Quoting Jacob Keller <[EMAIL PROTECTED]>:
The reason I called the phenomenon "resonant scattering" is because
that is the term used by
"Elements of Modern X-ray Physics" by Jens Als-Nielsen, Des
McMorrow. I prefer the term also
because this scattering is, as somebody has said, no longer really
"anomalous--" it fits well into
x-ray physical theory.
Let the heroes speak:
In 1994 D. H. Templeton wrote:
"The index of refraction of transparent materials for visible light
generally increases as the wavelength decreases and this dispersion is
said to be 'normal'. Near absorption bands there are intervals of
wavelength where the slope of n versus \lambda is positive, and the
dispersion is 'anomalous'. According to this convention and the
relation between n and f', x-ray dispersion is anomalous only in those
intervals where df'/d\lambda is negative. Yet 'anomalous dispersion'
and 'anomalous scattering' have come to be used for the effects of
absorption on x-ray optical properties at all wavelengths, or
sometimes perhaps only for those related to the imaginary term f".
These effects are significant for nearly all atoms at all wavelengths
commonly used for diffraction experiments, and therefore 'anomalous'
is somewhat inappropriate. I prefer 'dispersion' or 'resonant
scattering'."
(in 'Resonant Anomalous X-ray Scattering: Theory and Applications',
G.Materlik, C.J.Sparks & K.Fischer (eds.), Elsevier Science,
Amsterdam: 1994)
The editors (G.Materlik, C.J.Sparks & K.Fischer) of that same book
wrote in the preface:
"Since resonant interactions are characteristic of the interaction of
photons with matter, we suggest that 'resonant' better describes the
field than 'anomalous' scattering."
But note that they used the pleonasm "Resonant Anomalous X-ray
Scattering" as a title for their book ;-)
More recent review articles use the term "resonant" scattering or
"resonant" diffraction, e.g.
Hodeau JL, Favre-Nicolin V, Bos S, Renevier H, Lorenzo E, Berar JF
(2002). "Resonant diffraction". Chem Rev. 101, 1843--1867.
which includes a section on MAD phasing.
Thus, "resonant scattering" and "anomalous scattering" are synonyms
and it is almost a matter of taste which term one prefers. Both are
perfectly acceptable. The x-ray physics and crystallography
communities (except protein crystallography) have shifted from the
usage of "anomalous scattering" to "resonant scattering".
But then, as you write, if we want to keep the MAD SAD SIRAS etc
acronyms we are tied to "anomalous".
------------
Marc Schiltz
