[Ifeffit] Very large exafs amplitude in collected data - cause unknown (Jason Alvino)

[Jason F. Alvino]

Hi all,

There seemed to have been abit of a misunderstanding with the data format,
sorry about that. I was not really sure what format was going to be best to
share with people, I thought the raw data would have been useful so that
people were able to see all the channels.

I have attached a data file from average (AuNP_anatase_Au.D) which has the
data in columns of energy vs u(E).

The second data format (.ssrl) has data in the following columns;

1 = real time clock
2 = Mono angle (requested)
3 = Energy (eV)
4 = I0
5 = I1
6 = I2
7-9 = blank
10-44 - fluor channels (SCA)
45-79 - ICRs for the det element

I have attached these two data sets to this new email.

Now to try to answer some of the questions that have been asked of the data.

The samples are chemically synthesised Au8, Au9, Au11 clusters protected by
PPh3 ligands. We then support these samples on various oxides, in this case
anatase at a % weight loading of 0.17% cluster/anatase. The loading is
typically quite low as we discovered that higher loadings lead to greater
levels of Au agglomeration. The samples can then undergo various
treatments, in an attempt to remove the protective ligand groups, however
harsher treatments can also lead to agglomeration of the Au clusters. The
currently attached example is of Au9 on anatase at 0.17% loading after
being calcined at 200 C under an oxygen atmosphere. There was potentially
an issue initially about releasing all the info of the samples which has
 now been sorted out.

"Perhaps there is some sort of coupling in the signal chain between the Ge
detector and IO" - I wouldn't really know how to start with that one,
however I could have another chat with the beamline scientists over this
and see if they may have some ideas as to the cause.

We were actually analysing some of the data as we went,
but unfortunately not all of it. However this problem of large k2 or k3
weighted exafs amplitude only occurs in samples that we know begin to
agglomerate into larger particles of gold (this has been determined from a
similar study we have done at the SXR). We have multiple samples that
display this behaviour.

The values I get for sigma2 when trying to fit using XFIT are 2 or greater,
if I maintain debye waller factors greater than 0.001 and maintain sensible
coordination numbers <=12.

The samples were cooled to 12 K using a cryostat, however the Au foil data
is not cooled.

Also it seems my first response email directly replied to Bruce, sorry
about that I simply hit the reply button on the forum page instead of
writing a new email to the mailing list. I had attached a data file from
average (AuNP_anatase_Au.average) which had the data in columns of energy
vs u(E), to Bruce, which may have been overlooked.

If anything needs further clarification please let me know. As I'm sure you
are already aware this was only our first trip using XAFS so our experience
is quite limited.

Cheers for all the input so far!
Jason
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