[Ifeffit] Transmission EXAFS sample

Tue Nov 23 08:58:38 CST 2010

On 22.11.2010 19:00, ifeffit-request at millenia.cars.aps.anl.gov wrote:
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> Today's Topics:
>
>     1. Re: Transmission EXAFS sample (Scott Calvin)
>     2. Re: Transmission EXAFS sample (Scott Calvin)
>     3. maximum number of paths in Artemis (Yuan Ping)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Mon, 22 Nov 2010 07:13:42 -0800
> From: Scott Calvin<dr.scott.calvin at gmail.com>
> To: XAFS Analysis using Ifeffit<ifeffit at millenia.cars.aps.anl.gov>
> Subject: Re: [Ifeffit] Transmission EXAFS sample
> Message-ID:<5B5040D9-F501-4BA0-8F24-38630B43B943 at gmail.edu>
> Content-Type: text/plain; charset=US-ASCII; format=flowed; delsp=yes
>
> To my mind, when considering sample preparation the important thing is
> not so much the "right" thickness, as knowing the effects to guard
> against as the thickness deviates toward the thin or thick side.
>
> As transmission samples become thicker, the problem of "unwanted"
> photons becomes more severe. Those photons may be harmonics, photons
> scattered into the It detector, or photons from the tails of the
> resolution curve of the monochromator.
>
> As transmission samples become thinner, uniformity becomes more of an
> issue. If you play with the equations, you'll see that if your sample
> is a mixture of regions that have a thickness of 1.0 absorption
> lengths and regions that have a thickness of 2.0 absorption lengths,
> the spectrum is less distorted than if it is a mixture of 0.5 and 1.0
> absorption lengths.
>
> So if a sample is on the thick side, it is particularly important to
> guard against harmonics in the beam and scattered photons. If it is on
> the thin side, it is particularly important to guard against
> nonuniformity.
>
> To put it another way, problems are synergistic. With a well-
> conditioned beam, a uniform sample, and linear detectors, the
> thickness almost doesn't matter (within reason)--at a modern beamline,
> a total absorption of even 0.05 or 4.0 will work.
>
> But as each of those conditions deviates from the ideal, distortions
> become much more severe.
>
> There's an old joke about someone on a diet going in to a fast food
> joint and asking for a double bacon cheeseburger, a large fries...and
> a diet Coke. In XAFS measurements, that attitude actually kind of
> works, because of the synergies I just discussed.
>
> Personally, I trust my ability to condition the beam and minimize
> scattering more than I trust my ability to make a uniform sample, so I
> lean a little toward the thicker side.
>
> --Scott Calvin
> Faculty at Sarah Lawrence College
> Currently on sabbatical at Stanford Synchrotron Radiation Laboratory
>
>
>
> On Nov 22, 2010, at 5:13 AM, Welter, Edmund wrote:
>
>    
>> Dear Jatin,
>>
>> the optimum mued of 2.x is not just derived by simple photon counting
>> statistics. As Matt pointed out, for transmission measurements at a
>> synchrotron beamline in conventional scanning mode this is seldom a
>> matter. Nevertheless, one should avoid to measure subtle changes of
>> absorption at the extreme ends, that is, transmission near 0 % or
>> 100 %.
>> In optical photometry this is described by the more or less famous
>> "Ringbom plots" which describe the dependency of the accuracy of
>> quantitative analysis by absorption measurements (usually but not
>> necessarily in the UV/Vis) from the total absorption of the sample.
>>
>> This time the number is only near to 42, the optimum transmission is
>> 36.8 % (mue = 1). So, to achieve the highest accuracy in the
>> determination of small Delta c (c = concentration) you should try to
>> measure samples with transmissions near to this value (actually the
>> minimum is broad and transmissions between 0.2 and 0.7 are ok). In our
>> case, we are not interested in the concentration of the absorber,
>> but we
>> are also interested in (very) small changes of the transmission resp.
>> absorption in our samples. Or, using Bouger, Lambert Beer's law, in
>> our
>> case mue (-ln(I1/I0) is a function of the absorption coefficient
>> (mue0).
>> The concentration of the absorber and the thickness (d) of the sample
>> are constant.
>>
>> -ln(I1/I0) = mue0 * c * d
>>
>> But then: If the optimum is a mue between 0.35 and 1.6 why are we all
>> measuring successfully (ok, more or less ;-) using samples having a
>> mue
>> between 2 and 3? ...and 0.35 seems desperately small to me! Maybe
>> sample
>> homogeneity is an issue?
>>
>> Cheers,
>> Edmund Welter
>>
>>
>>      
>
>
> ------------------------------
>
> Message: 2
> Date: Mon, 22 Nov 2010 07:16:06 -0800
> From: Scott Calvin<dr.scott.calvin at gmail.com>
> To: XAFS Analysis using Ifeffit<ifeffit at millenia.cars.aps.anl.gov>
> Subject: Re: [Ifeffit] Transmission EXAFS sample
> Message-ID:<3730BF4F-966F-46E3-9784-B5FB8EF5310D at gmail.edu>
> Content-Type: text/plain; charset="us-ascii"; Format="flowed";
> 	DelSp="yes"
>
>
> On Nov 22, 2010, at 7:09 AM, Jatinkumar Rana wrote:
>    
>>>        
>> Hi Scott,
>>
>> Sorry for mixing up the things.
>>
>> For the case, when i have very limited amount of sample that i can not
>> cover 1sq.cm area, you, Matt and others have given very very clear
>> explanation about possible solutions and the probable effects on data
>> quality. I am really very thankful to all of you for sharing your
>> experience and expertise.
>>
>> My last post was with reference to the case when i have enough powders
>> (i.e., reference oxide compounds). It is just to be ensured that i am
>> doing things 100% exactly in a same way it has to be done.
>>
>> With best regards,
>> Jatin
>>
>> --
>> Jatinkumar Rana
>>
>>      
> Yes, Jatin, the procedure you described is fine. There is no "right"
> way to make samples, although there are many wrong ways.
>
> --Scott Calvin
> Faculty at Sarah Lawrence College
> Currently on sabbatical at Stanford Synchrotron Radiation Laboratory
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> ------------------------------
>
> Message: 3
> Date: Mon, 22 Nov 2010 09:43:52 -0800
> From: Yuan Ping<ping2 at llnl.gov>
> To: XAFS Analysis using Ifeffit<ifeffit at millenia.cars.aps.anl.gov>
> Subject: [Ifeffit] maximum number of paths in Artemis
> Message-ID:<C90FEBD8.110A%ping2 at llnl.gov>
> Content-Type: text/plain;	charset="US-ASCII"
>
>
> Dear Ifeffit experts,
>
> Hopefully this is a short question: how can I increase the path number limit
> for fitting in Artemis? Currently it's 512, and I need to at least double
> the number.
>
> Thanks in advance.
> Yuan
>
>
>
>
>
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> End of Ifeffit Digest, Vol 93, Issue 18
> ***************************************
>
>
>    
Dear all,

Thank you so much for the detailed practical explanation about sample 
preparation for EXAFS, and also, sharing all the Pros and Cons 
associated with it. It is of great help to me when i go next time to 
beamline for aquiring EXAFS data.

Thank you all !!

With best regards,
Jatin

-- 
Jatinkumar Rana

Helmholtz-Zentrum Berlin für Materialien und Energie GmbH
Institut für angewandte Materialforschung
FI-1 Mikrostruktur

Hahn-Meitner-Platz 1
D-14109 Berlin Germany
Tel: +49 30 8062-43217
Fax: +49 30 8062-43059
eMail: jatinkumar.rana at helmholtz-berlin.de

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