Line hyperfine structure

When NH4Y is activated at 400°C to form the decationated or HY zeolite, two types of V centers are observed after y irradiation in vacuo (274). Both types give a rather broad EPR line with the same g tensor, x 2.045, g2 = 2.005, and g3 = 2.002, but only one exhibits a six-line hyperfine structure (Ax not determined, A2 = 8.0, A3 = 7.5 G) due to an interaction with 27A1 nuclei (I = f) which is superimposed on the broad EPR line. These V centers are attributed to a positive hole (denoted by the symbol ) trapped either on an oxygen adjacent to both an A1 and a Si atom (V,) or on an oxygen adjacent to two Si atoms (V2). 

Different results are obtained when NH4Y is activated at higher temperatures (600°C). Vedrine et al. (266) showed that y irradiation in vacuo of such an activated zeolite leads to two types of signals. The signal with g = 2.0125, g = 2.0030, and a 12-line hyperfine structure with A aiso = 10.0 G was attributed to a positive hole (V center) trapped on an oxygen bridging two aluminum atoms ... 

It is interesting to note than an 11-line hyperfine structure with aiso = 10 G was observed at gjm = 2.007 when the Al-exchanged HY zeolite was irradiated in vacuo. This hyperfine structure was reported to be reversibly broadened by oxygen (103). Although the hyperfine structure appears to be similar to that observed by Vedrine et al. (266) for the V center associated with two aluminum atoms, it does not lead to the formation of OJ on adsorption of oxygen. 

Unpaired electron on that bridge interacts symmetrically with the nuclear spin 7/2 of both cobalt (III) atoms to give the 15-line hyperfine structure. Complexes with the y-superoxo bridges were not... 

Fe (/ = is substituted for Fe(/ = 0), a two-line hyperfine structure should occur (Figure 2). When, however, only partly enriched Fe is used, an unsplit line due to Fe will occur halfway between the Fe lines, at In a sample containing naturally abundant Fe (2.25%), the Fe satellites may appear as unresolved shoulders on the edges of the main line but will often not be observed. 

Most commonly used probe is nitroxide radical. A significant amount of work on this topic is available in the literature. Nitroxide free radical (probe) produces a three-line hyperfine structure whose properties such as peak shape and splitting depend upon the environment of the radical. The shape of the ESR signal depends also on the orientation of the magnetic field relative to the axis of the radical. 

In addition to this electron spin fine structure there are often still finer lines present. These are known as the hyperfine structure, which arises from the dilTerent weights of the isotopes of an element or from the spin of the nucleus. 

The spatial localization of H atoms in H2 and HD crystals found from analysis of the hyperfine structure of the EPR spectrum, is caused by the interaction of the uncoupled electron with the matrix protons [Miyazaki 1991 Miyazaki et al. 1991]. The mean distance between an H atom and protons of the nearest molecules was inferred from the ratio of line intensities for the allowed (without change in the nuclear spin projections. Am = 0) and forbidden (Am = 1) transitions. It equals 3.6-4.0 A and 2.3 A for the H2 and HD crystals respectively. It follows from comparison of these distances with the parameters of the hep lattice of H2 that the H atoms in the H2 crystal replace the molecules in the lattice nodes, while in the HD crystal they occupy the octahedral positions. 

Wolfgang Pauli (1900-1958 Nobel Prize 1945), at the age of 24, formulated the exclusion principle, which became famous as the Pauli principle. Accordingly, all electrons in an atom differ from each other, none are the same. His theoretical considerations led him to the existence of so-called nuclear spins, which also explained the hyperfine structures of spectral lines. His hypothesis was later unambiguously confirmed. As each element has its own... 

Early treatments of powder patterns attempted to deal with the spatial distribution of resonant fields by analytical mathematics.9 This approach led to some valuable insights but the algebra is much too complex when non-axial hyperfine matrices are involved. Consider the simplest case a single resonance line without hyperfine structure. The resonant field is given by eqn (4.3). Features in the first derivative spectrum correspond to discontinuities or turning points in the absorption spectrum that arise when dB/dB or dB/dcp are zero ... 

Hyperfine splitting. As was discussed above, one consequence of placing a free electron onto a molecule is to alter its 0-value. Another is that the electron spin comes under the influence of any magnetic nuclei present in the radical, with the result that the spectrum is split into a number of lines centred on the position of the single resonance expected for the simple /transition discussed above. This hyperfine structure is the most useful characteristic ofepr spectra in the identification of an unknown radical species. 

Fig. 12. Derivative curves of EPR in a highly dislocated As-doped germanium crystal grown in a H2 atmosphere. The magnetic field is oriented along the [100] direction. T= 2 K, /= 25.16 GHz. Note the sign reversal of the new lines as compared to the As-donor hyperfine structure. Dislocation density 2 x 104 cm 2. (Courtesy Pakulis and Jeffries, reprinted with permission from the American Physical Society, Pakulis, E.J., Jeffries, C D. Phys. Rev. Lett. (1981). 47, 1859.)...

Fig. 3. (a) Partially resolved nuclear hyperfine structure in the p.SR spectrum for Mu in GaAs in an applied field of 0.3 T. The structure occurs in the line corresponding to 0 = 90° and Ms = —1/2. (b) Theoretical frequency spectrum obtained by exact diagonalization of the spin Hamiltonian using the nuclear hyperfine and electric quadrupole parameters in Table I for the nearest-neighbor Ga and As on the Mu symmetry axis. Both Ga isotopes, 69Ga and 71Ga, were taken into account. From Kiefl et al. (1987). 

---