Re: Spin.f program for SPXAS

[Alex Ankudinov <alex@leonardo.phys.washington.edu>]

Dear David,

Below are some answers to your questions and some questions to you.
I probably would have more comments if I saw the measured SPXAS and
had some explanation, how you measured it.

Best wishes
Alexei Ankudinov

On Tue, 29 Apr 2003, David Eustace wrote:
> 
> As some of you may know, i'm working on spin dependent calculations of CrN 
> at the moment.
> I have a quick and simple question about the spin.f program used to 
> calculate overall SPXAS from two (spin-up and spin-down) xmu.dat files. I'm 
> looking at the nitrogen K-edge.
  I do not understand, how you can get SPXAS at N, since the spin on N is
zero. I would use Cr K or L3 edge instead. 

> 
> When i look at my SPXAS plot generated by spin.f, i get something close to 
> experimental data, but with one large peak in the negative intensity 
> region. This doesn't make sense as you cannot have negative energy loss in 
> a XAS plot.
> 
> But the spin.f program contains the lines:
> 
> elseif (icase.eq.3) then
> c         factor=0.5 always for SPXAFS
>             t1 = (y1*ap - z1*am)/2.0/xnorm
>             t2 = (y2*ap - z2*am)/2.0/xnorm
>             t3 = (y3*ap - z3*am)/2.0/xnorm
> 
> which are obviously subtractions, which will give negative values if, say, 
> z1>y1.
> Does this explain the origin of the 'negative' peak, and if so, i assume 
> its perfectly reasonable to convert all negative numbers generated to 
> positive values?
 
No, it is not reasonable. You have to adjust the spin.f to correspond
to the measured signal. First, since you measure N K-edge, I do not
understand how you can get spin-up and spin-down signals at all.

The spin.f assumes that one wants to know
the DIFFERENCE between spin-up and spin-down signal. The difference
obviously can have either sign.
  
> 
> This seems fairly obvious but i wanted to make sure. Let me know if i'm 
> wrong, please.
> I've attached a copy of the SPXAS plot i'm talking about.

Sorry, I could not open successfully your file. May be it got corrupted.
Could you send me the picture of N K-edge SPXAS again, probably 
as a postscript file. Then probably I will have more comments.
Probably, you should also explain how did you measure the SPXAS.