Electron powder spectra

Figure 4. First-derivative ESR spectra of a nitroxide spin probe with the x-axis (a), y-axis (b), and z-axis (c) aligned parallel to H. The rigid-limit "powder" spectrum (d) is also shown. The dashed line marks the free electron field position (g = 2.0027). Reproduced from Ref. (jl) with permission of D. Reidel Publishing Co.

To simplify terminology of axial systems, gzz is defined to be g(l (the g-value observed with the symmetry axis of Cu + parallel to the applied field), and gxx (= gyy) is defined to be gA (the g-value observed with the symmetry axis perpendicular to the applied field). An elongated z-axis (depicted in Figure 11 for Cu(H20)5 +) results in gjj > gj. For axially symmetric Cu + rigidly bound in a crystal, the g-value can then vary between the minimum (gj.) and maximum (g(,), depending on orientation of the crystal within the magnetic field. However, for axial Cu + bound in a powdered clay sample, all possible orientations, and therefore all g-values between gA and gj are represented in the "powder" spectrum. Therefore, electron spin resonance occurs only for field values, H, between Hjj and H, where ... 

The triangular planar (D3h symmetry) CO/ molecular ion with 24 electrons (AB324-type) in CaC03 is easily ionized by radiation to electron and hole centres self-trapped in the lattice or an oxygen vacancy type C02 molecular ion at the anon site. Molecular orbital schemes based on the general scheme of AB3 molecules with 25,24 and 23 electrons for atoms A (B, C, Si, N, P, As and S) and B (O) characterize their specific -factor. Hence, the anisotropic -factor of these radicals estimated from the powder spectrum has been to identify the radical species.1... 

Light (or near-ir and uv radiation) that is incident on opaque minerals is partly absorbed and partly reflected by them. There are two kinds of reflection processes that occurring when light is reflected from a flat polished surface of the mineral (specular reflectance) and that occurring when the light is reflected from the mineral after it has been finely powdered (diffuse reflectance). The latter arises from radiation that has penetrated the crystals (as in an electronic absorption spectrum) and reappeared at the surface after multiple scatterings in this case there will also be a specular component to the reflectance from light that is reflected from the surfaces of the particles. The specular reflectance of a flat polished surface of an opaque mineral measured at normal incidence can be related to the n and k terms of the complex refractive index (N) in which ... 

While the effective g value is expressed in terms of three principal values directed along three axes or directions in a single crystal, only the principal values of g can be extracted from the powder spectrum rather than the principal directions of the tensor with respect to the molecular axes. (Therefore it is more correct to label the observed g values as gi, g2, g3 rather than g gyy, in a powder sample.) In the simplest case, an isotropic g tensor can be observed, such that all three principal axes of the paramagnetic center are identical (x = y = z and therefore gi= gi = g-i). In this case, only a single EPR line would be observed (in the absence of any hyperfine interaction). With the exception of certain point defects in oxides and the presence of signals from conduction electrons, such high symmetry cases are rarely encountered in studies of oxides and surfaces. 

Several commercial and free-ware program packages can be downloaded electronically. Examples of software for general use are given in Table 3.5, while we refer to a literature survey [36] for programs dedicated to the powder spectrum simulation of free radicals and triplet state molecules. 

Detailed theoretical studies, based on EPR experimental data, on both trapped electron centres and on electron transfer toward adsorbed molecules on MgO have also appeared. In the latter paper, beside further investigation of the Fs +/N2 system, a detailed analysis of the Fs +/02" complex was reported with particular emphasis on the whole hyperfine structure of adsorbed O2 . The Fermi contact term was evaluated as = —20.3 G and the resulting dipolar tensor was = — 56 G, Byy = -1-27.5 G, B = +28.6 G. An experimental paper appeared some months later reporting a fully resolved hyperfine structure of adsorbed 2. The powder spectrum contained a large number of lines and was further comphcated by the presence of several off-axis extra features. The simulation of such a complex pattern of fines gave Uiso = 4.8 G and a dipolar tensor (J) with a value remarkably close to the theoretically calculated value. This vahdates the quahty of the model but also the capabihty of modern theoretical approaches in forecasting EPR hyperfine parameters. 

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