Re: [ccp4bb] Is anomalous signal a different wavelength?

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CCP4bb <-- 1999 <-- November 1999 <-- 30 November 1999

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Subject: Re: refmac crashes when ncs restraints are applied
From: Eleanor Dodson ccp4 {- at -} YSBL {- dot -} YORK {- dot -} AC {- dot -} UK
Date: 2007-05-31

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Subject: Re: Rfree into an existing mtz...
From: Patskovsky Yury yurip {- at -} MEDUSA {- dot -} BIOC {- dot -} AECOM {- dot -} YU {- dot -} EDU
Date: 2007-05-31

Subject: Re: Is anomalous signal a different wavelength?
From: Ethan A Merritt merritt {- at -} U {- dot -} WASHINGTON {- dot -} EDU
Date: 2007-05-31

On Thursday 31 May 2007 01:37, Marc SCHILTZ wrote:
> Ethan A Merritt wrote:
> > And please note that "resonant scattering" is not a standard term.
>
> "Resonant Scattering" is now the standard term accepted and used
> anywhere in the X-ray physics and crystallography literature, except in
> protein crystallography.

I stand corrected.

Is there a IUPAC or other standard definition of the term?
My attempts to find one via Google did not turn up anything definitive.

The literature examples that I found use the term to refer to the experiment
itself, or to the scattering process as a whole. However the original
poster used the term in such a way that made it sound as if the
"resonant scattering" was roughly a synonym for "imaginary component of the
scattering factor", which is not exactly the same thing. If we protein
crystallographers are to begin using the term, we should first pin down
what the definition is.

> It is the more adequate term since the X-ray phenomena under discussion
> involve resonant interactions of photons with matter and are actually
> not at all 'anomalous'.

The 'anomalous' behaviour that gave rise to this label is the violation
of the general principle that the strength of interaction between
light and solid matter decreases as the wavelength decreases.
At an absorption edge, the scattering curve is anomalous because
the strength of the interaction is not monotone decreasing with photon energy.
Instead it increases as you approach the edge from either direction.
The term was inherited from the description of experiments with visible
light that characterized the wavelength-dependence of a material's index of
refraction.

--
Ethan A Merritt
Biomolecular Structure Center
University of Washington, Seattle 98195-7742

CCP4bb navigation

CCP4bb <-- 1999 <-- November 1999 <-- 30 November 1999

Previous message:
Subject: Re: refmac crashes when ncs restraints are applied
From: Eleanor Dodson ccp4 {- at -} YSBL {- dot -} YORK {- dot -} AC {- dot -} UK
Date: 2007-05-31

Next message:
Subject: Re: Rfree into an existing mtz...
From: Patskovsky Yury yurip {- at -} MEDUSA {- dot -} BIOC {- dot -} AECOM {- dot -} YU {- dot -} EDU
Date: 2007-05-31