Weak field

In the previous sections we have described the interaction of the electromagnetic field with matter, that is, tlie way the material is affected by the presence of the field. But there is a second, reciprocal perspective the excitation of the material by the electromagnetic field generates a dipole (polarization) where none existed previously. Over a sample of finite size this dipole is macroscopic, and serves as a new source tenu in Maxwell s equations. For weak fields, the source tenu, P, is linear in the field strength. Thus,... 

Assuming that the system has no pennanent dipole moment, tire existence ofP(t) depends on a non-stationary j induced by an external electric field. For weak fields, we may expand the polarization in orders of the perturbation. 

Depending on tire sign of U and F, atoms in states whose energy increases or decreases witli magnetic field are called weak-field seekers or strong-field seekers , respectively. One could, in principle, trap atoms in any of tliese states. 

Weak-field, high-spin Strong-field, low-spin- ... 

Figure 7.40 (a) Weak-field, high-spin and (b) strong-field, low-spin configurations in d, d, d ... 

The Amount of Free Energy Lost by a Dielectric. The above considerations apply to fields of any intensity. When we are dealing only with ordinary weak fields, for which the polarization is proportional to the field (the straight part of the curve in Fig. 5), the substance under discussion is said to possess a dielectric constant. This will be denoted by t. In a vacuum e is set equal to unity and in a dielectric the polarization is proportional to (t — 1). The loss of free energy by the dielectric may be expressed in terms of e. In Note 1 of the Appendix at the end of this book it is shown that, when a homogeneous slab is introduced into a uniform field of initial intensity X, the free energy lost per unit volume amounts to... 

WEB For the species in Question 35, indicate the number of unpaired electrons with weak-field ligands. 

In molecular doped polymers the variance of the disorder potential that follows from a plot of In p versus T 2 is typically 0.1 eV, comprising contributions from the interaction of a charge carrier with induced as well as with permanent dipoles [64-66]. In molecules that suffer a major structural relaxation after removal or addition of an electron, the polaron contribution to the activation energy has to be taken into account in addition to the (temperature-dependent) disorder effect. In the weak-field limit it gives rise to an extra Boltzmann factor in the expression for p(T). More generally, Marcus-type rates may have to be invoked for the elementary jump process [67]. 

Predict the electron configuration of an octahedral eP complex with (a) strong-field ligands and (b) weak-field ligands, and state the number of unpaired electrons in each case. 

STRATEGY Decide from their positions in the spectrochemical scries whether the ligands are weak-field or strong-field. Then judge whether the complex is high spin or low spin. 

FIGURE 16.35 (a) A strong-field ligand is likely to lead to a low-spin complex (in this case, the configuration is that of Fe3+). (b) Substituting weak-field ligands is likely to result in a high spin complex. 

For a weak-field ligand, predict a high-spin /2(,4e(,2 configuration. 

Explain the difference between a weak-field ligand and a strong-field ligand. What measurements can be used to classify them as such ... 

Describe the changes that may take place in a compound s properties when weak-field ligands are replaced by strong-field ligands. 

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