Dear Colleagues,
The real issue is the 'anomalous' word introduced as an X-ray scattering theory
correction, which was not anomalous but the actual physical situation of
resonance scattering. Thus the most recent of the Anomalous Scattering
conferences was correctly called REXS2011. Ie Resonant Elastic X-ray
Scattering.
The 1975 Anomalous Scattering Conference book incidentally has the Hoppe and
Jakubowski Ni and Co K alpha two wavelength study either side of the Fe K edge
for phase determination of the erythrocruorin protein, in turn based on the
Okaya and Pepinsky 1956 formalism. These are MAD but 'simply' not synchrotron.
Francis Crick's autobiography 'What Mad Pursuit' will give you a further link
to MAD, based on weak ie small intensity changes.
Just to also mention I regularly refer to the 'Hendrickson Se-met MAD' method.
The history is interesting. Keith Hodgson is a must mention name, as is
Stanford Synchrotron Radiation Laboratory.
A most recent wrinkle in nomenclature in this area is the use in chemical
crystallography by some of Resonant scattering for off resonance ie in
determining the hand of organics. At present I see no way around correcting
such mentions but with the unfortunate term:-
Off-resonance resonance scattering Flack parameter determination of the hand.
Greetings,
John
Prof John R Helliwell DSc
On 19 Jan 2012, at 17:50, Ian Tickle wrote:
> Perhaps I could chime in with a bit of history as I understand it.
>
> The term 'dispersion' in optics, as everyone who knows their history
> is aware of, refers to the classic experiment by Sir Isaac Newton at
> Trinity College here in Cambridge where he observed white light being
> split up ('dispersed') into its component colours by a prism. This is
> of course due to the variation in refractive index of glass with
> wavelength, so then we arrive at the usual definition of optical
> dispersion as dn/dlambda, i.e. the first derivative of the refractive
> index with respect to the wavelength.
>
> Now the refractive index of an average crystal at around 1 Ang
> wavelength differs by about 1 part in a million from 1, however it can
> be determined by very careful and precise interferometric experiments.
> It's safe to say therefore that the dispersion of X-rays (anomalous
> or otherwise) has no measurable effect whatsoever as far as the
> average X-ray diffraction experiment (SAD, MAD or otherwise) is
> concerned. The question then is how did the term 'anomalous
> dispersion' get to be applied to X-ray diffraction? The answer is
> that it turns out that the equation ('Kramer-Kronig relationship')
> governing X-ray scattering is completely analogous to that governing
> optical dispersion, so it's legitimate to use the term 'dispersive'
> (meaning 'analogous to dispersion') for the real part of the
> wavelength-dependent component of the X-ray scattering factor, because
> the real part of the refractive index is what describes dispersion
> (the imaginary part in both cases describes absorption).
>
> So then from 'dispersive' to 'dispersion' to describe the wavelength
> dependence of X-ray scattering is only a short step, even though it
> only behaves _like_ dispersion in its dependence on wavelength.
> However having two different meanings for the same word can get
> confusing and clearly should be avoided if at all possible.
>
> So what does this have to do with the MAD acronym? I think it stemmed
> from a visit by Wayne Hendrickson to Birkbeck in London some time
> around 1990: he was invited by Tom Blundell to give a lecture on his
> MAD experiments. At that time Wayne called it multi-wavelength
> anomalous dispersion. Tom pointed out that this was really a misnomer
> for the reasons I've elucidated above. Wayne liked the MAD acronym
> and wanted to keep it so he needed a replacement term starting with D
> and diffraction was the obvious choice, and if you look at the
> literature from then on Wayne at least consistently called it
> multi-wavelength anomalous diffraction.
>
> Cheers
>
> -- Ian
>
> On 18 January 2012 18:23, Phil Jeffrey wrote:
>> Can I be dogmatic about this ?
>>
>> Multiwavelength anomalous diffraction from Hendrickson (1991) Science Vol.
>> 254 no. 5028 pp. 51-58
>>
>> Multiwavelength anomalous diffraction (MAD) from the CCP4 proceedings
>> http://www.ccp4.ac.uk/courses/proceedings/1997/j_smith/main.html
>>
>> Multi-wavelength anomalous-diffraction (MAD) from Terwilliger Acta Cryst.
>> (1994). D50, 11-16
>>
>> etc.
>>
>>
>> I don't see where the problem lies:
>>
>> a SAD experiment is a single wavelength experiment where you are using the
>> anomalous/dispersive signals for phasing
>>
>> a MAD experiment is a multiple wavelength version of SAD. Hopefully one
>> picks an appropriate range of wavelengths for whatever complex case one has.
>>
>> One can have SAD and MAD datasets that exploit anomalous/dispersive signals
>> from multiple difference sources. This after all is
