[Ifeffit] Is it legitimate to merge transmission datawithfluorescence data?

Zajac, Dariusz A. dariusz.zajac at desy.de
Tue Aug 11 02:17:00 CDT 2009

Hi all,
there is one point of discussion missing for me - does the signal from
transmision and fluorescence is the same?
The fluorescence signal is "more sensitive" to the surface in some
situations, thus if you expect some changes (e.g. oxidation) of the
surface of your sample you should be more carefull with data merging.
Otherwise as it is written in previuos posts.

>-----Original Message-----
>From: ifeffit-bounces at millenia.cars.aps.anl.gov 
>[mailto:ifeffit-bounces at millenia.cars.aps.anl.gov] On Behalf Of Matthew
>Sent: Tuesday, August 11, 2009 12:55 AM
>To: Bruce Ravel; XAFS Analysis using Ifeffit
>Subject: Re: [Ifeffit] Is it legitimate to merge transmission 
>datawithfluorescence data?
>A trick I've used when the fluorescence data are good but 
>affected by overabsorption, and the transmission data are noisy is to 
>reduce both data sets to post-edge normalized form (0 in 
>pre-edge, oscillating about 1 post-edge), then apply a simple 
>overabsorption model to 'correct' the fluorescence so it 
>matches the transmission.  This is a bit risky in that some of 
>the same 
>samples which exhibit overabsorption will also show hole 
>(thickness) effect, which looks enough like overabsorption so that you 
>under-correct the fluorescence.
>Otherwise, if the goal is to reduce noise, perhaps the best 
>way is to reduce both to post-edge normalized form, then do a weighted 
>sum.  What weights?  Look at the noise level on each and 
>weight it accordingly.  One way to do that is to fit a 
>polynomial and look 
>at the magnitude of the residuals and ask "how many 
>counts/point would an ideal Poissonian counter need to get the 
>residual this 
>low?".  The "2-column Editor" program available from the ALS 
>Beamline 10.3.2 website 
>http://xraysweb.lbl.gov/uxas/Beamline/Software/Software.htm -> 
>Utilities has that as a capability.  If you want to use it, download 
>the program and reply to me off-line for an explanation of how 
>to use the noise analysis; the on-line doc is outdated and doesn't 
>include that feature.
>    mam
>----- Original Message ----- 
>From: "Bruce Ravel" <bravel at bnl.gov>
>To: "XAFS Analysis using Ifeffit" <ifeffit at millenia.cars.aps.anl.gov>
>Sent: Monday, August 10, 2009 3:20 PM
>Subject: Re: [Ifeffit] Is it legitimate to merge transmission 
>data withfluorescence data?
>> On Monday 10 August 2009 06:13:32 pm jrkizews at ncsu.edu wrote:
>>> Dear XAFS community members,
>>> I have a question concerning data merge. We normally collect XAFS 
>>> data in both transmission mode and fluorescence mode. We normally 
>>> have to merge a few scans to enhance signal-to-noise ratio. For one 
>>> particular sample, I want to know if it is legitimate to merge its 
>>> transmission scans with its fluorescence scans to improve data 
>>> quality?
>> Hi Fiona,
>> There is no a priori reason not to do so and certainly not a 
>> reason.  From a numerical perspective, you can merge anything with 
>> anything!
>> My one concern is that the fluorescence and transmission data are 
>> really equivalent.  That is, if the fluo data are affected by 
>> significant self-absorption attenuation or if the transmission data 
>> suffer from significant pin-hole effects, then you run the risk of 
>> degrading the entire data set by doing the merge.  But if 
>the data are 
>> merely noisy, then I think you are safe doing so.
>> Think about it this way: when you use a multi-element detector, you 
>> are making several measurements which are presumed to be 
>identical. We 
>> routinely merge the channels of an MED.  You situation is, from a 
>> measurement theory perspective, analogous.  Assuming your 
>data do not 
>> suffer from the problems mentioned above, it seems all right to me.
>> B
>> --
>> Bruce Ravel  ------------------------------------ bravel at bnl.gov
>> National Institute of Standards and Technology
>> Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2 
>> Building 535A Upton NY, 11973
>> My homepage:    http://xafs.org/BruceRavel
>> EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/
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