July 2016 - Ifeffit - millenia.cars.aps.anl.gov

Postdoc Position
by Jeff Terry 29 Jul '16

29 Jul '16

Hi All, I never remember what the rules are here for posting postdoc positions but I have one position available. You can see more information about the position here: http://mrmac.mr.aps.anl.gov/~jterry/postdoc.html Please let anyone who might be interested know. Thanks, Jeff Jeff Terry Professor, Department of Physics Illinois Institute of Technology Life Sciences Bldg Rm 166 3101 S. Dearborn St. Chicago IL 60616 630-252-9708

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Multiple scattering path parameters
by Neil M Schweitzer 29 Jul '16

29 Jul '16

Are there standard ways of relating the parameters (deltaR and sigma^2) of multiple scattering paths to the parameters of the single scattering paths based on the type of MS path? For example, would deltaR_MS = deltaR_SS*2 if the MS was a "forward through" geometry? What about other geometries and paths that involve different types of atoms? Neil

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Re: [Ifeffit] determining reasonable fitting parameters
by Neil M Schweitzer 27 Jul '16

27 Jul '16

Thanks to everyone for all the previous responses to my emails, I've learned a lot in the few weeks I've been on this list! I was hoping to build off of the last point Bruce made in message 3 below by asking another question. I am fitting a CeO2 reference from paths generated by feff from a cif file. Since this is my starting point, I made SO2 the same for every path (and used the coordination numbers from the cif file). If I am generating what looks to be a reasonable fit, but my SO2 is in the 0.55-0.7 range, what is my fit trying to tell me? Am I doing something wrong in my initial background subtraction in Athena? I'm asking about the background subtraction because I recently discovered that there is a CeO2 reference provided in Hephaestus. I exported the data, and tried to fit it in Artemis using a similar methodology I used for my own sample, and I got an even lower SO2. So I don't think the small SO2 value is an artifact of the way the experiment was run (although I might be wrong, I wasn't present when the data was actually collected). The observed difference seems real because the magnitude of the signal in E-space, k-space, and R-space is lower for the Hephaestus sample compared to my experimental sample (see attached image... One note, I'm not sure why the signals are off set in the Energy plot. The Eo's are the same, and if I plot the spectra together in E-space alone, they are not offset. Maybe a bug?). Neil -----Original Message----- From: Ifeffit [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] On Behalf Of ifeffit-request(a)millenia.cars.aps.anl.gov Sent: Thursday, July 21, 2016 8:01 AM To: ifeffit(a)millenia.cars.aps.anl.gov Subject: Ifeffit Digest, Vol 161, Issue 20 Send Ifeffit mailing list submissions to ifeffit(a)millenia.cars.aps.anl.gov To subscribe or unsubscribe via the World Wide Web, visit https://urldefense.proofpoint.com/v2/url?u=http-3A__millenia.cars.aps.anl.g… or, via email, send a message with subject or body 'help' to ifeffit-request(a)millenia.cars.aps.anl.gov You can reach the person managing the list at ifeffit-owner(a)millenia.cars.aps.anl.gov When replying, please edit your Subject line so it is more specific than "Re: Contents of Ifeffit digest..." Today's Topics: 1. demeter installation in OSX El Capitan (Ping, Yuan) 2. Re: determining reasonable fitting parameters (Bruce Ravel) 3. Re: determining reasonable fitting parameters (Bruce Ravel) ---------------------------------------------------------------------- Message: 1 Date: Thu, 21 Jul 2016 05:37:10 +0000 From: "Ping, Yuan" <ping2(a)llnl.gov> To: "ifeffit(a)millenia.cars.aps.anl.gov" <ifeffit(a)millenia.cars.aps.anl.gov> Subject: [Ifeffit] demeter installation in OSX El Capitan Message-ID: <D3B5AD95.32BB2%ping2(a)llnl.gov> Content-Type: text/plain; charset="windows-1252" Dear all, I recently upgraded my mac to OSX El Capitan, and tried to install demeter following the steps at https://urldefense.proofpoint.com/v2/url?u=http-3A__bruceravel.github.io_de… XQuartz 2.7.9 (xorg-server 1.17.4), Xcode 7.3.1, command line tools, ?sudo port -v self update?, ?port upgrade outdated? were all OK. When I tried ?sudo port install xorg-server demeter?, it showed: ---> Computing dependencies for xorg-server ---> Cleaning xorg-server ---> Computing dependencies for demeter ---> Dependencies to be installed: ifeffit libgcc cctools llvm-3.8 ---> libcxx libffi llvm_select gmp isl ld64 ld64-latest libmpc mpfr ---> pgplot gcc5 ... (a long list) Then after many attempts to fetch libcxx-3.7.1_0.darwin_15.x86_64.tbz2 from various websites, it showed: Error: org.macports.archivefetch for port libcxx returned: archivefetch failed for libcxx @3.7.1_0 Error: Failed to install libcxx Please see the log file for port libcxx for details: /opt/local/var/macports/logs/_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_lang_libcxx/libcxx/main.log The log file is attached. What should I do to fix it? Any suggestion will be appreciated. Thanks. Yuan

