Reference: O. Zhou, J. E. Fischer, N. Coustel, S. Kycia, Q. Zhu, A. R. McGhie, W. J. Romanow, J. P. McCauley Jr., A. B. Smith III and D. E. Cox, Nature 351, 462 (1991).
C60 first isolated in macroscopic amounts in 1990.
When doped with alkali metals, C60 becomes superconducting at temperatures as high as 30K.
Structure of Cs6C60 determined by powder XRD at Brookhaven National Laboratory.
Body-centered cubic, with Cs atoms in the interstices between the C60's.
High resolution and flux at NSLS X10B allowed resolution of closely separated peaks, improved signal:background, measurement of weak peaks, and determination of crystal coherence length.
(Actually, the superconducting compound is Cs3C60, structure determined subsequently by Stephens et al).
Reference: B. C. Larson et al, Phys. Rev. Lett. 48, 337 (1982).
Implant surface of Si with boron or some other dopant. Resolve strains by laser annealing. Question: does the annealing happen via ``melting'' or ``electron plasma fluid'' mechanism?
Surface strains yield broadened XRD lineshape.
Watch evolution of lineshape as a function of time after laser pulse--gate laser pulse to happen ``just before'' x-ray pulse.
X-rays: 0.1ns pulse, 2.5ms between pulses,25000 photons per pulse, 2-3 scattered photons per pulse
Q-switched ruby laser, 15 ns pulse, 1.3J/cm**2.
Measure profile at 100 and 195 ns after laser pulse. (Requires many shots, moving the crystal after each shot and recharging laser).
Strain broadening decreases as a function of time.
Conclusion: melting model is more valid than plasma model.
Reference: J. Als-Nielson, F. Christensen, and P. S. Pershan, Phys. Rev. Lett. 48, 1107 (1982).
Coherent superposition of Fresnel reflection from the (homogeneous) surface and Bragg scattering from surface layers.
Surface tension induces smectic (layered) ordering at the surface.
Done first at synchrotron, but doable with rotating anode.
Also applied to polymers in water, Langmuir-Blodgett films, etc.
Reference: J. Wang, B. M. Ocko, A. J. Davenport, and H. S. Isaacs, Phys. Rev. B46, 10321 (1992).
Top layer of gold atoms undergoes reversible transition between the (1x1) bulk termination and a (p X sqrt(3)) striped phase, with p=23.
Used NaF, NaCl, and NaBr solutions.
Technical challenge: x-ray diffraction ``under water!'' (Requires high intensity).
Combine with AFM, STM to get full picture.
Reference: M. Sutton, S. G. J. Mochrie, T. Greytak, S. E. Nagler, L. E. Berman, G. A. Heidi and G. B. Stephenson, Nature 352, 608 (1991).
Visible coherent light: reflected light from inhomogeneous medium shows graininess known as speckle.
Time distribution of speckle pattern gives time correlation function of the inhomogeneity.
(Sometimes called light-beating spectroscopy, dynamic light scattering, or intensity fluctuation spectroscopy). Used to study, e.g., diffusion of particles in fluids.
Normally use laser, but can also use monochromatic incoherent source (such as arc lamp) with pinhole collimation.
Not possible with standard X-ray sources because they are incoherent.
Speckle interferometry possible with X-rays if sufficiently collimated and monochromatic.
Measured randomly arranged antiphase domains in single
crystal of Cu3Au using the X25 wiggler beamline at NSLS.
Experiments ideally suited to Advanced Photon Source.
Future application: mass-transport mechanisms on 1-100 nm scale in metal alloys and complex fluids.
Reference: N. C. Maliszewskyj, P. A. Heiney, J. Y. Josefowicz, T. Plesnivy, H. Ringsdorf, and P. Schuhmacher, Langmuir 11, 1666-1674 (1995).
Study monolayers and multilayers of star-shaped compounds:
Measurements made in both reflection and grazing incidence configurations
For preliminary investigations, collect 2D XRD patterns with Fuji imaging plate.
For quantitative measurements, collect diffracted photons
using analyzer crystal and scintillation detector.
Up: High Resolution X-ray Diffraction
Previous: Instrumentation for XRD:
Diffractometers and Detectors
Last updated December 30, 1996
Paul A. Heiney, heiney@dept.physics.upenn.edu