Showing papers by "Nicholas Winograd published in 1990"
TL;DR: It is concluded that the peak in the polar-angle distribution of neutral particles desorbed from ion-bombarded single crystals is mainly determined by the relative positions of surface atoms which influence the trajectory of an exiting particle via channeling and blocking.
Abstract: Energy-resolved angular distributions of Rh atoms desorbed by 5 keV Ar-ion bombardment of the Rh{l brace}100{r brace} surface are measured with use of a multiphoton resonance ionization technique. The results are shown to be in good agreement with molecular-dynamics simulations of the ion-impact event using the same interaction potential optimized previously to describe desorption from Rh{l brace}111{r brace}. In addition, by analyzing contour plots of the surface potential energy, the trend in the experimental results for Rh{l brace}100{r brace} and those previously published for Rh{l brace}111{r brace} are well explained. Based on this analysis, it is concluded that the peak in the polar-angle distribution of neutral particles desorbed from ion-bombarded single crystals is mainly determined by the relative positions of surface atoms which influence the trajectory of an exiting particle via channeling and blocking. Moreover, the anisotropy of the momentum imparted to the surface atoms in the last collision leads to an enhancement of ejection along certain crystallographic directions.
33 citations
TL;DR: The adsorption and thermal decomposition of C 2 H 2 on Rh{111} is compared to the atomically stepped Rh{331} surface over a temperature range of 300 to 800 K using X-ray photoelectron spectroscopy (XPS) as discussed by the authors.
21 citations
TL;DR: In this article, angle-resolved SIMS experiments have been employed to study the bonding geometry of Cl on Ag{001] in the presence of shadow-cone enhanced desorption.
14 citations
TL;DR: In this paper, the angular distribution of ions desorbed from the molecular-beam-epitaxy-grown GaAs{001}-(2\ifmmode\times\else\texttimes\fi{}4) surface by ion bombardment is presented.
Abstract: The angular distribution of ${\mathrm{Ga}}^{+}$ ions desorbed from the molecular-beam-epitaxy-grown GaAs{001}-(2\ifmmode\times\else\texttimes\fi{}4) surface by ion bombardment is presented. This distribution displays the highest degree of anisotropy, relative to the crystal direction of desorbed ions, which has been reported to date. The interpretation of the data is possible using physical arguments based on a simple geometric model of the desorption of ions from the surface. Further insight is provided by comparison to molecular-dynamics simulations of the keV-ion bombardment of metal and semiconductor surfaces. The experimental and calculated distributions of ${\mathrm{Ga}}^{+}$ ions desorbed from the (2\ifmmode\times\else\texttimes\fi{}4) surface are in reasonable qualitative agreement. The results indicate that the extreme anisotropy in the angular distribution results from a direct mechanism wherein a third-layer As atom collides with a second-layer Ga atom and thereby causes the Ga atom to eject along their mutual bond axis. This mechanism has been observed previously on Si and GaAs surfaces but is not commonly observed on metal surfaces, and can be ascribed to the directional bonding and open structure of covalent crystals. Other features of the angular distribution are related to blocking and channeling of the desorbed ${\mathrm{Ga}}^{+}$ ions. These features indicate that there is one, and only one, missing row of ${\mathrm{As}}_{2}$ dimers for every 16 \AA{} unit-cell length along the 4\ifmmode\times\else\texttimes\fi{} crystal direction. These results provide complementary information which is in excellent agreement with other studies of the GaAs{001}-(2\ifmmode\times\else\texttimes\fi{}4) surface.
13 citations
TL;DR: In this article, the authors used multiphoton resonance ionization spectroscopy to determine the polar-angle and the kinetic-energy distributions of rhodium atoms desorbed from ion-bombarded Rh{100} surface in the fine-structure components of a 4Fj (J=9/2 and 7/2) ground-state multiplet.
Abstract: Multiphoton resonance ionization spectroscopy has been used to determine the polar-angle and the kinetic-energy distributions of rhodium atoms desorbed from ion-bombarded Rh{100} surface in the fine-structure components of the a 4Fj (J=9/2 and 7/2) ground-state multiplet. The peak in the energy distribution of the metastable level (4F7/2 with excitation energy of 0.2 eV) is found to occur roughly at the same value as the ground-state (4F9/2) distribution but decays more gradually at higher energies. The measured spectra have been used to investigate the dependence of the excitation probability on the takeoff angle (θ) as well as the emission velocity (v). It is shown that the excitation probability depends strongly on these parameters, approaching an exponential dependence on l/[v cos(θ)] at higher velocities (> 5×l05cm/sec).