Hi All, I am a little bit surprised how my post had made a nice discussion ;)
I didn't want to discuss about reference spectra and their usefulness. I only wanted to point that when you merge data with marked reference groups you get inversed merged groups - in "merge" group reference and in "Ref merge" your real data. Let users decide if it is useful or not. For me it is! Especially when I work with many repeats, like e.g. in QEXAFS, when the signal from sample and from reference in single scan is to noisy. cheers darek >-----Original Message----- >From: ifeffit-boun...@millenia.cars.aps.anl.gov >[mailto:ifeffit-boun...@millenia.cars.aps.anl.gov] On Behalf >Of Scott Calvin >Sent: Friday, November 20, 2009 12:48 AM >To: XAFS Analysis using Ifeffit >Subject: Re: [Ifeffit] Bug in Athena? > > > > >On Nov 19, 2009, at 2:59 PM, Matt Newville wrote: > >> For this case, wouldn't it be better to measure the reference >> separately to determine the chemical shift, and not rely on the >> reference channel for this purpose? >> >> How often is the reference channel both noisy AND improved >by merging? >> That would imply a transmission measurement that was poor due to low >> flux. But if this is because the sample is thick as you >suggest, the >> x-rays hitting the reference could be dominated by >harmonics, and the >> reference data may just be bad, not noisy due to counting statistics. > > >It's a good point. But pick your poison. When I am trying to be >careful about chemical shift, I don't trust that the mono won't just >happen to skip a step between measuring the standard separately and >measuring the sample. So I do both. I measure a standard in >the sample >channel, with a reference in the reference channel. I then leave the >reference in the reference channel, and put my sample in. If the >sample is a "reasonable" thickness for transmission, but a bit on the >high side (say 2.3 absorption lengths), the photon count is down >pretty far by the time it gets to the reference. The reference >is also >often the worst detector and amplifier that a line has, as the good >stuff is used for I0, It, and If. So the reference channel may well >have a considerable amount of random noise which can be improved by >merging. > >If that's the case, and if my sample appears to be suffering no beam >damage (scans when aligned, lie on top of each other), then I align >used the sample data. I then merge the sample data and the reference >data. By comparing the sample to the reference and the previous scans >where I measured the standard to the reference, I can see if there's >been any energy shift between scans. As far as harmonics, this >procedure should detect them. If the merged reference looks different >from sample to sample (including the case where a standard was >also in >the sample channel), that suggests that there are issues with >harmonics. If those issues move the first peak of the first >derivative, I know they're going to affect my determination of >chemical shift. Also, if I get a nonzero chemical shift from this >procedure for the standard, I know there's an issue. If not, they're >not a problem. > >The net result is that I have good confidence that I'm getting >accurate chemical shifts, as loss of energy calibration, harmonics, >and noise should all become evident by this procedure. > >I'm not recommending this procedure over others; it's just what I do >in some cases. But it doesn't seem like an unreasonable >procedure to me. > >--Scott Calvin >Sarah Lawrence College > >_______________________________________________ >Ifeffit mailing list >Ifeffit@millenia.cars.aps.anl.gov >http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit > _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit