Mossbauer parameters nuclear energy states

Figure 2A. Schematic diagram of Mossbauer parameters isomer shift (6), quadrupole splitting (AEq) and magnetic dipole splitting of the nuclear energy states of 57pe leading to various hyperfine splitting in Mossbauer spectra.

In a conventional Fe Mossbauer experiment with a powder sample, one would observe a so-called quadrupole doublet with two resonance lines of equal intensities. The separation of the lines, as given by (4.36), represents the quadrupole splitting The parameter Afg is of immense importance for chemical applications of the Mossbauer effect. It provides information about bond properties and local symmetry of the iron site. Since the quadrupole interaction does not alter the mean energy of the nuclear ground and excited states, the isomer shift S can also be derived from the spectrum it is given by the shift of the center of the quadrupole spectrum from zero velocity. 

Since one is dealing with the same set of energy levels in each experiment one should look for a model consistant with each experiment. In the case of the iron transport compounds the experimental results are parameterized by the coupling coefficients D, X, p. A, P in the spin Hamiltonian to be discussed in the next section. The parameter P in 57 Fe can be measured only with Mossbauer spectroscopy since it comes about through the excited nuclear state which is not available in an ESR experiment. 

The cormection between tbe quadrupole splitting energy and the separation of the absorption lines is less obvious if in addition to the excited state, the ground nuclear state is also subject to byperfine quadrupole interaction. This is the case, e.g., for where the Mossbauer transition occurs between the excited state with 4 = 5/2 and the ground state with 4 = 7/2. The quadrupole splittings of this transition are treated below by assuming that the asymmetry parameter of the EFG is zero (7/ = 0, see Eqs. (25.68) and ( 25.73)). 

Relaxation effects in Mossbauer spectroscopy are of a different nature from those in NMR. The term relaxation effects or relaxation spectra in nuclear gamma resonance spectroscopy refers to averaging effects that occur in the hyperfine spectrum when the hyperfine interactions fluctuate at a rate more rapid than the nuclear frequency characteristic of the hyperfine interaction itself. This situation is a consequence of the rapid relaxation of the host ion among its energy levels, and the relaxation time for such effects is characteristic of the ion and not of the nuclear spins. The relaxation processes involved also affect electron spin resonance spectra, and their discussion is best considered in that context (see sections 3.3. and 3.4.). In the following subsections the principal interactions which contribute to the nuclear spin relaxation times in NMR experiments are briefly considered, and the connections between these and the parameters characterizing the steady-state spectrum are outlined. 

The shift due to Coulomb interactions is of the order of 10 of the transition energy. The value of the shift for every nuclear level depends on the chemical state of the atom. This is characterized by the IV (0)l s parameter which is the electron density at the nucleus in the absorber (a) or in the source (s). In a Mossbauer spectrum this part of the full electrostatic interaction manifests itself as the isomer (chemical) shift d between the centre of gravity of the... 

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