Fundamental Aspects of Energetic Particle/Solid Interactions

The interaction of keV particles with solids has been characterized by the measurement of the angle and energy distribution of sputtered secondary ions and neutrals. The results are compared to classical dynamics calculations of the ion impact event. Examples using secondary ions are given for clean Ni{001}, Cu{001} reacted with O2, Ni{001} and Ni{7 9 11) reacted with CO, and Ag{111} reacted with benzene. The neutral Rh atoms desorbed from Rh{001} are characterized by multiphoton resonance ionizaton of these atoms after they have left the surface

Energetic Ion Bombardment of Ag Surfaces by C60+ and Ga+ Projectiles

The ion bombardment-induced release of particles from a metal surface is investigated using energetic fullerene cluster ions as projectiles. The total sputter yield as well as partial yields of neutral and charged monomers and clusters leaving the surface are measured and compared with corresponding data obtained with atomic projectile ions of similar impact kinetic energy. It is found that all yields are enhanced by about one order of magnitude under bombardment with the C60+ cluster projectiles compared with Ga+ ions. In contrast, the electronic excitation processes determining the secondary ion formation probability are unaffected. The kinetic energy spectra of sputtered particles exhibit characteristic differences which reflect the largely different nature of the sputtering process for both types of projectiles. In particular, it is found that under C60+ impact (1) the energy spectrum of sputtered atoms peaks at significantly lower kinetic energies than for Ga+ bombardment and (2) the velocity spectra of monomers and dimers are virtually identical, a finding which is in pronounced contrast to all published data obtained for atomic projectiles. The experimental findings are in reasonable agreement with recent molecular dynamics simulations

Direct Comparison of Au3+ and C60+ Cluster Projectiles in SIMS Molecular Depth Profiling

The sputtering properties of two representative cluster ion beams in secondary ion mass spectrometry (SIMS), C60+ and Au3+, have been directly compared. Organic thin films consisting of trehalose and dipalmitoylphosphatidylcholine (DPPC) are employed as prototypical targets. The strategy is to make direct comparison of the response of a molecular solid to each type of the bombarding cluster by overlapping the two ion beams onto the same area of the sample surface. The ion beams alternately erode the sample while keeping the same projectile for spectral acquisition. The results from these experiments are important to further optimize the use of cluster projectiles for SIMS molecular depth profiling experiments. For example, Au3+ bombardment is found to induce more chemical damage as well as Au implantation when compared with C60+. Moreover, C60+ is found to be able to remove the damage and the implanted Au effectively. Discussions are also presented on strategies of enhancing sensitivity for imaging applications with cluster SIMS

Inhibition of HMG CoA reductase reveals an unexpected role for cholesterol during PGC migration in the mouse

<p>Abstract</p> <p>Background</p> <p>Primordial germ cells (PGCs) are the embryonic precursors of the sperm and eggs. Environmental or genetic defects that alter PGC development can impair fertility or cause formation of germ cell tumors.</p> <p>Results</p> <p>We demonstrate a novel role for cholesterol during germ cell migration in mice. Cholesterol was measured in living tissue dissected from mouse embryos and was found to accumulate within the developing gonads as germ cells migrate to colonize these structures. Cholesterol synthesis was blocked in culture by inhibiting the activity of HMG CoA reductase (HMGCR) resulting in germ cell survival and migration defects. These defects were rescued by co-addition of isoprenoids and cholesterol, but neither compound alone was sufficient. In contrast, loss of the last or penultimate enzyme in cholesterol biosynthesis did not alter PGC numbers or position in vivo. However embryos that lack these enzymes do not exhibit cholesterol defects at the stage at which PGCs are migrating. This demonstrates that during gestation, the cholesterol required for PGC migration can be supplied maternally.</p> <p>Conclusion</p> <p>In the mouse, cholesterol is required for PGC survival and motility. It may act cell-autonomously by regulating clustering of growth factor receptors within PGCs or non cell-autonomously by controlling release of growth factors required for PGC guidance and survival.</p

Theoretical studies of ion bombardment: Many-body interactions

Many-body forces obtained by the embedded-atom method are incorporated into the description of low-energy collisions and surface ejection processes in molecular dynamics simulations of sputtering from metal targets. Bombardment of small, single-crystal Cu targets (400–500 atoms) in three different orientations ({100}, {110}, {111}) by 5-keV Ar + ions have been simulated. The results are compared to simulations using purely pairwise additive interactions. Significant differences in the spectra of ejected atoms are found

Angular distributions of ejected particles from ion-bombarded clean and reacted single-crystal surfaces

The expected angular distributions of ejected particles from ion-bombarded clean and reacted single-crystal surfaces are calculated using classical dynamics to model the momentum dissipation. For oxygen atoms adsorbed on a Cu(100) lattice in A-top and fourfold and twofold bridged geometries, preferred ejection angles are found for the higher-kinetic-energy particles (>20 eV). Because of scattering mechanisms which cause atoms to move through gaps in the lattice surface, we find that the patterns for different overlayer registries are easily distinguishable.This work was supported by the National Science Foundation (Grant no. MPS75-9308), the Materials Research Program (Grant No. DMR-77-23798), and the U.S. Air Force Office of Scientific Research (Grant No. AF762974)Portions of the computations were supported by the Foundation Research Program of the Naval Postgraduate School with funds provided by the Chief of Naval Research, and by the Lawrence Berkeley Laboratory supported by the U.S. Department of Energy.Approved for public release; distribution is unlimited