[Ifeffit] calibration/alignment

[Scott Calvin]

Hi Ornel,

Alignment is used to compensate for monochromators that do not  
maintain stable energy calibration between scans. In conventional  
measurements, you'll generally have several scans that are supposed to  
be of the same sample under identical conditions, and those scans may  
need to be aligned with each other. That is not the case for a time  
series, which is what I think you are saying you have.

So in a time series, how do you compensate for any energy drift of the  
monochromator? If you are recording a simultaneous reference spectrum,  
you can align the reference spectra to each other. (Athena  
automatically will shift the sample spectra by the same amount that  
the reference spectra are shifted.)

If you have a time series but don't have a simultaneous reference  
spectra, it becomes tougher. If you collected a reference spectrum  
before and after the time series, you could try to interpolate any  
shift that's seen, although that's dicey; shifts sometimes occur in  
jumps. But if there's no shift, you're probably OK!

If you have a time series and no reference at all, or a reference only  
before the series, you're out of luck. You're relying then on the  
assumption of energy stability, which on some beamlines might be  
OK...but it is best to confirm that by at least measuring a reference  
before and after.

--Scott Calvin
Sarah Lawrence College

On Oct 14, 2010, at 2:46 AM, ornella smila castro wrote:

> Hi everyone,
>
> I am trying to do some data processing with Athena but I am already  
> stuck at the first step. The thing is: I read the worked example  
> section of the "Athena's user guide" and on the example on the iron  
> foil, it is mentioned to calibrate the data at the right energy  
> (until here evrything is fine) but then it is said to align the  
> data. Can anyone explain to me what does "alignment" exactly means,  
> and what is the aim of "aligning the data".   The data that I have  
> collected were through a channel through which a solution flow (~200  
> microliters/hr) so I am not convinced that alignment makes sense.
>
> Many thanks,
> Ornel
> <ATT00001..txt>