIES IN ATOMIC 9 APPLIED PHYSICSApril 1980 i INTRODUCTION RIn an earlier paper, 1 we presented data on uranium sputtered from a uranium tetrafluoride target. The beams employed were heavy ions with energies near the peak in the electronic stopping power. Our purpose was to test a prediction by Haff 2 that good insulators may exhibit large sputtering yields associated with track formation when bombarded with energetic heavy ions. Very large shuttering yields were found which could not be explained by standard sputtering theory. 3 Other investigators have seen enhanced sputtering from a number of insulating targets when bombarded under similar conditions. 4-9 Metals, however, do not appear to show the enhanced sputtering effect. 10,11 These data support the suggestion byHaff, that enhanced sputtering due to energ, -ric heavy ion bombardment and track formation arise from the same mechanism. This mechanism is generally thought to be either thermal in nature, 4,7'8'12 with an electron-phonon interaction supplying heat to the lattice atoms, or collisiona1 5,6,13 with an " ion-explosion" leading to a weak collision cascade.In this paper we shall attempt to determine which of the above mechanisms is responsible for enhanced sputtering in UF 4 . An energy spectrum of sputtered particles presented in Reference 1 was fit to a curve of the formThis function is expected to describe collision cascade sputtering typical of low energy ion bombardment. E is the surface binding energy and n is generally close to 3 (Rei-ence 14). However, when fit to our energy spec-19 + trum taken with 4.74 Mei F 2 ion bombardment, a least squares analysis gave values of Eb = 1.2 eV and n = 6.1. The large value of n reflects a very rapid decrease in the yield with increasing energy. This would suggest 1 5 emely weak collision cascade or could be indicative of a thermal I .. t spike. Another time-of-flight (TOF) spectrum which incorporates a number of improvements is presented here. The resolution has been improved by a factor of two, more data were accumulated which decreased the statistical uncertainty, and a voltage drop between the target and detector was used to separate the charged from the neutral particles. This last change also allowed us to separate charged molecular clusters having different mass...

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Summary:We present scanning tunneling microscopy (STM) results that show the superstructure on the highly oriented pryrolytic graphite (HOPG) surface. The superlattice appears to be the result of an apparent twist boundary dislocation. The crucial point of the observation is that the superlattice changes simultaneously its constant from 18 to 72 nm when we go around the screw dislocation in the plane perpendicular to the dislocation vector. Based on available models of the graphite surface top layers, we are able to simulate observed STM images. Our STM observation of the superstructure lattice, whose lattice constant varies continuously, gives strong evidence that the superlattice is of Moiré pattern nature, but a correct model can be built only if the graphite electronic structure is taken into consideration.

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