Dear Jon (and others),
The "standard" reference for X-ray Lp corrections is the paper by L.
Azaroff, Acta Cryst., 8,701-704 (1955). It applies equally well for both
incident beam and diffracted beam monochromator cases and is independent of
choice of Debye-Scherrer, Bragg-Brentano or for that matter single crystal
diffraction (hence also applies to transmission geometry - but there may be
a separate "foot print" effect that depends on the optics. See below.). The
L (Lorentz) part is applied to CW neutron data (usually Debye-Scherrer)
since there is no neutron polarization effect. There are three "options" in
GSAS for x-ray Lp corrections. The first two are equivalent except for
scaling and the third is no correction at all; some folks wanted this as
they apply the correction beforehand. However, they must do proper error
propagation for this correction and use the "ESD" or "ALT" form for the raw
data input to GSAS. The third equation in the GSAS Manual was incorrect, it
was deleted sometime ago.
The crucial thing to establish for transmission geometry is whether the
illuminated (and seen!) sample volume is independent of scattering angle.
If it is independent then either GSAS Lp correction will work just fine. If
not, then an additional correction must be applied for the change in
sampling volume - this correction is not in GSAS as I don't know what it is.
It should also be noted that the sampling volume error can occur in
Bragg-Brentano geometry when the sample isn't "infinitely thick" or doesn't
cover completely the beam foot print at all measured scattering angles.
This is a particular problem for diffraction from organic solids with low
absorption or for thin "smears" of sample on a glass slide or samples that
are too small for complete beam coverage at low angles. GSAS doesn't have
anything to deal with these elementary experimental errors.
Bob Von Dreele
At 11:44 AM 7/11/00 +0100, you wrote:
>On Mon, 10 Jul 2000, Brian H. Toby wrote:
>
> > There are three polarization functions listed in the manual, but only 2
> > when I try to select the function in EXPEDT. I seem to remember that the
> > 1st two functions are equivalent, other than scaling, but I am not going
> > to work this mystery today.
>
>My apologies for such a public display of ignorance, I have a related
>mystery to ask about. Having had a quick look at the manual, all three
>corrections are for parallel geometry, I'm guessing "no monochromator",
>then "incident beam mono." and then "diffracted beam mono.". OK.
>
>The question is what about flat plate transmission (STOE) type data versus
>Bragg-Brentano versus Debye-Sherrer? Doesn't a factor of cos(theta) at
>least turns up in the transmission data. This is where I start to feel
>rather stupid, but how can you specify these cases? It doesn't appear to
>come into the instrument parameter file or the lorentz corrections
>dicussed here?
>
>Any hints or advice would be appreciated. Probably Bob has the best idea
>of what GSAS is doing, and how it's doing it. I've heard rumours of
>manual "corrections" being applied to data before presenting it for
>refinement, which sounds ugly. Fullprof has a flag for transmission
>geometry, hence my question.
>
>Thanks in advance,
>
>Jon Wright
>
>PS : If someone could refer me to simple derivations of the LP
>corrections for all the various geometries, I'd be very grateful.
>
>============================================================================
> Dept. of Chemistry, Lensfield Road, Cambridge, CB2 1EW, UK
>
