Carlos Frazao wrote:
Hi,
I have once heard and recently read that "the diffraction event results
from the fact that both the X-rays wavelength and the atomic distances
are of the same magnitude". Although such a relation seems appealing I
am unsure if this is not a mere coincidence. Could someone clarify or
lead me to a relevant reading.
Cheers,
Carlos
The diffraction event does not <<< result from >>> the fact that both
wavelength and atomic differences are of the same magnitude. But as many
interestingly different yet related answers have indicated you need such
a wavelength to resolve the atomic details you are interested in.
Another way to think about it, the phase difference between the
scattering of two atoms at distance d depends on d, the wavelength, and
the angle of diffraction. If the wavelength is long relative to d then
the phase difference becomes too small and you can thus not resolve the
small details (I believe in microscopy the smallest visible detail is
the wavelength divided by 2 or the square root of 2 depending on the
method of illumination)
At the other end of the spectrum, making the wavelength much smaller
than the atomic distance does not create a similar problem. I believe
that in electron diffraction on 2-dimensional crystals the wavelength of
the electron beam is actually many orders smaller than the atomic
distances. The problem is that the scattering power is proporsional to
the cube of the wavelength. So reducing the wavelength by a factor of 10
will reduce the diffraction intensity by a factor of a 1000. Very hard
X-rays are also harder to generate and detect. So it is attractive to
use wavelengths that are short enough to resolve the details of interest
but not much shorter than that. Hence the predominant use of wavelengths
in the 1 to 1.5 Angstrom range.
Bart
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Bart Hazes (Assistant Professor)
Dept. of Medical Microbiology & Immunology
University of Alberta
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phone: 1-780-492-0042
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