[Ifeffit] a question about the white line

[Matt Newville]

Shelly, 

That seems like a lot of steps to me, and it almost seems like
you're mimicking by hand the use of a standard chi(k) with
autobk/spline().  Do you find very different results using a
standard chi(k) with autobk/spline()?  If so, do you have any
suggestions for how the more automated version could be improved?

My experience with strong white lines is to adjust E0, kweight
(=1, 0, or 2), now the low-k clamp, and sometimes kmin (I know
others regularly use kmin >1, but I prefer to have kmin<0.5 and
adjust E0).  I adjust these until the background mu0(E) goes
through the mu(E) in a way that "looks right" (yep, it's
subjective!).  This isn't a particularly strong white line (only
going to 1.45 of the normalized mu(E), not 3 or 4 that you can get
with Re L-edges), but this is the kind of result I'm looking for:
    http://cars.uchicago.edu/ifeffit/misc/bkg_xanes.png

I know it's completely unfair to show one favorable example and
say "it should be like this", so I'll say that this is meant as
one favorable example, and it will not always be like this. What
I'm looking for is that the background goes smoothly and fairly
"straight" to the main absorption edge without swooping up or down
too much.  This may not always be the best background, but it's
the best I can come up with for a 'rule of thumb'.  This kind of
background isn't always easy to get: I don't mind swooping up to
part way up the white line (say to 1.2 or so on the above plot or
up a little higher on stronger white lines), but I try to avoid
swooping down or swooping too far up above the white line.

I should emphasize that I usually start with the simplest
background possible and plow ahead to the firs shell analysis,
coming back to background subtraction only after I have a sense of
what the first shell is doing.  And then I often use a rough first
shell standard chi(k) with spline(), or just refining the
background with feffit().  That's probably consistent with your
(Shelly's) advice, but I think it's easy for beginners to get
trapped into spending way too much time on the background removal.

Hope that helps,

--Matt