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Re: [Ifeffit] determining reasonable fitting parameters
by Neil M Schweitzer 27 Jul '16

27 Jul '16

Thanks for the responses. Seems like it is a problem without a simple solution, especially after the experiment has already been performed. When my users rerun the samples, I'll have to make sure they are very careful. In terms of the question about fitting the Hephaestus reference, when you plot it in Hephaestus it only shows out to ~50eV, but when you save the data file the data goes out much further. You can see from the plots I had included that the data goes out past k=10 A^-1. Neil -----Original Message----- From: Ifeffit [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] On Behalf Of ifeffit-request(a)millenia.cars.aps.anl.gov Sent: Wednesday, July 27, 2016 4:09 PM To: ifeffit(a)millenia.cars.aps.anl.gov Subject: Ifeffit Digest, Vol 161, Issue 39 Send Ifeffit mailing list submissions to ifeffit(a)millenia.cars.aps.anl.gov To subscribe or unsubscribe via the World Wide Web, visit https://urldefense.proofpoint.com/v2/url?u=http-3A__millenia.cars.aps.anl.g… or, via email, send a message with subject or body 'help' to ifeffit-request(a)millenia.cars.aps.anl.gov You can reach the person managing the list at ifeffit-owner(a)millenia.cars.aps.anl.gov When replying, please edit your Subject line so it is more specific than "Re: Contents of Ifeffit digest..." Today's Topics: 1. Re: determining reasonable fitting parameters (Matthew Marcus) 2. Re: determining reasonable fitting parameters (Bruce Ravel) 3. Re: determining reasonable fitting parameters (Matthew Marcus) 4. Re: determining reasonable fitting parameters (Bruce Ravel) ---------------------------------------------------------------------- Message: 1 Date: Wed, 27 Jul 2016 13:39:03 -0700 From: Matthew Marcus <mamarcus(a)lbl.gov> To: XAFS Analysis using Ifeffit <ifeffit(a)millenia.cars.aps.anl.gov> Subject: Re: [Ifeffit] determining reasonable fitting parameters Message-ID: <c2c71f99-44d9-f17f-ab52-399f53d8bb83(a)lbl.gov> Content-Type: text/plain; charset=windows-1252; format=flowed I'm not sure I get your point about ion-chamber response. Shouldn't that normalize out in post-edge spline? If the I0 chamber gets less sensitive as a function of energy, then the post-edge background rises by the same amount as the wiggles get bigger, so it divides out. Similarly for probe-depth effects, depending on concentration. Another couple of possible sources of amplitude error: 1. The model for the pre-edge background is inaccurate, so that its extrapolation to the EXAFS region is incorrect. For instance, if there's a lot of elastic scatter getting into the fluorescence detector, then the pre-edge is curved, and if you model that with a straight line fitted near the edge, them you'll be off far from the edge. Similarly, in a long transmission scan, both pre- and post-edge are curved. My background-sub program has simple models for thse shapes which aren't perfect but help. This effect is really hard to control for because you have no real way to know what the 'right' answer is, most of the time. 2. Overabsorption will definitely reduce the amplitude of the wiggles, and I'm surprise Bruce didn't menntion it. It looks much like pinhole effect. Harmonics do similar things as well in transmission. I still haven't heard how you do EXAFS analysis using the short spectrum from Hepheastus. mam On 7/27/2016 1:23 PM, Bruce Ravel wrote: > On 07/27/2016 03:53 PM, Neil M Schweitzer wrote: >> Thanks to everyone for all the previous responses to my emails, I've >> learned a lot in the few weeks I've been on this list! I was hoping >> to build off of the last point Bruce made in message 3 below by >> asking another question. I am fitting a CeO2 reference from paths >> generated by feff from a cif file. Since this is my starting point, I >> made SO2 the same for every path (and used the coordination numbers >> from the cif file). If I am generating what looks to be a reasonable >> fit, but my SO2 is in the 0.55-0.7 range, what is my fit trying to >> tell me? Am I doing something wrong in my initial background >> subtraction in Athena? >> >> I'm asking about the background subtraction because I recently >> discovered that there is a CeO2 reference provided in Hephaestus. I >> exported the data, and tried to fit it in Artemis using a similar >> methodology I used for my own sample, and I got an even lower SO2. So >> I don't think the small SO2 value is an artifact of the way the >> experiment was run (although I might be wrong, I wasn't present when >> the data was actually collected). The observed difference seems real >> because the magnitude of the signal in E-space, k-space, and R-space >> is lower for the Hephaestus sample compared to my experimental sample >> (see attached image... One note, I'm not sure why the signals are off >> set in the Energy plot. The Eo's are the same, and if I plot the >> spectra together in E-space alone, they are not offset. Maybe a >> bug?). > > I don't remember much about the provenance of that CeO2 spectrum. I > don't remember how calibration was done (or even if it is reliable). > I don't remember much about the sample -- it was a pellet, but I have > no way of knowing if it was homogeneous or even if the particles were > of the size reported. > > I guess what I am trying to say is this -- if you trust your CeO2 > measurement, you should have more faith in that (or the one Matthew > just sent) than in the one from Hephaestus. > > As for your S02 question, here's some of the standard litany: > > + the mean free path used by feff was too long. > > + S02 and sigma^2 are highly correlated. > > + you did a lousy job normalizing the data (although it sure doesn't > look that way from your picture). > > + your sample had a lot of pinholes. > > + there is some source of loss in CeO2 that is neglected in Feff > (which is, practically speaking, the same as the mean free path > comment). You could try fixing S02 and floating an Ei parameter. > Or floating both S02 and Ei, although they will be highly > correlated. > > + There is also an energy response to the ionization chambers at such > a low energy -- that is, the absorption of the gas in the detector > is weaker at the end of the spectrum than at the beginning. In > fluorescence, that serves to attenuate the edge0step-normalized > spectrum in a way that looks like an enhancement to sigma^2. But > it's an attenuation, which could show up in S02. > > I should mention that in my favorite teaching example -- FeS2 -- by > the end of the lecture I have a S02 of something like 0.69. And I > just walk away from the lecture at that point :) The reason I do so is > that all the rest of the parameters are defensible and I chalk the > small S02 up to one of the points above (along with some enthusiastic > hand waving -- usually it's time for coffee or lunch by the time I've > finished the FeS2 demo!). > > B > > ------------------------------ Message: 2 Date: Wed, 27 Jul 2016 16:45:26 -0400 From: Bruce Ravel <bravel(a)bnl.gov> To: XAFS Analysis using Ifeffit <ifeffit(a)millenia.cars.aps.anl.gov> Subject: Re: [Ifeffit] determining reasonable fitting parameters Message-ID: <e0a93edf-8a8b-97a5-aa14-f595fc385c31(a)bnl.gov> Content-Type: text/plain; charset=windows-1252; format=flowed On 07/27/2016 04:39 PM, Matthew Marcus wrote: > I'm not sure I get your point about ion-chamber response. Shouldn't > that normalize out in post-edge spline? Athena edge-step normalizes (i.e. normalizes by a constant) by default. It does not do a functional (or energy dependent) normalization by default in the way you are suggesting. > If the I0 chamber gets less sensitive as a function of energy, then > the post-edge background rises by the same amount as the wiggles get > bigger, so it divides out. Similarly for probe-depth effects, > depending on concentration. Another couple of possible sources of > amplitude error: > 1. The model for the pre-edge background is inaccurate, so that its > extrapolation to the EXAFS region is incorrect. > For instance, if there's a lot of elastic scatter getting into the > fluorescence detector, then the pre-edge is curved, > and if you model that with a straight line fitted near the edge, > them you'll be off far from the edge. Similarly, in > a long transmission scan, both pre- and post-edge are curved. My > background-sub program has simple models for thse > shapes which aren't perfect but help. This effect is really hard > to control for because you have no real way to know > what the 'right' answer is, most of the time. > > 2. Overabsorption will definitely reduce the amplitude of the > wiggles, and I'm surprise Bruce didn't menntion it. It looks > much like pinhole effect. Harmonics do similar things as well in > transmission. My brain is really tiny and it's getting to be late in the afternoon here. It's amazing that many bullet points actually made it into the email! B -- Bruce Ravel ------------------------------------ bravel(a)bnl.gov National Institute of Standards and Technology Synchrotron Science Group at NSLS-II Building 743, Room 114 Upton NY, 11973 Homepage: https://urldefense.proofpoint.com/v2/url?u=http-3A__bruceravel.github.io_ho… Software: https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_bruceravel&… Demeter: