Surface Structural Information Research Articles

Dose-dependent cesium ion beam damage effects on chemically modified poly(methyl methacrylate) films using secondary ion mass spectrometry and x-ray photoelectron spectroscopy

The Cameca IMS-3f ion microscope was used to obtain negative SIMS spectra of poly(methyl methacrylate) (PMMA) and chemically modified PMMA films to assess its ability to provide surface structural information from organic samples. Negative ion spectra generated by Cs/sup +/ bombardment produced structurally specific fragment ions, particularly for Cs/sup +/ doses of < 1 X 10/sup 15/ ions/cm/sup 2/. At larger doses, the progressive degradation of the native polymer structure was reflected by an increasing hydrocarbon-like fragmentation pattern. The primary ion dose dependence of various secondary ion intensities, as influenced by beam damage effects, was established. Small-area X-ray photoelectron spectroscopy (XPS) was used to confirm these results. The PMMA was chemically modified with pentafluorobenzaldehyde (PFBA) and dissolved in a solvent in which the polymer was insoluble, to provide tag molecules at the polymer's surface. Although XPS indicated that only 6% of the near-surface ester side chains were labeled with PFBA, tag-related molecular fragment ions such as C/sub 6/F/sub 5//sup -/ exhibited strong intensities in the immediate surface region. 24 references, 7 figures, 5 tables.

Low energy electron channeling observed by current image diffraction (CID)

Electron channeling patterns observed at LEED energies contain surface structural information. Calculations indicate that surface layer relaxations contribute to the width of the channeling lines and large relaxations can result in line splitting. Possible evidence of this has been observed on aluminum. CID images containing lines due to elastic scattering from different sets of planes can be used to determine the inner potential and planar separation simultaneously at a specified primary electron beam energy. Generally, these channeling patterns observed experimentally are in good agreement with computations based on the simple Bragg theory.

Rare gas titration studies of Si(111) surfaces

Ultraviolet photoemission (UPS) measurements of the coverage-dependent electron binding energies of adsorbed argon and xenon atoms on Si(111) surfaces are used to obtain surface structural information. We distinguish three submonolayer types of adsorption sites on Si(111)-7×7 which fill sequentially and whose coverage relative to a full monolayer are determined with UPS: an initial site (∼4%), a majority site (∼76%), and another minority site (∼20%) which fills last. These results exclude most structural models for the 7×7 but are consistent with recent vacuum tunneling micrographs and the structure proposed by Binnig and Rohrer. The sensitivity of rare gas UPS binding energies to other structural differences induced by impurities, laser annealing, and sputtering are described.

Single-scattering cluster calculations and Fourier-transform analyses of normal photoelectron diffraction

The results of single-scattering cluster (SSC) calculations of normal photoelectron diffraction (NPD) from the S $1s$ level in $c(2\ifmmode\times\else\texttimes\fi{}2)\mathrm{S}$ on Ni(001) are compared with multiple-scattering (MS) calculations by Tong and co-workers over the energy range 100 to 600 eV. It is found that the kinematical $\ensuremath{\chi}(E)$ curves are in good agreement with the multiple-scattering curves over the energy range \ensuremath{\sim} 160 to \ensuremath{\sim} 400 eV but in poor agreement elsewhere. The range of agreement between the SSC and MS curves can be directly associated with a relatively strong peak in the backscattering amplitude over essentially the same 160- to 400- eV range. The SSC curve for an adsorbate vertical height of $z=1.35$ \AA{} is also in good agreement with experimental data of Hussain and co-workers over the range of \ensuremath{\sim} 170-430 eV. Thus, it appears that this simple SSC model can be fruitfully used in analyzing NPD data, even if it may not be quantitative enough for refined structural determinations. The effects of cluster size are examined within the single-scattering model, and it is suggested that approximately eight Ni layers are needed to describe NPD at electron energies \ensuremath{\gtrsim} 300 eV. Inclusion of instrumental angular broadening is also found to be significant, and in particular increases the agreement between SSC and MS curves. Fourier-transform analysis of both the single- and multiple-scattering curves shows that peaks in the magnitude of the Fourier transform are most directly related to path-length differences between the direct wave and various scattered waves and not to perpendicular interlayer distances as previously suggested. These results thus indicate that the Fourier transformation of normal photoelectron diffraction data is not a particularly reliable method of obtaining surface structural information unless a very limited number of path-length differences are strongly predominant in the scattering.

