Electron Spin Resonance
So far we have confined our attention to nuclear magnetic resonance, although many of the basic principles apply to electron spin resonance. We have also considered questions concerning the electrons, such as the quenching of orbital angular momentum and
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11.1 Introduction So far we have confined our attention to nuclear magnetic resonance, although many of the basic principles apply to electron spin resonance. We have also considered questions concerning the electrons, such as the quenching of orbital angular momentum and the magnetic coupling of the nuclear spin to that of the electron. In this chapter we shall add a few more concepts that are important to the study of electron spin resonance l but which are not encountered in the study of nuclear resonance. Probably the major difference between electron and nuclear magnetic resonance is the fact that the nuclear properties such as spin, magnetic moment, and quadrupole moment are to a very high degree of approximation unaffected by the surroundings, whereas for electronic systems, the relatively much greater physical size and the much smaller energy to excited states make the system strongly dependent on the surroundings. An atom, when placed in a crystal, may have angular momentum, magnetic moment, and quadrupole moment values entirely different from those of its free atom. It is as though in nuclear resonance we had to compute 'Yn, I, and Q for each material in which the nucleus was to be studied. The fact that the state of an atom in a solid or liquid is very different from that when it is free means that we cannot predict the properties or even the existence of a resonance from the free atom electronic angular momentum and magnetic moment. For example, a sodium atom has zero orbital magnetic moment and angular momentum, but it has a spin of and a corresponding spin magnetic moment. The magnetic properties can be studied by the method of atomic beams. In sodium metal, the valence electrons form a conduction band, with substantial pairing of spins. However, there is a weak electronic spin magnetization whose spin resonance has been studied. In sodium chloride, the sodium gives up its outermost electron to complete the unfilled p-shell of the chlorines. The result is a zero spin magnetization and no electron spin resonance. Even if one has atoms whose bonding is covalent, as in molecular hydrogen, there is usually
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See references to "Electron Spin Resonance" listed in the Bibliography.
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C. P. Slichter, Principles of Magnetic Resonance © Springer-Verlag Berlin Heidelberg 1990
no net spin magnetization because the electron spins pair off into a spin singlet. There are exceptions, of course, such as the oxygen molecule. As we remarked in connection with chemical shifts, the orbital angular momentum is often quenched, so that there is no first-order orbital contribution to a resonance. We see that most insulators will not exhibit a resonance, unless one takes special pains to unpair the spins. Some atoms, such as those in the iron group or rare earths, have incomplete inner shells. Even when ionized, they still possess a net moment. Thus neutral copper has a configuration (3d) 104s. Cu++ has (3d)9, which is paramagnetic. In an ionic substance such as euS04 • 5H20 (copper sulfate), the copper atoms are pa
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