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Oeq 2.37 Å

Oeq 2.37 Å. Oax 1.76 Å. EXAFS Fitting with Multiple scattering John Bargar July 1, 2011. Oeq 2.37 Å. Oax 1.76 Å. Question set #1. START SixPack: EXAFS FITTING MODULE LOAD WORKSPACE: UAQ29MMP3_fl_Feb99.chi (click LOAD) Set parameters so that they look like this:

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Oeq 2.37 Å

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  1. Oeq 2.37Å Oax 1.76Å EXAFS Fitting with Multiple scattering John Bargar July 1, 2011

  2. Oeq 2.37Å Oax 1.76Å Question set #1 • START SixPack: EXAFS FITTING MODULE • LOAD WORKSPACE: UAQ29MMP3_fl_Feb99.chi (click LOAD) • Set parameters so that they look like this: • Click: Plot K, then Plot R. • About how many shells of backscattering atoms are present? • * What are they?/How many shells do you expect to see? • * How do I know if a FT peak is “real”?

  3. Oeq 2.37Å Oax 1.76Å Question set #2: Information content * How many independent data points are present? * What does this mean? * What is the minimum resolvable distance between neighboring shells? Nind = (2/π)xΔk Δr + 2 = Δr = π/2kmax =

  4. Oeq 2.37Å Oax 1.76Å Question set #2: Information content * How many independent data points are present? * What does this mean? * What is the minimum resolvable distance between neighboring shells? Nind = (2/π)xΔk Δr + 2 = 0.64x10x3 = 19.1 Δr = π/2kmax = 0.16 Å

  5. Oeq 2.37Å Oax 1.76Å Fit First Shell Click on VARIABLES tab Set parameters so they look like this Click FF2chi Click Plot R and Plot K (be sure that the Fit Plot option is checked) Click Fit – then replot What is Chi Sq? What is R factor? View uncertainties on RESULTS tab

  6. Oeq 2.37Å Oax 1.76Å Fit Second Shell(s) Go Back to the PATHS tab Click ADD PATH Add variables as shown below Click Define Path Go Back to the VARIABLES tab Click ADD ALL VARIABLES Input seed values, Click FF2CHI, plot Fit – What is R?, What are your uncertainties?

  7. Is this a good fit according to Scott Calvin’s criteria? Criterion 1: Figure it out Criterion 2: Figure it out Criterion 3: Figure it out Criterion 4: Figure it out Criterion 5: Figure it out Criterion 6: Figure it out Criterion 7: Figure it out Criterion 8: Figure it out

  8. Is this a good fit according to Scott Calvin’s criteria? Criterion 1: Statistical quality – NOT A GOOD PARAMETER Criterion 2: Closeness of fit – R = 0.012 - YES Criterion 3: Sensible results - YES Criterion 4: Defensible model - YES Criterion 5: Stability - YES Criterion 6: Precision - YES Criterion 7: More data is better – Can do this too Criterion 8: Agreement outside the fitted range - YEP

  9. Oeq 2.37Å NOTE:σ2 = … Oax 1.76Å ~ 2 x σ2_O_ax O U O FEFF0016.dat ~ 2 x σ2_O_ax FEFF0015.dat O U O ~ 4 x σ2_O_ax FEFF0017.dat O U O Fit Multiple Scattering Peaks NOTE: R=2xR_Oax N=2 (each path) But, σ2 is more complicated… σ2 = ¼[∑σj]2

  10. Oeq 2.37Å Oax 1.76Å Add Multiple Scattering Shell(s) Go Back to the PATHS tab Define the 3 key MS paths Go to Variables tab Add All Variables Constrain your Variables Example: CN for each shell should have a “fit value” of “2*R_Oax”, and float option should be set to “DEFINE”. Click FF2CHI, plot Fit – What is R?, What are your uncertainties?

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