Secondary electron emission (SEE) refers to the ejection of
low-energy (< \sim 50 eV) electrons from solid surfaces as a
result of energetic electron or ion bombardment. Empirical
descriptions of SEE generally regard the process as a single
phenomenon; fundamentally, however, SEE is the result of
multiple underlying mechanisms involving a variety of
electron-electron and electron-lattice momentum exchange
processes. Details of these processes are as yet poorly
understood. Though fundamental theoretical formulations have
been offered, their level of detail has thus far exceeded
the confirming ability of available data. AR SE spectra
measurements may aid in assessing the role of bulk and
surface plasmon decay into single-electron excitations, and
provide a unique means for investigation of the angular
distribution of various subgroups of the SE energy
population. A full set of AR spectra for 1.5 keV electrons
normally incident on polycrystalline gold has been obtained
at USU. Preliminary results are discussed. Funding provided
by NASAÕs Graduate Student Researchers Program.
[EC17.02] High resolution radial distribution function of pure amorphous silicon prepared by ion implantation
K. Laaziri, S. Roorda, M. Chicoine (Université de Montréal, Canada), S. Kycia (CHESS, Cornell University), L.J. Robertson (Oak Ridge National Laboratory), J. Wang, S.C. Moss (University of Houston)
The Radial Distribution Function (RDF) of pure amorphous silicon (a-Si) has been determined with a high degree of precision. Amorphous Si membranes of 10 \mum thickness were prepared by ion implantation and chemical etch. High energy x-ray diffraction measurements were performed to characterize annealed (600^\circC, 1hour) and as-implanted self-supporting a-Si membranes. In order to obtain high resolution atomic structural information, a large region of reciprocal space, up to Q = 55 Åwas covered. A reference measurement on c-Si powder was performed under similar experimental conditions, so as to determine a high resolution RDF of c-Si powder. Calculation of the first neighbor shell coordination (C_1) as a function of maximum Q indicates that measurement of S(Q) out to at least 40 Åis required to reliably determine the RDF. A 2% change in C_1 and subtle changes in the rest of the RDF were observed upon annealing, consistent with point defect removal. After annealing, C_1 = 3.88, which would explain why a-Si is less dense than c-Si.
Work at Houston was supported by the DOE/BES on DE-FG05-8745325.
[EC17.03] Phonons from neutron powder diffraction
D. A. Dimitrov, D. Louca, H. Röder, A. R. Bishop (LANL)
The spherically averaged structure function S(|\mathbfq|) obtained from pulsed neutron powder diffraction contains both elastic and inelastic scattering via an integral over energy. The Fourier transformation of S(|\mathbfq|) to real space, as is done in the pair density function (PDF) analysis, regularizes the data, i.e. it accentuates the diffuse scattering. We present a technique which enables the extraction of off--center (|\mathbfq| \neq 0 ) phonon information from powder diffraction experiments by comparing the experimental PDF with theoretical calculations based on standard interatomic potentials and the crystal symmetry. This procedure (dynamics from powder diffraction(DPD)) has been \emphsuccessfully implemented for two systems, a simple metal, fcc Ni, and an ionic crystal, CaF_2. Although computationally intensive, this data analysis allows for a phonon based modeling of the PDF, and additionally provides off--center phonon information from powder neutron diffraction.
[EC17.04] Diffusing Wave Spectroscopy Applied to Multiple Scattering in a Cylindrical Geometry
J. P. McClymer (Department of Physics and Astronomy, University of Maine), G. A. Zimmerli (National Center for Microgravity Research on Fluids and Combustion, NASA Lewis Research Center)
The photon density and autocorrelation signal is measured
for a turbid suspension of sub-micron spheres suspended in
water in a cylinder with a length to diameter ratio of 5. In
this experiment an incident laser beam travels up through
the center of the sample and a moveable fiber optic probe
measures the static and dynamic properties of the multiply
scattered light. From this information the photon density is
measured and compared to the predictions of Diffusing Wave
Spectroscopy (DWS). Three experimental situations are
studied, in two cases the cylinder consists of a glass
walled vessel suspended in air and then in water, thus
changing the reflectivity coefficient. In the third case the
cylinder is made of an absorbing material. In each case the
predictions of DWS in these differing situations are
compared to our experimental results.
