Chemistry Reference
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Fig. 2.2 Electric monopole interaction between electrons and protons perturbs the energy levels
of the nuclear ground and excited states (with different radii). The energy changes are different in
the source (S) and absorber (A) as a result of different electron densities at the source and absorber
nuclei, the result is manifested as isomer shift in the Mössbauer spectrum
effects of p-, d- and f-electrons, which are not capable (if neglecting relativistic
effects) of penetrating the nuclear field. Results from Hartree-Fock calculations of
the contributions of s-orbitals to the total electron density at the iron nucleus as a
function of oxidation state and configuration have shown that (a) nominally the
largest contributions originate from the filled 1s and 2s shells (1s * 10 4 au -3 ,
2s * 10 3 au -3 ,3s* 10 2 au -3 ), and (b) significant changes in the electron den-
sities arise from the noticeably different contributions from the 3s shell populations
due to different shielding effects of 3d n . The reason becomes apparent on
inspecting the strongly overlapping distribution functions of 3s and 3d electrons.
Chemical bonds between metal ion and ligands in coordination compounds can
be viewed as the result of the balance between r-donation (s-electrons from ligands
are donated into s-orbitals of the metal) and d p -p p back donation (d-electrons move
from d-orbitals of the metal to empty p-orbitals of the ligands). Both bonding
mechanisms influence the isomer shift in the same direction, but to different extent,
depending on the nature of the ligands and thus on the weight of the atomic orbitals
of the metal and ligands participating in the molecular orbitals (covalency effects).
This is the reason why isomer shift scales for different compounds of the same
oxidation state often cover a broad range of values. The most valuable information
derived from isomer shift data refers to the valence state of a Mössbauer-active atom
embedded in a solid material as shown in Fig. 2.3 .
2.2.2 Electric Quadrupole Interaction: Quadrupole Splitting
Electric quadrupole interaction occurs if at least one of the nuclear states involved
possesses a quadrupole moment eQ (which is the case for nuclear states with spin
I [ 1/2) and if the electric field at the nucleus is inhomogeneous. In the case of
57 Fe the first excited state (14.4 keV state) has spin I = 3/2 and therefore also an
electric quadrupole moment.
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