Thanks for the response! I should clarify, the data are collected in fluorescence, concentration is perhaps in the hundreds of ppm to tenths-of-percent range in this case. Personally I haven't had great luck with self-absorption correction in the past (actually I've had basically no luck), so for concentrated samples I generally follow Scott Calvin's "cut twice, measure once" rule and that has been helpful. When the concentrations get _too low_ though, it seems that I run into this problem... Cheers, Mike

On Jan 15, 2019, at 4:31 PM, "daltonabdala@gmail.com" wrote:

Hello, Mike et al.,

you can correct for self-absorption using the "self-absorption correction tool" in Athena, as long as the chemical composition of your sample is known.

In case it's not possible to determine the chemical composition of you sample, and your sample is conductive, then running your experiment in total electron yield (TEY) mode would be an alternative.

If you're sample isn't suitable for TEY, and the only choice you have left is to run it in fluorescence mode, then you should dilute your sample to a total concentration of P of ~ 0.1 - 0.15% to avoid self-absorption.

It's very important to think through your experiment/samples before you get to the beamline as it avoids you running into such issues, saving you a lot of time.

Hope it helps ;)

- Dalton Abdala _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit

Thanks Riti and Carlo. Riti: will give it a go, thanks! I think the data are salvageable, it's just a question of how... Carlo: Interesting point. In terms of how I'm fitting it, I just use the LCF window to do a two-component fit. Nothing super fancy. But what you're suggesting is that maybe I have a misunderstanding in the way I'm normalizing my data. That's entirely possible. I generally try to get a nice flat background by following the "flat part" of the spectra but in cases like this the pre- and post-edge lines end up not parallel. I can force it to be parallel, and that helps the fit to behave better, but ends up creating other normalization issues. Much obliged for the responses so far. Mike

On Jan 15, 2019, at 4:42 PM, Carlo Segre wrote:

Hi Mike:

How are you trying to fit it? It looks to me like the post edge line is not in the right place. You need to tune it by hand when you have such a short energy range after the edge since the normalization is critically dependent on how you set this line.

carlo

...

On Tue, 15 Jan 2019, Mike Massey wrote:

Hi All,

Does anyone have advice for LCF of spectra with a small edge step?

I've recently found myself with more and more issues fitting spectra like the one shown in the attached photo, where the spectrum is of good quality, but the fits end up not working out due to normalization (as far as I can tell).

Sadly, I have so far ended up just throwing these fits out, but I'm hoping there is a better solution out there that someone might be able to recommend.

One solution is "get more of the element of interest," as a beamline scientist recently told me. I'm just reluctant to really load it up in the data collection, because at 2 keV I get nervous about artifacts like self-absorption.

Is there a better way? What am I missing?

Thanks for your thoughts,

Mike

-- Carlo U. Segre -- Duchossois Leadership Professor of Physics Interim Chair, Department of Chemistry Director, Center for Synchrotron Radiation Research and Instrumentation Illinois Institute of Technology Voice: 312.567.3498 Fax: 312.567.3494 segre@iit.edu http://phys.iit.edu/~segre segre@debian.org _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit

Hi Scott, I collect up to 2240 eV often, because that's about where the mirror stops, but end up truncating it at 2200 eV before fitting because sometimes I end up with all sorts of non-systematic background things at higher energies, depending on the sample. I still collect the data because it helps me to troubleshoot, but I chop it before fitting because 2200 eV is the limit of what I normally consider consistently "good data." I end up with spectra that, to my eye anyway, are comparable. For dilute samples, I don't usually trust it beyond that. For concentrated samples, I don't have a problem. Thanks, Mike

On Jan 15, 2019, at 5:12 PM, Scott Calvin wrote:

Hi Mike,

Is there a reason the data stops at 2200 eV—e.g., is there another edge just above that?

—Scott

...

On Jan 15, 2019, at 8:02 PM, Mike Massey wrote:

Thanks Riti and Carlo.

Riti: will give it a go, thanks! I think the data are salvageable, it's just a question of how...

Carlo: Interesting point. In terms of how I'm fitting it, I just use the LCF window to do a two-component fit. Nothing super fancy.

But what you're suggesting is that maybe I have a misunderstanding in the way I'm normalizing my data. That's entirely possible. I generally try to get a nice flat background by following the "flat part" of the spectra but in cases like this the pre- and post-edge lines end up not parallel. I can force it to be parallel, and that helps the fit to behave better, but ends up creating other normalization issues.

Much obliged for the responses so far.

Mike

...

On Jan 15, 2019, at 4:42 PM, Carlo Segre wrote:

Hi Mike:

How are you trying to fit it? It looks to me like the post edge line is not in the right place. You need to tune it by hand when you have such a short energy range after the edge since the normalization is critically dependent on how you set this line.

carlo

...

On Tue, 15 Jan 2019, Mike Massey wrote:

Hi All,

Does anyone have advice for LCF of spectra with a small edge step?