[EC17.05] Polarized Light Scattering Measurements of Particles on Silicon Wafers
Lipiin Sung (Univ. of Maryland and National Institute of Standards and Technology (NIST)), George Mulholland, Thomas Germer (NIST)
Bidirectional ellipsometry has been applied to the characterization of surface roughness and particulate contamination on silicon wafers. Employing incident light with fixed polarization, the principal polarization of light scattered into directions out of the plane of incidence has been shown to yield information about the sources of optical scatter. Theoretical models have predicted that the polarization of light scattered by particles should also be different than that scattered by subsurface defects and microroughness. In this presentation, experimental results will be presented which show good agreement with these models for different sizes of polystyrene latex spheres on silicon wafers. The results demonstrate that the polarization of light scattered by particles can be used to determine the size of particulate contaminants on silicon wafers and other smooth surfaces.
[EC17.06] Estimate of Second-Order Scattering Amplitude for Light Scattering from Roughness in a Dielectric Layer
Jeff Chabot (Rochester Institute of Technology), Thomas Germer (National Institute of Standards and Technology)
Light scattering from surfaces is a valuable industry tool for determining surface characteristics of important substrates such as silicon wafers, optical materials, and magnetic disks. The polarization can provide information about the scatterer, and can be used to distinguish between particles, roughness, and subsurface defects, when a single interface is involved. With multiple interfaces, first-order perturbation theory can predict the polarization of scattered light, yielding polarizations which depend upon the correlation between the roughnesses at the different interfaces. Measurements were carried out using an oxide layer grown on a photolithographically-generated pseudorandomly rough surface. Although details of the theory were observed in the data, the agreement was not as good as that observed for a single interface. In this talk, second-order scattering will be evaluated as a possible mechanism for the disagreement between the experimental data and the first-order scattering theory.
[EC17.07] Optical Scattering Cross Section Measurement of Fractal Aggregates in a Colloidal System
W.B. Hageman, G.M. Wang, C.M. Sorensen (Kansas State University)
We are testing the theory of optical scattering cross
sections of fractal aggregates. To do this we look at laser
light scattering from an aggregating system of small (14nm)
polystyrene spheres suspended in water. Through the use of
dynamic light scattering (DLS) we are able to size particles
in the colloid and measure the intensity of scattered laser
light from them as they aggreagte from single spheres to
aggregates of over 100 spheres. This data is then analyzed
using the data obtained from aggreaged spheres as a
calibration. According to light scattering theory, at a
given scattering angle the normalized scattered intensity
should be proportional to the hydrodynamic radius to the
fractal dimension (R_m ^D). Using static light
scattering techniques to separately measure the fractal
dimension D, we have found that, within the Rayleigh limit,
the optical scattering cross section of fractal aggregates
appears to conform well to theory.
[EC17.08] Particle Sizing in Concentrated Suspensions Using the One-Beam Cross-Correlation Dynamic Light Scattering Technique
A. J. Adorjan (Kent State University), J. A. Lock, T. W. Taylor (Cleveland State University), P. Tin (National Ohio Aerospace Institute NASA Lewis Research Center), W. V. Meyer (National Center for Microgravity Research NASA Lewis Research Center), A. E. Smart (NASA Lewis Research Center)
Studying light scattered by colloids has applications in
both space research and industry. One application is to use
the measured diffusion coefficient for determining particle
size. Ideally, a reliable light scattering device is capable
of measuring a wide range of particle sizes over a wide
range of concentrations. However, when measuring light
scattered from concentrated systems, multiple scattering and
low signal-to-noise ratios have, until recently, made
particle sizing a challenging task. While several approaches
have attempted to address this problem, ours provides
simplicity, robustness, ease of use, and gives the correct
results over both a large range of concentrations and
correlator delay times. The work we present will show
results obtained by extending a single wavelength
cross-correlation technique developed in our laboratory.
This paper will also discuss the techniques we have
developed for particle sizing with both dilute and
concentrated solutions.
