Direct exchange interaction, effect

It must be said that the idea of a correlation between mobile and localized electrons appeared in the thirties of the last century and it was proposed by two soviet scientists Schubin and Vonsovsky in the paper [28]. In this model it is supposed that the localized electrons (d-electrons) are in magnetic order due to, for example, direct exchange interaction, while conduction electrons (s-electrons) are magnetized due to s-d interaction, which leads to a number of peculiarities in the behavior of the last. A detailed description of this model and the various effects that appear when it is used can be foimd in the monograph [29]. 

Wliile the earliest TR-CIDNP work focused on radical pairs, biradicals soon became a focus of study. Biradicals are of interest because the exchange interaction between the unpaired electrons is present tliroiighoiit the biradical lifetime and, consequently, the spin physics and chemical reactivity of biradicals are markedly different from radical pairs. Work by Morozova et al [28] on polymethylene biradicals is a fiirther example of how this method can be used to separate net and multiplet effects based on time scale [28]. Figure Bl.16.11 shows how the cyclic precursor, 2,12-dihydroxy-2,12-dimethylcyclododecanone, cleaves upon 308 mn irradiation to fonn an acyl-ketyl biradical, which will be referred to as the primary biradical since it is fonned directly from the cyclic precursor. The acyl-ketyl primary biradical decarbonylates rapidly k Q > 5 x... 

A new additive dose method is proposed to obtain the age directly without extrapolating the growth curve.1115 The experimental growth curve at the artificial irradiation dose rate gives simply the defect production efficiency (G-value) from the initial growth and the interaction distance, d, between spins from the saturation behaviour. The latter involves the effect of magnetic dipolar and exchange interactions of similar and dissimilar spins and also destabilization of a spin in a distorted area by a local lattice distortion. 

Besides interacting with suspended particles, a chemical also undergoes direct exchange at the sediment surface by diffusion and advection into the hyporheic zone. Furthermore, resuspension followed by exchange between water and particles also adds to the sediment-water interaction. These processes have been extensively discussed in Chapter 23, especially in Box 23.2. There we concluded that the effect from the different mechanisms can be combined into a flux of the form (see Eq. 23-25) ... 

The proposed approach to static cooperative Jahn-Teller effect is based on the exact effective Hamiltonian (7), acting in the reduced space of active one-centre distortions only. It involves effective force constants, which are analytically related to the parameters of the full microscopic Hamiltonian. Direct electronic interactions between sites, such as orbital-dependent electrostatic and exchange interactions [28], can be added to the effective Hamiltonian without modifying it. This approach proves to be especially efficient in the case of strong Jahn-Teller distortions, when the effects of second-order Jahn-Teller coupling become important. 

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