I've recently found myself with more and more issues fitting spectra like the one shown in the attached photo, where the spectrum is of good quality, but the fits end up not working out due to normalization (as far as I can tell).

Sadly, I have so far ended up just throwing these fits out, but I'm hoping there is a better solution out there that someone might be able to recommend.

One solution is "get more of the element of interest," as a beamline scientist recently told me. I'm just reluctant to really load it up in the data collection, because at 2 keV I get nervous about artifacts like self-absorption.

Is there a better way? What am I missing?

Thanks for your thoughts,

Mike

-- Carlo U. Segre -- Duchossois Leadership Professor of Physics Interim Chair, Department of Chemistry Director, Center for Synchrotron Radiation Research and Instrumentation Illinois Institute of Technology Voice: 312.567.3498 Fax: 312.567.3494 segre@iit.edu http://phys.iit.edu/~segre segre@debian.org _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit

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Hi Chris, Thanks for the suggestion, I'll give it a try on the morrow. Interestingly enough, I loaded up the non-truncated version of this spectrum for comparison purposes, and...The extended background is pretty flat but not really increasing. If I use it for normalization, the edge jump computed by Athena is negative. I'm going to try using Athena for normalization, and another program for LCF, and see how that goes. In the majority of cases, Athena seems to work just fine...But a few points of the particular datasets I've been working with recently have a pesky negatively-sloping pre-edge. Looks fine when I line up the normalized plots, but the fitting doesn't seem to work out. Thanks to all for your input, I've been working at the low-Z XAS for 4-5 years now and it feels like I learn new ways to screw it up (and new solutions!) every day. If you have any thoughts, don't hesitate to reach out. Mike

On Jan 15, 2019, at 7:42 PM, Christopher Patridge wrote:

Hi Mike,

I would suggest that, as Carlo mentions, that because you cut off at 2200 eV, the post edge background function in purple might be skewed a bit. I might be use values ~2190 eV and 2164 eV? Though it appears to be a “flat” region where you place the points, it may actually not be all that flat if data was collected further out. Even if you were to collected new data, if this short range is used and the same approach to normalization is used, I don’t know if LCF would improve.

Chris

...

On Jan 15, 2019, at 8:23 PM, Mike Massey wrote:

Hi Scott,

I collect up to 2240 eV often, because that's about where the mirror stops, but end up truncating it at 2200 eV before fitting because sometimes I end up with all sorts of non-systematic background things at higher energies, depending on the sample.

I still collect the data because it helps me to troubleshoot, but I chop it before fitting because 2200 eV is the limit of what I normally consider consistently "good data." I end up with spectra that, to my eye anyway, are comparable. For dilute samples, I don't usually trust it beyond that. For concentrated samples, I don't have a problem.

Thanks,

Mike

...

On Jan 15, 2019, at 5:12 PM, Scott Calvin wrote:

Hi Mike,

Is there a reason the data stops at 2200 eV—e.g., is there another edge just above that?

—Scott

...

On Jan 15, 2019, at 8:02 PM, Mike Massey wrote:

Thanks Riti and Carlo.

Riti: will give it a go, thanks! I think the data are salvageable, it's just a question of how...

Carlo: Interesting point. In terms of how I'm fitting it, I just use the LCF window to do a two-component fit. Nothing super fancy.

But what you're suggesting is that maybe I have a misunderstanding in the way I'm normalizing my data. That's entirely possible. I generally try to get a nice flat background by following the "flat part" of the spectra but in cases like this the pre- and post-edge lines end up not parallel. I can force it to be parallel, and that helps the fit to behave better, but ends up creating other normalization issues.

Much obliged for the responses so far.

Mike

...

On Jan 15, 2019, at 4:42 PM, Carlo Segre wrote:

Hi Mike:

How are you trying to fit it? It looks to me like the post edge line is not in the right place. You need to tune it by hand when you have such a short energy range after the edge since the normalization is critically dependent on how you set this line.

carlo

...

On Tue, 15 Jan 2019, Mike Massey wrote:

Hi All,

Does anyone have advice for LCF of spectra with a small edge step?

I've recently found myself with more and more issues fitting spectra like the one shown in the attached photo, where the spectrum is of good quality, but the fits end up not working out due to normalization (as far as I can tell).

Sadly, I have so far ended up just throwing these fits out, but I'm hoping there is a better solution out there that someone might be able to recommend.

One solution is "get more of the element of interest," as a beamline scientist recently told me. I'm just reluctant to really load it up in the data collection, because at 2 keV I get nervous about artifacts like self-absorption.

Is there a better way? What am I missing?

Thanks for your thoughts,

Mike

-- Carlo U. Segre -- Duchossois Leadership Professor of Physics Interim Chair, Department of Chemistry Director, Center for Synchrotron Radiation Research and Instrumentation Illinois Institute of Technology Voice: 312.567.3498 Fax: 312.567.3494 segre@iit.edu http://phys.iit.edu/~segre segre@debian.org _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit

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