Nils Swanson

Direct Verification of Hydrogen Termination of the Semiconducting Diamond (111) Surface

Low‐energy, high‐resolution electron energy loss spectroscopy has been used to identify the vibrational modes of hydrogen on the semiconducting diamond surface providing the first direct evidence that the (111) 1×1 surface is terminated by hydrogen. The vibrational loss spectrum from the ’’as‐polished’’ surface shows two major losses near 160 meV (CH3 deformation), a major loss at 360 meV (CH3 stretch), and two minor losses at 520 and 720 meV (combinations and overtones). All of these losses disappear from the spectrum after heating the sample to ∠1000 °C (which has been established by other experiments to be sufficient to reconstruct the surface to 2×2/2×1). The loss spectrum for the reconstructed surface is indicative of a two‐dimensional metallic state of the dangling‐bond surface states for clean diamond. Exposure of this reconstructed surface to atomic hydrogen results in a loss spectrum which is essentially identical to that for the as‐polished surface. Further verification that the loss spectrum re...

Excitation of π Electrons in Polystyrene and Similar Polymers by 20‐keV Electrons

Excitation of the π electrons in thin films of polystyrene and some similar polymers has been observed in the characteristic energy loss spectra of 20‐keV electrons scattered at zero angle. The observed energy loss of approximately 7 eV corresponds to the strong uv absorption near 1800 A in benzene and its derivatives. An energy loss of 6.97±0.10 eV was measured in films of atactic and isotactic polystyrene, styrene ethylene copolymer, and poly 2,3,4,5,6‐pentadeuterostyrene, and an energy loss of 7.25±0.15 eV was measured in poly 2,3,4,5,6‐pentafluorostyrene. These energy losses appeared as sharp peaks superimposed on a very broad and more intense loss peak with a maximum at 21.3±0.3 eV in all the electron energy loss spectra obtained. The results for the energy loss attributed to the π electrons are correlated with relevant uv absorption data. A discernible decrease in the measured ∼7‐eV loss occurred during electron bombardment of the specimens, and is attributed to film contamination and breakdown. The...

Specular and off‐specular high resolution electron energy loss spectroscopy of acetylene and ethylene on tungsten (100)

High resolution electron energy loss spectroscopy (EELS) in both specular and off‐specular directions has been used to identify the vibrational modes of acetylene and ethylene on tungsten (100). The off‐specular data were essential to this study since, at low coverages, some of the vibrational modes were detectable only for off‐specular scattering. In addition, analysis of the relative intensities of the specular and off‐specular loss peaks allows us to infer orientations of molecular dipole derivatives and bond directions for the adsorbed species. We find at 135 K that ethylene on tungsten (100) dissociates to acetylene and hydrogen for exposures less than 1 L. At saturation coverage molecular ethylene is also adsorbed. Warming of this adsorbed ethylene to 300 K causes dissociation to acetylene. Analysis of specular and off‐specular loss intensities suggests a geometry with the C–H bonds lying parallel to the surface. Previous UPS data for this adsorption system can be reinterpreted giving a C–C bond len...

The diamond (111) surface: A dilemma resolved

Abstract A dilemma due to the experimental observation of a “clean” unreconstructed elemental semiconductor surface without band gap states is resolved. Results from photon stimulated ion desorption, high resolution low energy electron loss spectroscopy and photoemission spectroscopy find that the conventionally polished (in olive oil) diamond (111) 1 × 1 surface is atomically terminated and electronically stabilized by hydrogen. Thermal desorption of hydrogen upon heating (∼1000°C) results in a reconstructed 2 × 2 2 × 1 surface with filled electronic surface states in and near the fundamental gap. Exposure of the reconstructed surface to atomic hydrogen (or dueterium) is found to again terminate the surface and remove the near band gap surface states. Apparent inconsistencies (with respect to the experimental literature) in the understanding of the diamond:hydrogen interaction are resolved in terms of our work.

Resonant structure in electron impact excitation of CO near threshold

Electron impact excitation functions of numerous states in CO have been measured at 45° scattering angle with resolutions of 16–23 meV FWHM. The decay peak of the 10.04 eV resonance can be seen in the results for the a 3Π, a′ 3Σ+, and A 1Π vibrational levels. There was no evidence of resonant excitation of the a′ 3Σ+ state near 8 eV as suggested by Newton and Thomas. Excitation functions of the b 3Σ+, B 1Σ+, C 1Σ+, c 3Π, and E 1Π states, and a previously unobserbed state at 11.26 eV show numerous sharp resonances in the first few eV above threshold. Energy loss spectra in the 8–14 eV loss region show peaks corresponding to known states as well as to unidentified states. No sign of the metastable state at about 9.5 eV seen by Wells, Borst, and Zipf could be detected in direct excitation, but an indirect excitation process involving the A 1Π state is consistent with the data.