[Ifeffit] FT transform

[Matt Newville]

Dear Li Dangdang,

On Wed, 11 Feb 2004, Li Dangdang wrote:

> Hi, i wonder if there is anyone to tell me what is the advantage of using 
> real and imaginary part of  transform in data and fit, compared with 
> magnitude plot.
> 
> Best, DD

The real and imaginary parts of the Fourier transform include both
the magnitude and phase terms.  For XAFS, the phase term is much
more sensitive than the magnitude to the distance and atomic species
of the scattering atom.  

In principle one could fit to the phase and magnitude terms instead
of the real and imaginary terms.  My experience was (many years ago
now) that this was not quite as robust as using the real and
imaginary terms, mostly because of the 2*pi ambiguity in the phase.

--Matt