Chemistry Reference
In-Depth Information
Fig. 2.34
[Fe(phen)
2
(NCS)
2
] undergoes thermal ST (MAS spectra on the left). The analogous
Co
II
57
compound doped with
Co and used as Mössbauer source (or the corresponding iron
57
compound doped with
Co as source which gives the same results) yield the temperature
dependent Mössbauer emission spectra (MES) shown on the right. The main result is that in the
temperature region, where the MAS spectra reflect the transition to the LS state, the MES spectra
still show the typical HS signals arising from short-lived excited ligand field states [
64
]
gives the same results) yield the temperature dependent MES spectra shown on the
right [
64
]. The main result is that in the temperature region, where the MAS
spectra reflect the transition to the LS state, the MES spectra still show the typical
HS signals arising from excited ligand field states.
The mechanism of the formation of the excited ligand field states as a conse-
quence of the EC nuclear decay of radioactive
57
Co is well understood. It is much
related to that of the LIESST phenomenon (Fig.
2.14
) and has therefore been termed
Nuclear Decay-Induced Excited Spin State Trapping (NIESST) [
59
]. The main
difference between the two phenomena lies in the primary step of excitation, which
is the application of a light source in the case of LIESST, whereas in NIESST the
nuclear decay serves quasi as an intrinsic molecular excitation source. The lifetime
or probability of observing the metastable HS (
5
T
2
) state within the Mössbauer time
scale (given by the lifetime of the 14.4 keV nuclear state of
57
Fe) at a given tem-
perature is governed by the ligand field strength felt by the nucleogenic
57
Fe ion: The
weaker the ligand field strength, the longer the lifetime of the metastable HS (
5
T
2
)
state. It is worth emphasizing that this technique, Mössbauer emission spectroscopy,
is most effective for the study of chemical and physical after-effects of nuclear decay
processes, referred to as ''Hot Atom Chemistry'', in solids [
63
].
