Bedzyk Research Group - Techniques


 
In situ X-ray Synchrotron Techniques
2-beam Interference Approach
    We combine x-ray scattering and x-ray spectroscopy into a single hybrid experiment that allows us to study interfacial processes at the atomic-scale. Here instead of using one, we use two coherently-coupled (interfering) x-ray plane- waves as our primary excitation field to induce atomic transitions. Prime examples include spectroscopically observing fluorescence x-rays or photo-electrons while
generating an X-ray standing wave (XSW) above a reflecting interface or an evanescent wave below a reflecting interface.

X-ray Standing Waves
    An XSW with period ( D = λ/(2sinθ) = 2π/Q ) can be generated by Bragg diffraction from a single crystal1, 2 or periodic multilayer3 and by total external reflection (TER) from a mirror surface4. In each of these cases it is the superposition of the incident and reflected plane waves that gives rise to the XSW. The XSW acts as a periodic positional probe that is used to pin-point the position of
selected atoms at the interface5. This is done by observing the changes in the XRF yield while adjusting the phase, direction and period of the XSW.

XSW Atomic Imaging
    Due to the periodic nature of the XSW, the measured modulation in the XRF yield Y(Q), while scanning through a reflection, is a direct measure of the Fourier transform F(Q) of the atomic distribution. (Unlike convention diffraction techniques the Fourier phase is not lost.) A Fourier inversion of the XSW data produces a model-independent image of the atomic distribution6, 7. For the TER and the multi-layer Bragg cases this constitutes a 1D projected
profile along the interface normal direction. For the single crystal Bragg case, hkl Fourier components8, 9 are measured, which upon inversion leads to a 3D atomic map10-13 projected into the primitive unit cell. The result does not require long-range-order within the XRF-selected atomic distribution.

Enhanced X-ray Intensity at the interface
    At the TER critical angle (typically 0.1 to 0.5°) there is a very strong build-up of the X-ray intensity at the reflecting surface14, 15 that can dramatically enhance surface sensitivity for x-ray spectroscopy and x-ray scattering measurements. The enhancement is due to the fact that the first antinode of the XSW is aligned with the reflecting interface at the critical angle and that the x-ray intensity below the interface is characterized by an evanescent wave with a nanometer-like penetration depth. By applying the reciprocity theorem it is possible to gain this same type of surface sensitivity by observing x-ray emission at escape angles at or below the critical angle15. The evanescent wave absorption and emission phenomenon are the basis for total reflection x-ray fluorescence (TRXRF)16, 17 and grazing incidence x-ray scattering (GIXS)18, 19.

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3. M.J. Bedzyk, D.H. Bilderback, G.M. Bommarito, M. Caffrey, J.S. Schildkraut, Science 241, 1788 (1988). PDF
4. M.J. Bedzyk, G.M. Bommarito, J.S. Schildkraut, Phys. Rev. Lett. 62, 1376 (1989). PDF
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6. M.J. Bedzyk and L. Cheng, "X-ray standing wave studies of minerals and mineral surfaces; principles and
    applications", in Reviews in Mineralogy and Geochemistry, edited by P.A. Fenter, M.L. Rivers, N.C. Sturchio, S.R.
    Sutton (Mineralogical Society of America, Washington, D.C.), Vol. 49, 221-266 (2002). PDF
7. L. Cheng, P. Fenter, M.J. Bedzyk and N.C. Sturchio, Phys. Rev. Lett. 90, 255503-1 -4 (2003). PDF
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9. M.J. Bedzyk, G. Materlik, Phys. Rev. B 31, 4110 (1985). PDF
10. J.S. Okasinski, D.A. Walko, C.-Y. Kim, M.J. Bedzyk, Phys Rev B 69, 041401(R) (2004). PDF
11. Z. Zhang, P. Fenter, L. Cheng, N.C. Sturchio, M.J. Bedzyk, M.L. Machesky, D.J. Wesolowski, Surf. Sci. Lett. 554,
     L95-L100 (2004). PDF
12. Z. Zhang, P. Fenter, L. Cheng, N.C. Sturchio, M.J. Bedzyk, M. Predota, A. Bandura, J. Kubicki, S.N. Lvov, P.T.
     Cummings, A.A. Chialvo, M.K. Ridley, P. Bénézeth, L. Anovitz, D.A. Palmer, M.L. Machesky, D.J. Wesolowski,
     Langmuir 20, 4954-4969 (2004). PDF
13. A.A. Escuadro, D.M. Goodner, J.S. Okasinski, and M.J. Bedzyk, Phys. Rev. B 70, 235416-1-7 (2004). PDF
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