26
R. Rüffer and A. I. Chumakov
Fig. 1.8 Simulations of corresponding spectra of Mössbauer spectroscopy (left panel, with
Γ 0 =/τ 0 ) and NFS (right panel) for thin (t eff = 1) and thick (t eff = 25) samples. For the single lines
(panel a and b) the thin sample leads to an exponential decay whereas the spectrum from the thick
sample is further modulated by the dynamical beats. In panel c the energy spectra of a thick sample
(t eff = 25) with one (solid line) and two (dotted line) transition lines, respectively, are shown. In
panel d the corresponding time spectra are shown. The sample with the two lines shows in addition
to the dynamical beats the fast, equidistant quantum beat structure superimposed (from [86])
In case of electric quadrupole interaction (E 2 ) the excited nuclear state, I = 3/2
in case of
57 Fe, splits to the ± 3/2 and ± 1/2 levels. This gives rise to two absorption
lines in Mössbauer spectroscopy (see dotted line in Fig. 1.8 c). The corresponding
spectrum in NFS shows up as an interference pattern of these two transitions with
a single frequency Ω of the quantum beats (see dotted line in Fig. 1.8 d). Due to
the thick sample with an effective thickness t eff = 25, the dynamical beat structure
is strongly modulating the quantum beats as an envelope. It is clearly seen that
the quantum beat structure is equidistantly spaced whereas for the dynamical beat
structure the distance of the minima increases with time. This can be described for
NFS in a good approximation by
I NFS (t) ∝
t eff
t/τ 0
cos
2
Ωt
2
· e
−
t
τ 0 · J 1
4t eff t/τ 0
2
(1.26)
and for SRPAC by
I SRPAC (t) ∝ e
−
t
τ 0 {1 − A 22 cos
2
Ωt
2
}.
(1.27)