Chemistry Reference
In-Depth Information
vanish the quadrupole interaction. Generally the magnetic substance has non-cubic
symmetry having the electric field gradient and shows the quadrupole interaction.
In this case the
57
Fe Mössbauer spectrum shows a combined quadrupole and
magnetic hyperfine interactions. For the analysis of the combined quadrupole and
magnetic
interaction,
the
several
methods
and
computer
programs
for
the
numerical analysis have been published from rather old time [
22
,
23
].
1.2.2 Dynamical Effects on the Spectral Shape
Previous section, the static hyperfine interactions have been concerned about the
application to the material research. One might imagine that the Mössbauer spec-
troscopy can apply to study of the dynamics of the lattice in which the Mössbauer
active atoms is embedded. Atomic jump associated with the atomic diffusion of the
Mössbauer active atoms can be also studies by the Mössbauer spectroscopy [
24
].
For the study of lattice dynamics, recoilless fraction f (
1.17
) indicates the
vibrational dynamics of the lattice that concerns the mean square displacements of
Mössbauer atoms \x
2
[ . Similarly the second order Doppler shift (SOD) that is
given by
SOD (Second order Doppler shift)
¼
v
2
2c
;
where \v
2
[ is the mean square velocity of Mössbauer active atoms in lattice.
These \x
2
[ and \v
2
[ can be evaluated using various models for lattice dynam-
ics, for example, using Debye model.
Mössbauer spectroscopy can be used to observe the effects of diffusive motion
in solids under three different conditions; (1) when the diffusing atom itself is a
Mössbauer active atom, (2) when a Mössbauer active atom experiences a changing
hyperfine interaction as a result of the motion of a more rapidly diffusing species in
the material, and (3) when the c-ray is scattered by some material and the
Mössbauer effect is used for energy analysis of the scattered c-ray. In the first case,
if the atom with resonant nucleus moves rapidly enough to travel a distance that is
large relative to c-ray wavelength in a time that is short relative to the nuclear
lifetime, one can expect a line broadening since the motion destroys the coherence
of the c-ray.
If the direction of the main component V
zz
of the electric field gradient (EFG)
fluctuates rapidly as a result of diffusion, the Mössbauer spectrum may be changed
in shape. Two atomic sites have very different isomer shift values and the jump
between these two states also may show the time-dependent Mössbauer spectra as
a function of the jump frequency. The time scales over which the Mössbauer effect
can be used to observe the dynamical effect is determined by the characteristic
times
associated
with
the
resonance;
the
natural
lifetime
and
the
Larmor
precession times of the hyperfine interactions.