Electron microscope studies of the surface microstructures of layer-lattice silicates

Microstructural features of crystal surfaces of the silicate minerals muscovite, margarite, montmorillonite, beidellite, and kaolinite have been investigated by studying the way in which gold atoms, deposited from the vapor phase, coalesce on these surfaces. Although detailed interpretation of the experimental data is in some cases open to speculation, the results obtained from these investigations show marked differences between the surface structures of the different silicate minerals and reveal microstructural features which would be difficult to detect by any other means. It would seem that this experimental technique can be used to obtain quantitative information concerning the composition of silicate surfaces provided the precautions are taken to preclude postdepositional migration and coalescence of the metal particles. The coalescence of gold particles partially embedded in a carbon film is promoted by aqueous salt solutions and acids. This phenomenon imposes serious practical restrictions on the way in which this type of study can be carried out in order to obtain quantitative surface structural information and does not seem to have been taken into account in many previous experimental investigations.

Single-reflection layer-scattering theory of low energy electron diffraction spectra

A simple general method for computing medium and low energy electron diffraction (LEED) spectra of layered materials has been developed for application to transition metal dichalcogenides. Electron transmission and reflection coefficients for each individual sheet of atoms are computed; the separate sheets are grouped into sandwiches which are stacked upon one another to form the crystal; the intersheet scattering is treated with the single-reflection approximation. For an unrelaxed lattice whose sandwiches are separated by the distance b and whose intrasandwich layers are separated by a, the scattered electron intensity is ‖ f (a) ‖ 2 S (b), where f (a) is the single-sandwich form factor for the outermost sandwich. Lattice relaxation of the surface sandwich slightly modifies this result. The static structure factor S (b), which is a spiked function of energy and contains mainly bulk structural information, produces the principal peaks in the intensity versus energy spectrum, but the surface sandwich reflectivity ‖ f (a) ‖ 2 smoothly modulates the peak intensities and contains most of the surface structure information. This method of fitting ‖ f (a) ‖ 2 should be contrasted with most LEED analyses that concentrate on fitting peaks [which are related to S (b) and actually contain little surface structure information]. This simple single-reflection analysis of the surface structure is confirmed by a complete multiple scattering calculation.

The Surface Sensitivity of MeV Ion Scattering

400 keV helium ions have been scattered in the single channeling mode from a Ni(001) surface covered with an adsorbed ordered overlayer of tellurium. The atomic surface structure of the Ni(001)-C(2 x 2)-Te system was checked by low-energy electron-diffraction (LEED) and confirmed to be that reported earlier. The channeling results however, have not at this point in time yielded atomic surface structure information which can unambiguously confirm or deny the LEED results.

Low-energy ion scattering (LEIS) for composition and structure analysis of the outer surface

The energy distribution of low-energy ions (1–2 keV) scattered at some specific angle from a solid surface, provides information on the mass, or identify, and the number of surface atoms, through the energy position and magnitude, respectively, of peaks in the spectrum. The sampling depth is restricted to one or two atom layers. In the case of single-crystal targets, surface structure or atom location information can be derived from shadowing and multiple scattering effects. Examples are included of applications to equilibrium surface segregation in alloys and the location of sulphur atoms on the Ni (001) surface. The importance of inelastic effects is discussed.

Extraction of Surface Structural Information from Angle-Resolved Ultraviolet Photoemission Spectroscopy

The dependences of photoionization cross section on incident photon energy and electron emission on polar exit angle are used to determine the location of chemisorption sites. Calculations are made for the $c(2\ifmmode\times\else\texttimes\fi{}2)$ S-Ni(001) and CO-Ni(001) systems. Comparing with experiment, it is found that angle-resolved ultraviolet photoelectron spectroscopy data are very sensitive to the coordination number of the adsorption site and less sensitive to the overlayer-substrate interlayer spacing.

A dual-memory information processing interpretation of sentence comprehension

A dual-memory, information-processing model for comprehension of sentences is developed for tasks where a person has to answer questions about sentences. In two experiments, subjects either read (Experiment I) or listened to (Experiment II) 32 paragraphs of four sentences each and answered a question about one sentence following each paragraph. The voices (active-passive) of the sentences and questions were varied orthogonally. Latency data, accounted for by the models, indicated that surface information was available in short-term store and that either surface or deep structure information was available in long-term store. Sentences and questions were stored in the format of case grammar, verb (agent, recipient).