Samir J. Anz

The Higher Fullerenes: Isolation and Characterization of C76, C84, C90, C94, and C70O, an Oxide of D5h-C70.

The toluene extract of the fluffy carbon material produced by resistive heating of graphite contains a variety of molecules larger than C60 and C70 in a total amount of 3 to 4% by weight. Repeated chromatography of this material on neutral alumina has led to the isolation of stable solid samples of C76, C84, C90, and C94. The characterization, which includes mass spectrometry, 13C nuclear magnetic resonance, electronic absorption (ultraviolet/visible) and vibrational (infrared) spectroscopy identifies these all-carbon molecules as higher fullerenes. In addition, C70O, a stable oxide, has been isolated that is structurally and electronically closely related to D5h-C70. This compound forms during the resistive heating process and probably has an oxygen atom inserted between two carbon atoms on the convex external surface of the C70 skeleton.

Atomic Force Microscope Studies of Fullerene Films: Highly Stable C60 fcc (311) Free Surfaces

Atomic force microscopy and x-ray diffractometry were used to study 1500 �-thick films of pure C60 grown by sublimation in ultrahigh vacuum onto a CaF2 (111) substrate. Topographs of the films did not reveal the expected close-packed structures, but they showed instead large regions that correspond to a face-centered cubic (311) surface and distortions of this surface. The open (311) structure may have a relatively low free energy because the low packing density contributes to a high entropy of the exposed surface.

A versatile time-of-flight ion scattering and recoiling spectrometer system

Abstract We have enhanced our existing ion scattering apparatus to perform time-of-flight ion scattering and recoiling experiments. The ion source is capable of producing almost any type of ion at energies up to 12 keV. The beam is chopped to deliver effective pulse widths as short as 40 ns, although under normal conditions pulse widths closer to 75 ns are used for data collection. Both the sample manipulator and the entire scattering chamber can be translated and rotated to ensure that the sample and detectors are properly aligned with respect to the ion beam. There are two detection legs, one for backscattered ions and one for forward scattered ions and recoils. A free flight region located before each microchannel plate detector allows the separation of charged and neutral yields that scatter or recoil from the sample surface. We discuss a method to transform the raw time-of-flight distributions to energy distributions without knowing the actual energies of the scattered ions, and we present some recent data showing the sensitivity, versatility and various implementations of the instrument.

Detection of millimonolayers of oxygen on GaAs(001): observation of edge-effect rainbow enhancement in impact collision ion-scattering spectroscopy

Abstract Time-of-flight impact collision ion-scattering spectroscopy (TOF-ICISS) data obtained with 12 keV Ar + scattered from a GaAs(001) surface revealed the presence of millimonolayer amounts of oxygen. This magnitude of detection sensitivity is the result of an edge-effect rainbow singularity initiated by the forward scattering of argon by oxygen. Analysis of the TOF-ICISS data shows that the oxygen atoms are bonded primarily to surface gallium atoms. Published by Elsevier Science B.V.

Path-dependent neutralization of Ne+ and Na+ scattered from Ag(001)

Recent surface crystallographic studies involving impact collision ion scattering spectroscopy (ICISS) have utilized beams of Ne+ ions as the probe and an electrostatic analyzer as the detector. The data were analyzed under the assumption that the path dependent neutralization could be neglected for scattered Ne+ ions of at least 4 keV incident kinetic energy. We have investigated this assumption by collecting energy and angle dispersive ICISS distributions with a time-of-flight analyzer while separating the ion and neutral yields by application of a potential to the free flight region. This investigation shows that both 5 and 10 keV Ne+ scattered from Ag(001) display considerable path-dependent neutralization, but that of Na+ is very small.

Giant yield enhancements for keV ion scattering: the edge-effect rainbow

The three-dimensional (3D) differential cross-section for a double-scattering sequence in which the first target particle is lighter than the projectile is derived. The resulting cross-section exhibits a rainbow singularity that dramatically enhances the yield of backscattered projectiles by more than a factor of 10 over that of a simple binary scattering process. This classical resonance can be used for the detection of light atomic mass adsorbates on heavy atomic mass substrates with very high sensitivity.