https://urldefense.proofpoint.com/v2/url?u=http-3A__bruceravel.github.io_de… ------------------------------ Message: 3 Date: Wed, 27 Jul 2016 13:56:41 -0700 From: Matthew Marcus <mamarcus(a)lbl.gov> To: XAFS Analysis using Ifeffit <ifeffit(a)millenia.cars.aps.anl.gov> Subject: Re: [Ifeffit] determining reasonable fitting parameters Message-ID: <3ee04494-5113-2994-3d96-e9e5bd2ebecc(a)lbl.gov> Content-Type: text/plain; charset=windows-1252; format=flowed Oh, so it assumes that chi(k) = (mu(E(k))-spline(k))/const.? Mine does (mu(E(k))-spline(k))/spline(k), which I guess has its own problems - any wiggles driven in the spline will multiply the EXAFS. I haven't seen any effect from this, though. Maybe the right thing is (mu(E(k))-spline(k))/smooth(E(k)) where smooth is a smooth post-edge background such as a polynomial or spline with fewer knots than the subtraction spline or even a tensioned spline. I'd be willing to bet that this refinement will make no detectable difference, especially if you do the reference data the same way. I've found that you can do the most amazingly bogus things and get away with it as long as you do them equally to your reference and unknown, especially if the reference is very similar to the unknown. Of course, you can't rely on that. mam On 7/27/2016 1:45 PM, Bruce Ravel wrote: > On 07/27/2016 04:39 PM, Matthew Marcus wrote: >> I'm not sure I get your point about ion-chamber response. Shouldn't >> that normalize out in post-edge spline? > > Athena edge-step normalizes (i.e. normalizes by a constant) by default. It does not do a functional (or energy dependent) normalization by default in the way you are suggesting. > >> If the I0 chamber gets less sensitive as a function of energy, then >> the post-edge background rises by the same amount as the wiggles get >> bigger, so it divides out. Similarly for probe-depth effects, >> depending on concentration. Another couple of possible sources of >> amplitude error: >> 1. The model for the pre-edge background is inaccurate, so that its >> extrapolation to the EXAFS region is incorrect. >> For instance, if there's a lot of elastic scatter getting into >> the fluorescence detector, then the pre-edge is curved, >> and if you model that with a straight line fitted near the edge, >> them you'll be off far from the edge. Similarly, in >> a long transmission scan, both pre- and post-edge are curved. My >> background-sub program has simple models for thse >> shapes which aren't perfect but help. This effect is really hard >> to control for because you have no real way to know >> what the 'right' answer is, most of the time. >> >> 2. Overabsorption will definitely reduce the amplitude of the >> wiggles, and I'm surprise Bruce didn't menntion it. It looks >> much like pinhole effect. Harmonics do similar things as well in >> transmission. > > My brain is really tiny and it's getting to be late in the afternoon here. It's amazing that many bullet points actually made it into the email! > > B > > ------------------------------ Message: 4 Date: Wed, 27 Jul 2016 17:08:58 -0400 From: Bruce Ravel <bravel(a)bnl.gov> To: XAFS Analysis using Ifeffit <ifeffit(a)millenia.cars.aps.anl.gov> Subject: Re: [Ifeffit] determining reasonable fitting parameters Message-ID: <a632df68-e115-cda0-3ace-4c066230b85f(a)bnl.gov> Content-Type: text/plain; charset=windows-1252; format=flowed On 07/27/2016 04:56 PM, Matthew Marcus wrote: > Oh, so it assumes that chi(k) = (mu(E(k))-spline(k))/const.? Mine > does (mu(E(k))-spline(k))/spline(k), which I guess has its own > problems Exactly. Both solutions have problems. The potential for trouble with "/const" seems less severe to me and easier to support (in the sense of supporting the use of a software package). So that's why Athena does what she does. B -- Bruce Ravel ------------------------------------ bravel(a)bnl.gov National Institute of Standards and Technology Synchrotron Science Group at NSLS-II Building 743, Room 114 Upton NY, 11973 Homepage: https://urldefense.proofpoint.com/v2/url?u=http-3A__bruceravel.github.io_ho… Software: https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_bruceravel&… Demeter: https://urldefense.proofpoint.com/v2/url?u=http-3A__bruceravel.github.io_de… ------------------------------ Subject: Digest Footer _______________________________________________ Ifeffit mailing list Ifeffit(a)millenia.cars.aps.anl.gov https://urldefense.proofpoint.com/v2/url?u=http-3A__millenia.cars.aps.anl.g… Unsubscribe: https://urldefense.proofpoint.com/v2/url?u=http-3A__millenia.cars.aps.anl.g… ------------------------------ End of Ifeffit Digest, Vol 161, Issue 39 ****************************************

