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EXPERIMENTAL METHODS

The DAFS experiments described in this chapter were performed at the National Synchrotron Light Source (NSLS), using beamline X23A2. The synchrotron radiation source provided an intense continuous energy spectrum, strong collimation, small source size, and a high degree of polarization. An XAFS beamline was chosen because rapid and precise energy scanning is essential for DAFS measurements. The beamline was adapted by adding a 2-circle spectrometer to give it diffraction capabilities. The 2-circle spectrometer had limited reciprocal space coverage, but it was sufficient for all the experiments described in this chapter. All of the work reported here was done using specular Bragg reflections. The configuration of the X23A2 beamline during the DAFS experiments is shown in Fig. 4.

 figure276

In the fixed exit monochromator, Si(220) crystals were used for the Cu measurements and Si(311) crystals were used for the In0.2Ga0.8As and YBa2Cu3O6.6 measurements. The energy calibration procedure is described in reference [21]. During the DAFS experiments, the beamline produced a measured flux of about tex2html_wrap_inline1556 photons per second in a 1 mm high by 5 mm wide beam, with a measured energy spread of about 2 eV.

The Bragg diffracted radiation was selected with a 3 mm high by 5 mm wide slit located 27 cm from the sample. This provided adequate suppression of the fluorescence from the sample while accepting essentially all of the diffracted beam. At each energy, the diffractometer was adjusted to keep the momentum transfer fixed; to obtain reliable intensity measurements we found it essential to accurately track the Bragg peak versus energy. For the thin epitaxial film samples we studied, which had broad and smooth mosaic distributions, the peak intensities were proportional to the integrated intensities, and we report here our peak intensity measurements.

We used integral (current mode) techniques to maximize the number of detected photons. Because commercial NaI(Tl) scintillation detectors can only count up to about tex2html_wrap_inline1564 photons per second and the samples in this study often had diffracted intensities greater than tex2html_wrap_inline1566 photons per second, both the incident and diffracted x-ray intensities were measured using ionization chambers. The noise, set by photon counting statistics, is proportional to the square-root of the number of detected photons and should be less than the fine structure signal. Since the fine structure signal can be as low as tex2html_wrap_inline1568, at least tex2html_wrap_inline1570 detected photons are required just to reduce the noise to the level of weak fine structure signals. To reduce the sensitivity of the signal and monitor ionization chambers to the second or third harmonics of the 9 Kev fundamental, the gas in each ion chamber was chosen to produce an absorption of about 50% at 9 Kev.

To compensate for the incident intensity variations, we divided the diffracted intensity at each energy by the incident beam monitor signal. The fluorescence from the sample was measured simultaneously with the DAFS signal using a 10 cm diameter ionization chamber [22]. This measured fluorescence signal was used as an energy reference and to allow comparisons of the XAFS and DAFS signals. The measured fluorescence was also used to calculate the absorption corrections for the Cu metal and YBa2Cu3O6.6 samples.




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Next: Optimization of DAFS measurements Up: Diffraction anomalous fine structure: Previous: The polarization dependence of