Low Field Effect

At very weak fields, it is sometimes possible to observe an increase in the rate of S-T mixing, which in Fig. 8.6 corresponds to an increase in geminate product yield. This is known as a low-field effect (LFE) and has been a topic of interest recently due to the interest in the effect of environmental electromagnetic fields on human health and... 

Although the low field effects on chemical reactions through radical pairs had been explained by the LCM, Timmel et al. [14] proposed that the so-called low field effects arose also fi om coherent superpositions of degenerate electron-nuclear spin states in a radical pair in zero field. They made some model calculations for their mechanism. At first, let us consider the case of a radical pair with a single spin-1/2 nucleus, e.g., a proton. When the exchange term is not included (J= 0 J), its spin Hamiltonian (H) can be expressed from Eq. (3-3) as... 

Because ps(0 is a real quantity, we can choose the initial condition as shown by Eq. (15-15). In order to explore limiting cases to discover how large the low field effect could be under the right conditions, Timmel et al, [15] adopted an approximation that the singlet and triplet radical pairs disappear with first-order kinetics with a rate constant of A In this approximation, the singlet product yield (d ) is given by... 

The very low field effect mobilities reported in Table 5.2-26, show that the Fermi level is pinned in the gap because of the large concentration of surface states. Pinning positions of the Fermi level with respect to the top of the valence band at the surface are given in Table 5.2-27 for various semiconductors. 

Flood (1978) has measured the magnetization and magnetic susceptibility of ErHj, finding Curie-Weiss behavior from 4.2 to 2K and evidence for an antiferromagnetic transition. The low-field effective moment is 6.77 + 0.27/Zb-... 

A), respectively. These spacings correspond to the chain distances of a well-organized molecular layer structure. PDT showed a much weaker diffraction peak at 3.2° 20 (27.10A), indicating lower crystallinity or orientation. In addition, its higher volume fraction of insulating side chains may also contribute to its low field-effect mobility. 

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