[Ifeffit] Cadmium K-edge

[Scott Calvin]

Hi Alan,

(To the list: I discussed Alan's system a bit with him when he came to  
SSRL, but don't recall the details. That's why in this reply I have a  
little more knowledge about it than what he posted yesterday.)

I suspect the peak at 2.3 A does indeed represent structure of some  
kind. Your k-space data for sample 1 has very little noise, so I don't  
think it's spurious in that sense.

What strikes me about your data is that the FT's have so little  
structure beyond the 1.8 A peak. I don't recall exactly the nature of  
your system: it was cadmium loaded on to the surface of a substrate,  
but I don't recall what the substrate was. If it's low-Z (for example,  
silica), then the scattering off of whatever the oxygen is bonded to  
(in my example, silicon) would show up pretty weakly, and that could  
be the peak at 2.3 A. It's absence in sample 2 could represent the  
cadmium ion being less selective about what surface site it bonds to,  
so that the Cd-Si distance (in my example) varies and the peak washes  
out.

That's all just an example of the kind of thing that could be  
responsible for what you're seeing. I agree that a good plan is to use  
a Cd-O structure to at least let you model the first peak. For the  
second peak, though, you may have to take more inspiration from the  
structure of the substrate than the structure of Cd-O.

It's also intriguing that the white line in your sample is larger than  
in the CdO standard from the database. Someone with more XANES  
experience than I might be able to suggest why that is--something  
about these being bound to the surface?

Is my recollection of your system correct? If so, what was the  
substrate?

--Scott Calvin
Faculty at Sarah Lawrence College
Currently on sabbatical at Stanford Synchrotron Radiation Laboratory


On Jan 5, 2011, at 11:23 PM, Alan Du wrote:

>
> Hi all,
>
> I processed my XAS data and compare them with standards from Farrel  
> Lytle Database. (http://img717.imageshack.us/i/xasw.jpg/).
>
> In the RSF, both sample 1 and 2 has a major peak at R = ca. 1.8 A.  
> sample 1 has an additional peak at R = ca. 2.3 A. I wonder if this  
> tiny peak is significant.
>
> It seems both samples has peak positions similar to those of CdO  
> standard, which sound logical because the cadmium will bind to the  
> surface oxygen of the material.
>
> I looking at fitting sample 1 and 2 using whatever crystallographic  
> info of CdO I can get my hands on.
>
> Wonder if the plan make sense.
>
> Happy new year,
> Alan