Energy state Zeeman levels

This gives a value of ni/ 2 = 0.9986. The populations of the two Zeeman levels are therefore almost equal, but the slight excess in the lower level gives rise to a net absorption. However, this would very quickly lead to the disappearance of the EPR signal as the absorption of energy would equalize these two states. Gonse-quently there has to be a mechanism for energy to be lost from the system. Such mechanisms exist and are known as relaxation processes. 

Af fhe end of the 1960s and 1970s, all spectroscopists agreed upon the fact that MCD was very powerful and a beautiful technique as it describes the ultimate intrinsic electronic properties of matter the characterization of a Zeeman level by a signal fhat has a sign. This is not the case in EPR or classical Zeeman spectroscopy which measure the energy difference between two states (e.g., AM = 1) but which cannot tell which one is... 

Nuclear magnetic resonance (NMR) experiments subject a sample to a strong, static homogeneous magnetic field B = (0,0, B) that splits the energy levels of degenerate nuclear spin states. Transitions between these Zeeman levels are induced with an oscillating field (radio frequencies around 10 cm i). 

The simplest spin system available for magnetic resonance has a single spin, I, with two energy states, or Zeeman levels , as in Figure 4.2. The upper state has a spin population nj, and the lower state n. We may first consider the case of thermal equilibrium. At equilibrium, the net upwards flow of spins must equal the net downwards flow, so that... 

Transitions between the two spin states may be induced by the application of electromagnetic radiation of the appropriate frequency, v, such that the applied energy, 2jr vfi exactly matches that of the Zeeman-level separation, that is... 

To the extent that the Zeeman energy predominates, then the quadrupole interaction can be treated as a perturbation on the Zeeman levels and it lifts the degeneracy of these (21 + 1) states. Figure 3(b) illustrates this for I = 1 and 1 = 3/2. Apart from the 1/2 to -1/2 transition for half-integral spins, the quadrupole shifts in the levels and hence the splittings observed in the spectra are orientation dependent. In a powder sample, therefore, broad "powder" spectra are observed, their breadth often making observation difficult by pulse techniques. Indeed, for half-integral spins often only the 1/2 to -1/2 transition is observable. 

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