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Re: [Ifeffit] Athena Question
by tyson 27 Jul '16

27 Jul '16

Thanks for the fast response.TAT Sent via the Samsung GALAXY S® 5, an AT&T 4G LTE smartphone -------- Original message -------- From: Bruce Ravel <bravel(a)bnl.gov> Date: 7/27/2016 12:34 PM (GMT-06:00) To: "Tyson, Trevor A." <trevor.a.tyson(a)njit.edu> Cc: XAFS Analysis using Ifeffit <ifeffit(a)millenia.cars.aps.anl.gov> Subject: Re: Athena Question On 07/27/2016 12:01 PM, Tyson, Trevor A. wrote: > Hi Bruce, > > I know that you are very busy building beaminles but I could not > resist adding more to your load. I have an Athena question. I have > aligned a large set of data to a reference. Now for each data set > Athena records the energy shift. Is it possible to export the list of > shifts? Thanks. > > Trevor > Hi Trevor, Hope you don't mind that I am CCing this to the Ifeffit mailing list. The question seems broadly interesting, so leaving an answer in a place that can be found by google seems like a good idea. There are a several ways of doing this: 1. Right click on the label that says "Energy shift". one of the options is "Show energy shifts of all groups". That will post a dialog like the one in the attached picture. Click the "Save" button to write the contents as a text file. 2. Click on the File menu, then on the Export submenu. Select "Excel report on marked groups". This will write an XLS file. You can open that up in Excel or any other spreadsheet program. Column E has the energy shifts. 3. For the sake of completeness, if you are a command line kinda guy, you can use the "dlsprj" program that gets installed when you install the rest of demeter: dlsprj myproject.prj --params=bkg_eshift,bkg_delta_eshift This will print the shifts and their uncertainties to the screen. B -- Bruce Ravel ------------------------------------ bravel(a)bnl.gov National Institute of Standards and Technology Synchrotron Science Group at NSLS-II Building 743, Room 114 Upton NY, 11973 Homepage: http://bruceravel.github.io/home/ Software: https://github.com/bruceravel Demeter: http://bruceravel.github.io/demeter/

