On Wednesday 06 June 2007, MarkBondin wrote:
1)Does the equation for bond distance resolution (r = pi/2deltak) only apply to SS? I have held the opinion that this can be applied to MS analysis however I have recently been informed that this equation does not correctly describe distance resolution in MS analyses. The paper in Coord. Chem. Rev. 2005, 249, 141-160 describes this and is this concordant with the views of the wider EXAFS community?
Mark, Strictly speaking, that equation doesn't have anything to do with EXAFS. That is the equation that tells you what your Fourier component resolution is in a general Fourier analysis problem. It just so happens that, in the case of single scattering EXAFS analysis, that equation is easily interpreted in terms of photoelectron wavenumber k and SS path length r. The equation is neither different nor incorrect for MS analysis. That's true becuase MS analysis isn't any different from SS analysis. In either case, you do a Fourier transform. In either case, you attempt to model Fourier components using the contributions from some number of scattering geometries as computed by theory. In either case, you are asking yourself if you can actually resolve small differences in phase of the various things that contribute to the fit. The only difference lies in how you *interpret* the physical meaning of the Fourier components. And even then, things aren't so very different. In the case of SS analysis, you assert that the R axis is a measure of "bond length" while for MS analysis the R axis is a measure of "half path length" -- acknowledging, of course, that there is a phase shift in the EXAFS equation such that the R axis actually measures something a bit shorter than the bond or half path length. Off the top of my head, I don't recall the paper you cite and I most certainly cannot speak for the "wider EXAFS community". Speaking for myself, the physical interpretation of the equation for Fourier component resolution may change when you consider MS paths, but to claim that a property of the Fourier transform somehow becomes invalid when you change the details of the fitting model is just silly. B -- Bruce Ravel ---------------------------------------------- bravel@anl.gov Molecular Environmental Science Group, Building 203, Room E-165 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007 Argonne National Laboratory phone and voice mail: (1) 630 252 5033 Argonne IL 60439, USA fax: (1) 630 252 9793 My homepage: http://cars9.uchicago.edu/~ravel EXAFS software: http://cars9.uchicago.edu/~ravel/software/