[Ifeffit] Distortion of transmission spectra due to particle size

[Matt Newville]

On Mon, Nov 22, 2010 at 4:55 PM, Scott Calvin <dr.scott.calvin at gmail.com> wrote:
> Some follow-up.
> This, for example, is from an excellent workshop presentation by Rob
> Scarrow:
>
> Errors from large particles are independent of thickness

Yes... one can have a sample that is uniform, or made of small
particles, and still too thick. In that sense, having large particles
or sample with widely varying thickness is a separate issue from
having a sample that is too thick.

> The relative (%) variation in thickness depends on the ratio (particle
> diameter / avg. thickness), so it is tempting to increase the avg. thickness
> (i.e. increase μx) as an alternative to reducing the particle diameter.
>
> However, simulations of MnO2 spectra for average Δμ0x = 1, 2 or 3 show that
> the errors in derived pre-edge peak heights and EXAFS amplitude factors are
> significant when diameter > 0.2 / Δμ0, but that they are not affected by the
> average sample thickness. (Δμ0 refers to the edge jump)
>
> The equation at right is given by Heald (quoting earlier work by Stern and
> Lu). D is particle diameter, μ1 is for just below the edge, and Δμ =μ(above
> edge) - μ1.
>
> I've seen similar claims elsewhere, although Scarrow's is particularly clear
> and unambiguous.

OK.  Are you saying there something wrong with this?   Did he say that
spheres were stacked directly on top of one another?  I'm not seeing
that assumption in what you quote.  I read it as saying that you can't
have spheres that are too thick.

--Matt