The Mechanism of Absorption Resonance

When the magnetic field is applied, the nucleus begins to precess about its own axis of spin with angular frequency co, which is sometimes called its Larmor frequency. The frequency at 

FIGURE 3.7 (a) A top processing in the earth s gravitational field (b) the precession of a spinning nucleus resulting from the influence of an applied magnetic field. 

FIGURE 3.8 The nuclear magnetic resonance process absorption occurs when u = m. 

FREQUENCIES AND FIELD STRENGTHS AT WHICH SELECTED NUCLEI HAVE THEIR NUCLEAR RESONANCES 

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Nnclear Magnetic Moments 106 Absorption of Energy 107 The Mechanism of Absorption (Resonance) 109 Popnlation Densities of Nnclear Spin States 111 The Chemical Shift and Shielding 112 The Nnclear Magnetic Resonance Spectrometer 114... 

The ionization of ammonia clusters (i.e. multiphoton ionization,33,35,43,70,71 single photon ionization,72-74 electron impact ionization,75 etc.) mainly leads to formation of protonated clusters. For some years there has been a debate about the mechanism of formation of protonated clusters under resonance-enhanced multiphoton ionization conditions, especially regarding the possible alternative sequences of absorption, dissociation, and ionization. Two alternative mechanisms63,64,76,77 have been proposed absorption-ionization-dissociation (AID) and absorption-dissociation-ionization (ADI) mechanisms see Figure 5. 

This remark is associated with the amount of calculation performed and is not intended as a criticism. This work provides a valuable quantum mechanical analysis of a three-dimensional system. The artificial channel method (19,60) was employed to solve the coupled equations that arise in the fully quantum approach. A progression of resonances in the absorption cross-section was obtained. The appearance of these resonances provides an explanation of the origin of the diffuse bands found... 

The Forster mechanism is also known as the coulombic mechanism or dipole-induced dipole interaction. It was first observed by Forster.14,15 Here the emission band of one molecule (donor) overlaps with the absorption band of another molecule (acceptor). In this case, a rapid energy transfer may occur without a photon emission. This mechanism involves the migration of energy by the resonant coupling of electrical dipoles from an excited molecule (donor) to an acceptor molecule. Based on the nature of interactions present between the donor and the acceptor, this process can occur over a long distances (30—100 A). The mechanism of the energy transfer by this mechanism is illustrated in Figure 11. 

In complex molecules, such as the Rhodonines, it is important to recognize that both the dipole molecular and resonance molecular absorption can coexist. It is only resonance absorption is functionally important in vision. The spectral absorption associated with both may be observed in the laboratory. However, the dipole absorption is normally only observed in the liquid crystalline state. The mechanics of the resonant molecular process will be detailed in Section 5.4.3. 

Application of B at the resonance frequency results in both energy absorption (+ nuclei become -and emission (— nuclei become + ). Because initially there are more + than -1 nuclei, the net effect is absorption. As B irradiation continues, however, the excess of +5 nuclei disappears, so that the rates of absorption and emission eventually become equal. Under these conditions, the sample is said to be approaching saturation. The situation is ameliorated, however, by natural mechanisms whereby nuclear spins move toward equilibrium from saturation. Any process that returns the z magnetization to its equilibrium condition with the excess of spins is called spin-lattice, or longitudinal, relaxation and is usually a first-order process with time constant T. For a return to equilibrium, relaxation also is necessary to destroy magnetization created in the xy plane. Any process that returns the X and y magnetizations to their equilibrium condition of zero is called spin-spin, or transverse, relaxation and is usually a first-order process with time constant T2. 

Experimentally, water exchange rate constants are mainly determined from nuclear magnetic resonance measurements [6, 7]. Other techniques are restricted to very slow reactions (classical kinetic methods using isotopic substitution) or are indirect methods, such as ultrasound absorption, where the rate constants are estimated from complex-formation reactions with sulfate [3]. The microscopic nature of the mechanism of the exchange reaction is not directly accessible by experimental methods. In general, reaction mechanisms can be deduced by experimentally testing the sensitivity of the reaction rate to a variety of chemical and physical parameters such as temperature, pressure, or concentration. 

It is not clear why the earlier workers observed only the yellow or brown color and not the blue color. This may be due to differences between the anhydrous and hydrated materials and/or to differences in the production or stability of the 670 nm band. Since this band was found to be unstable at room temperature [59], it is possible that it was sufficiently unstable in their samples so as to be unobservable. It is also evident that under some irradiation conditions the band is not produced, so that the band may be due to an impurity which is not present in all samples. It is necessary to determine the defect responsible for this band before it is possible to resolve this matter. There is evidence that a band at 565 nm in KN3 is associated with the N2 defect [69], and it is tempting to associate the 670 nm band in Ba(N3)2 with this defect. The observations that after X-irradiation at room temperature or at 78°K the NJ defect is not observed and that the samples are yellow-brown, thus indicating the absence of the 670 nm band, tend to support the speculation. The NJ defect was also observed in Ba(N3)2 H20, but there is no information regarding color or optical properties [37]. There is, in addition, reason to associate the 300-nm band in Ba(N3)2 with the NJ molecular ion. Marinkas found an excitation band for conversion of NJ to N3 in this same region [52]. Correlated studies by magnetic resonance and optical absorption of Ba(N3)2 should further identify and characterize the irradiation-induced disorder. Studies by these techniques and of gas evolution will then undoubtedly enable the mechanism of decomposition to be established. 

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