>...So absorption is only a problem when the peaks disappear.
???
A comment on the various statements about absorption... For classical X-ray
or neutron powder diffraction there are tables of corrections for absorbtion.
However, if mu*R ~< 1 and cylindrical samples, one can easily show that the
effect of absorption is simply to underestimate the over-all Debye-Waller
factor eg Acta Cryst A35,248, 1979).
This does not mean that you MUST keep mu*R < 1, only that the angular
dependence is more complicated if mu*R >~ 1 (but still rather smooth).
Very strongly absorbing samples can be handled with constant wavelength
neutron machines eg location of Hydrogen in Europium as done on D20.
(Kohlmann et al. Ang. Chemie Int. Ed. English, 38 2029-2032, 1999)
For TOF data (pulsed neutron sources) absorption can be much more complicated
as Bob says; if you have "high absorption" you may be near an absorption
edge,
where it varies rapidly with wavelength ie with d-spacing. Under those
conditions, I guess that it might affect more than the Debye-Waller factor
before the peaks disappear :-)
I doubt that it is easy to either calibrate/measure or refine the effect of
strong absorption under these conditions; the only way to do it might be to
input the wavelength dependence for the absorbing elements, as Bob plans
(or else perform the experiment with constant wavelength :-)
Alan.
Alan Hewat, ILL Grenoble, FRANCE <[EMAIL PROTECTED]> tel (33) 4.76.20.72.13
ftp://ftp.ill.fr/pub/dif fax (33) 4.76.20.76.48 http://www.ill.fr/dif/