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Re: [Ifeffit] Athena Question
by Bruce Ravel 27 Jul '16

27 Jul '16

On 07/27/2016 12:01 PM, Tyson, Trevor A. wrote: > Hi Bruce, > > I know that you are very busy building beaminles but I could not > resist adding more to your load. I have an Athena question. I have > aligned a large set of data to a reference. Now for each data set > Athena records the energy shift. Is it possible to export the list of > shifts? Thanks. > > Trevor > Hi Trevor, Hope you don't mind that I am CCing this to the Ifeffit mailing list. The question seems broadly interesting, so leaving an answer in a place that can be found by google seems like a good idea. There are a several ways of doing this: 1. Right click on the label that says "Energy shift". one of the options is "Show energy shifts of all groups". That will post a dialog like the one in the attached picture. Click the "Save" button to write the contents as a text file. 2. Click on the File menu, then on the Export submenu. Select "Excel report on marked groups". This will write an XLS file. You can open that up in Excel or any other spreadsheet program. Column E has the energy shifts. 3. For the sake of completeness, if you are a command line kinda guy, you can use the "dlsprj" program that gets installed when you install the rest of demeter: dlsprj myproject.prj --params=bkg_eshift,bkg_delta_eshift This will print the shifts and their uncertainties to the screen. B -- Bruce Ravel ------------------------------------ bravel(a)bnl.gov National Institute of Standards and Technology Synchrotron Science Group at NSLS-II Building 743, Room 114 Upton NY, 11973 Homepage: http://bruceravel.github.io/home/ Software: https://github.com/bruceravel Demeter: http://bruceravel.github.io/demeter/

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Problem with Artemis opening an Athena project
by Irina Pi 27 Jul '16

27 Jul '16

Dear all, I have a problem opening an Athena project with Artemis. When I have selected it, it appears this error: Use of uninitialized value in addition (+) at C:/strawberry/perl/site/lib/Demeter/UI/Artemis/Prj.pm line 132 And after selecting OK and import selected data, everything closes. This Prj.pm file is attached in the mail, if it is of any help. Thank you. Best regards, Irina Pi.

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about sigma square
by Bruce Ravel 26 Jul '16

26 Jul '16

Shaofeng Wang asked this question on the mailing list that some may not have seen because it got marked as spam somewhere along the line. Kinda weird. Not quite sure why that happened.... > I have a question about sigma square. Must this parameter be in the > range of 0.003-0.02? I obtained some of it in the range of > 0.0001-0.002. Is this reasonable? Could please provide some > reference? There are no hard rules for anything. That sigma^2 is 0.00something is a good guideline and often the case, but not an actual rule. The problem with a sigma^2 in the range of 0.0something is that it tends to attenuate the path so much that it doesn't really have an impact on the quality of the fit. In that case the uncertainty is often very large precisely because the path no longer contributes significant spectral weight to the fit. A sigma^2 in the range of 0.000something means that the bond is very stiff. In some situations, that would be correct. As an example, consider the sort of negative thermal expansion material that is a network of metal-ligand octahedra. The NTE effect happens in that case because, as the material heats, up the octahedra tilt within the network, pulling the network inwards. This happens because the metalligand (usually metal-oxygen) bond is very strong -- much stronger than the forces connecting the octahedra. In an EXAFS measurement of an NTE material, you will likely find a very small sigma^2 for the first shell. (And a very large one for the second shell.) Regardless of the size of sigma^2, the bottom line is that the value is defensible. This means that you can understand and explain why sigma^2 is the size that it is. It also means that the uncertainty is such that you can support your conclusion. While the red line might overplot the blue with sigma^2=0.0003+0.001, that may be a troubling result because sigma^2 is not positive definite! Executive summary: a reasonable value is a defensible value. If you are willing to explain a result to your thesis committee or let one of us review it in a manuscript, then the result is probably reasonable. HTH, B -- Bruce Ravel ------------------------------------ bravel(a)bnl.gov National Institute of Standards and Technology Synchrotron Science Group at NSLS-II Building 743, Room 114 Upton NY, 11973 Homepage: http://bruceravel.github.io/home/ Software: https://github.com/bruceravel Demeter: http://bruceravel.github.io/demeter/

