[Ifeffit] Self-Absorption Corrections

[Bruce Ravel]

On Monday 28 July 2003 02:48 pm, Peter Pfalzer wrote:
> You're right. The "simple" correction with Corwin's approach will give a 
> correction factor of (at least) the right order of magnitude with any 
> detector. And as the selfabsorption doesn't do anything but smoothly 
> reducing the EXAFS amplitude (at least if the sample's not too thin), 
> one should be able to fit the remaining deviation with a slight change 
> in S_0^2 - but keeping in mind that some cases could require more 
> thorough treatment.

My two cents worth:

Peter has hit the nail on the head in this paragraph.

The point that the SA correction mostly affects the amplitude is
indeed an important point.  (The rest of the effect will be in
sigma^2, also an amplitude term.)  One can continue to measure bond
length even *ignoring* SA corrections.

It is not hard, though, to come up with a case where SA correction
needs to be done well, though.  Suppose, to make up an example from th
top of my head, you are looking at some effect in the XANES in a
series of ternary phase single crystals (i.e. something of the sort
A_{1-x} B_x C).  And suppose you are measuring the edge of the atom B.
As x approaches 1, the SA correction becomes larger.  The SA
correction must be applied consistently to make meaningful conparisons
between the data.

So, any of the approaches out there in the literature (including doing
nothing!) are possibly "adequate" if it is sufficient to measure the
phase terms in the EXAFS equations.  However, to try to make sense of
a series of data sets, a consistent and stable SA correction is
necessary.  In that case it would be prudent for the experimenter to
measure a toy system in which the SA correction is predictable to
understand the limitations of any correction scheme.

B

-- 
 Bruce Ravel  ----------------------------------- ravel at phys.washington.edu
 Code 6134, Building 3, Room 222
 Naval Research Laboratory                          phone: (1) 202 767 5947
 Washington DC 20375, USA                             fax: (1) 202 767 1697

 NRL Synchrotron Radiation Consortium (NRL-SRC)
 Beamlines X11a, X11b, X23b, X24c, U4b
 National Synchrotron Light Source
 Brookhaven National Laboratory, Upton, NY 11973

 My homepage:    http://feff.phys.washington.edu/~ravel 
 EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/