Brian H. Toby wrote:
> ... <SOAPBOX>Besides, Q
> is the Fourier conjugate of x (the coordinates); d-spaces have no
> physical significance. D-spaces can mislead a novice into correlating
> reciprocal space distances with real space distances.</SOAPBOX>
D-spacings are distances between lattice planes and provide me at least
with a nice physical picture of Bragg's law. In this way I guess I am
correlating real space distances with reciprocal space distances, but I
think I prefer remain a novice than give up what little understanding I
have :)
>and thus cannot be compared between techniques and not d-space because
>that overemphasizes the low Q (low angle) portion of the pattern which
>has minor leverage on most results and further is where one rarely needs
>to worry about optimizing instrumental resolution.
High resolution at low Q is handy for indexing, and is essential for
tackling moderately complex magnetic structures with powder neutron data.
ToF diffractometers which use slow neutrons (eg OSIRIS at ISIS) appear to
be doing particularly well for resolution in the d=3-10 Angstrom region (I
hope we eventually see such machines at SNS and ESS). Peak broadening
effects and splittings can be easier to interpret where there is less peak
overlap, so high resolution in this range can again be useful. It's a
question of applying the right instrument or setup to the particular
problem in mind.
>In the plot on my web page, low angle asymmetry is subtracted out -- but it
>should not be. For very short wavelength machines, the asymmetry can be
>severe for the bulk of the pattern.
There are a lot of different trade offs to take into account when choosing
a wavelength, with x-rays at least. If low angle asymmetry is a problem for
the experiment in question then you can use a small beam on sample and/or
narrower receiving slits. My recent experience at ESRF is that a lot of the
asymmetry can be removed without destroying the count rate even with a
large unit cell and fairly hard x-rays. (nb. removed by slits, not
deconvolution)
For ToF neutrons the moderator asymmetry is less dominant as ToF increases,
and this is where the resolution is maximised anyway. It can be fitted very
well with the right programs and the advantages of going to very low
d-spacings (high Q) also has to be taken into account.
Comparing resolution functions only tells part of the story, the real
question is how much information did you gain from the experiment. This can
depend on a lot of other factors, not least background, Q-range and count
rate which are rarely compared in quite the same way. Remember the sample
can still ruin the background and the resolution regardless of the
instrument :)
Cheers,
Jon