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Help with Athena
by Irina Pi 26 Jul '16

26 Jul '16

Dear all, I am just getting started with XAFS projects and I am using Athena software for processing and analysing the data. I am learning how to use it, and I have a little problem. The thing is that when I plot the XANES data, the automatic created plot is not labeled and plus it doesn't have the option of selecting a point of the spectrum. I mean, when I want to choose E0 for example, I need to do it changing the number in the Athena screen, and I don't know exactly what I am choosing because it doesn't appear anything in the graph. Was I clear? Do you know any way which I could solve it? Thank you very much, Regards, Irina Pi.

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Re: [Ifeffit] Regarding time resolved XAS
by Chris Milne 26 Jul '16

26 Jul '16

Hi Mazhar, Usually time-resolved (pump-probe) XAS is measured in data set pairs at each X-ray energy: one with laser on, one with laser off. This means that you can always directly reference your laser-excited measurement to your laser un-excited measurement. To calibrate to an absolute scale you would then use a reference foil as normal and shift both data sets according to the reference values. If you had significant delays between your laser on/off measurements then drifts and other noise sources will affect your data, making it very difficult to determine a reliable difference signal. You could try to use a reference foil measurement to correct for these kinds of drifts but you need to be very careful with the corrections to ensure you aren’t just seeing an artifact. Ideally you would be measuring laser on/off differences on a millisecond to microsecond timescale between acquisitions, this gets rid of most of the possible noise sources. Best, Chris > On 30 May 2016, at 19:00 , ifeffit-request(a)millenia.cars.aps.anl.gov wrote: > > Send Ifeffit mailing list submissions to > ifeffit(a)millenia.cars.aps.anl.gov > > To subscribe or unsubscribe via the World Wide Web, visit > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit > or, via email, send a message with subject or body 'help' to > ifeffit-request(a)millenia.cars.aps.anl.gov > > You can reach the person managing the list at > ifeffit-owner(a)millenia.cars.aps.anl.gov > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Ifeffit digest..." > > > Today's Topics: > > 1. Regarding time resolved XAS (mazhar iqbal) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Mon, 30 May 2016 06:20:36 +0000 (UTC) > From: mazhar iqbal <mazharphy_05(a)yahoo.com> > To: "ifeffit(a)millenia.cars.aps.anl.gov" > <ifeffit(a)millenia.cars.aps.anl.gov> > Subject: [Ifeffit] Regarding time resolved XAS > Message-ID: > <349705754.1395970.1464589236411.JavaMail.yahoo(a)mail.yahoo.com> > Content-Type: text/plain; charset="utf-8" > > hello.. I am sorry if am not posting on right place. > I want to ask about the aligning the XAS spectra that is given in Athena. Can we do the alignment of time resolved spectra? As you know that the resolved spectra (IR pump X-ray probe) may have edge shift due to laser pumping. Do you think that the same method of the alignment can be applied here as well. An additional piece of information is that ?we have a reference fluorescence line always on the screen that well behind the K absorption edge. > Hoping to see your response.? > Best Regards >

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