[EC17.09] Anisotropic Small Angle Neutron Scattering from Nanodroplet Aerosols
Gerald Wilemski (University of Missouri-Rolla), Barbara Wyslouzil (Worcester Polytechnic Institute), Janice Cheung (Boston College), Reinhard Strey (University of Cologne), John Barker (NIST)
Our latest efforts to apply small angle neutron scattering
(SANS) to study the properties of nanodroplet aerosols will
be reported. Our new aerosol SANS technique uses a
supersonic nozzle to generate the aerosol particles in a
high speed isentropic expansion. Because of the relative
motion of the neutrons and aerosol particles, the laboratory
SANS intensity patterns are anisotropic. To test our ability
to correct our results for this anisotropy, we conducted
SANS experiments using a constant source of D_2O aerosol
with droplet velocities of 400-450 m/s and neutron speeds of
267, 400, and 800 m/s. The theoretically predicted
anisotropy of the laboratory scattering intensities was
found to agree well with the experimental results. The
properly interpreted scattering patterns yield valuable
information about the size distribution and structure of the
aerosol particles.
[EC17.10] Patterns in Mie Scattering
D. J. Fischbach, C. M. Sorensen (Department of Physics, Kansas State University)
When plotted intensity vs. \Theta, Mie scattering functions for light scattering from spheres grow increasingly complex as the radius, R, and the relative index of refraction, m, increase from the Rayleigh limit. However, if intensity is plotted vs. q, where q=4\pi\lambda^-1sin(\Theta/2), order and patterns arise out of the complexity. Moreover, when plotted versus the dimensionless parameter qR universal trends appear. We investigated light scattered from a single dielectric sphere using standard Mie scattering programs. Mie curves were computed and trends were examined. If we ignore the Mie ripples, we see two different power laws which evolve from the Rayleigh-Debye-Gans limit, (qR)^-4 and (qR)^-2. The transition between the power laws depends upon the Rayleigh Parameter, 2kR(m-1). These patterns allow for physical interpretation of Mie scattering and simplify scattering analysis. They also provide a method to determine information embedded in the Rayleigh Parameter, namely the particle size and index of refraction.
[EC17.11] Scaling Approach for the Structure Factor of a Generalized System of Scatterers
C.M. Sorensen, C. Oh (Department of Physics and Program for Complex Fluid Flows, Kansas State University)
A scaling approach for understanding the broad, general features of scattering of waves from particulate systems (e.g., colloids, aerosols, and gels) is presented. The approach uses the concept of a system of scatterers with arbitrary length scales, mass and surface fractal dimensions, and correlations between scatterers. It is based on comparing q^-1, where q is the magnitude of the scattering wave vector, to the various length scales of the system of scatterers to determine whether the waves are scattered coherently or not. This comparison along with the fact that only fluctuations in the density of the scatterers scatter waves, yields power laws and cross over points which make up the structure factor. The system of scatterers can represent single spheres, fractal aggregates or ensembles of such entities in a scattering volume. Hence a large range of experimental situations can be described and unified.
[EC17.12] Fractal and Rayleigh Particle Scattering
Y.T. Tang, J.W.L. Lewis, C.G. Parigger (UTSI)
Results are reported of static and dynamic light scattering for the measurements of the size distribution function of soot particles. Particles of nominal size greater than 30 nm were represented as fractal objects, and the scattering and extinction kernels were obtained using the discrete dipole array method, were angle-averaged, and are compared with mean-field fractal scaling results. Cryodeposit samples of soot particles were obtained from a stoichiometric CH_4/O_2 flame, and the melted samples were centrifuged to separate spatially the size distribution. Optical measurements were performed and reported of particles of nominal size greater than 30 nm as well as the smaller Rayleigh-range particles.
[EC17.13] Measuring the mean-free-path of light in a turbid medium
David L. Everitt, X. D. Zhu (University of California, Davis)
The scattering mean-free-path is determined from measurements of the unscattered intensity transmitted through a thin wedge of the medium. The transport mean-free-path is determined from measurements of the diffuse flux transmitted through a thick sample of the medium. These results provide a test of models for the light diffusion boundary conditions.