Chemistry Reference
In-Depth Information
Figure1.30.
Dependence of the decrement
δ
on the wavelength
λ
for the elements (a) copper and
(b) gold. The theory is based on forced oscillations of the atomic electrons—forced by the
electromagnetic radiation of X-rays. At the“resonance”wavelengths or absorption edges, the
decrement shows a strong variation. The asymptotic approximation for short wavelengths is
represented by a dashed line. Data from Ref. [75], reproduced with permission from Forschungs-
zentrum Jülich.
metals. The material constant contains (
N
A
/
A
), which represents the reciprocal
volume of a single atom. This dependency becomes apparent in the figure.
Equation 1.58 leads to the simple formula
C
m
λ
2
δ
(1.59)
For given
λ
values of the X-ray region, the
δ
values can easily be calculated.
Because these values are quite small (between 2
×
10
7
and 5
×
10
3
), the real
part 1
δ
of the refractive index deviates only slightly from unity (between
0.995 and 0.9999998). The minus sign in 1
δ
reflects the fact that the bound
electrons follow the excitant photons only slowly, that is, with phase
opposition
.
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