Intensities spin-correlated

In the spin-correlated RP the two radicals interact via electron-electron dipolar and exchange interaction which leads to line splitting. The ET process creates the RP in a strongly spin-polarized state with a characteristic intensity pattern of the lines that occur either in enhanced absorption (A) or emission (E).144 145 The spectrum is therefore very intense and can directly be observed with cw EPR (transient EPR) or by pulse methods (field-swept ESE).14 To study the RPs high field EPR with its increased Zeeman resolution proved to be very useful the first experiment on an RP was performed by Prisner et al. in 1995146. From the analysis of the RP structure detailed information about the relative orientation of the two radicals can be extracted from the interaction parameters. In addition kinetic information about the formation and decay of the RP and the polarization are available (see references 145,147). 

Fig. 5-11 also shows the CIDEP spectra calculated with this model (a) 7 > 0 J (w > 0 J) and (b) 7 < 0 J (w < 0 J). As clearly seen in this figure, each spectrum has two anti-phase doublets with an A/E (E/A) pattern for 7 > 0 J (7 < 0 J). Similarly, one can show that it has an E/A (A/E) one for a radical pair generated from an S-precursor for 7 > 0 J (7 < 0 J). The splitting of each anti-phase doublet corresponds to 27. This is a novel method to determine the 7 values of radical pairs and biradicals in solution. Let us consider the conditions for detecting the CIDEP of spin correlated radical pairs. When 3 (= 7/ ) is much larger than Q, little polarization can be obtained because Ipg becomes very small from Eq. (5-43). When 3 (= 7 ti ) is smaller than the line width of each ESR line, the CIDEP signals also vanishes because the anti-phase components of each doublet cancel out with each other. For intermediate 3 (= 7/ft ), CIDEP due to the SCM becomes intense. Each of the anti-phase... 

To explain the held dependence, we have to assume that the applied held has an influence on v hether the spin selection rules can be obeyed in the fusion process. A kinetic explanation by Merrifleld and Johnson [49, 50] assumes in the reaction Ti + Ti (Ti Ti) Si + So the existence of an intermediate pair state (Ti Ti), in v hich the tv o excitons repeatedly collide before they react. The possible spin correlations in this pair state have both triplet as well as singlet character. The triplet fraction in the pair state is also influenced by an applied magnetic field via the Zeeman interaction of the coupled individual spins with the field. The strength and direction of the field thus determine the relative fraction of triplet and singlet states in the pair. The singlet fraction leads to the states Si and So, and thus to delayed emission. Therefore, the intensity and lifetime of the emission can be modulated by an applied magnetic field. This holds for all biexcitonic processes in which two triplet states participate. 

If there is no analysis of the scattered neutron energy then (within the static approximation) the measured intensity is proportional to the Fourier transform of the instantaneous correlation function which is essentially a snapshot of the spin correlations in reciprocal space. At non-zero energy transfer, the spin dynamics of the system under study are probed. In a magnetically ordered rsystem of localised spins, the elementary magnetic excitations are spin waves. 

It should be kept in mind that the covariance spectrum can contain false cross-peaks, which have intensities larger than the noise level but have nothing to do with spin correlation. Such an artifact is caused by the nature of the covariance processing and presumably can be one of possible origins of reluctance of some researchers to employ the covariance approach. Nevertheless, one can readily identify the false cross-peaks in many cases where correlation experiments are performed with various... 

Here F(t)is the recombination rate of the ion pair, 9 is the fraction of spin-correlated pairs, which is assumed to be constant but can vary for different chemical systems and pss (t) is the time dependence of the singlet state population of the spin-correlated pair. The second term in Eq. (8.2) includes the contribution of the singlet component of the spin-uncorrelated pairs to the fluorescence intensity. Typically, the results of the TR MFE decay are presented as a ratio of the fluorescence intensity at an applied field (Ib ) and at zero field (7o), which becomes independent of the unknown function Fit). 

It was found that contact interactions of the intermediates [reactions (11)-(17)] (i) acted as an extra source of spin relaxation for spin-correlated D+ / e pairs and (ii) decreased the frequency of annihilation for D. The former is due to spin exchange in reactions (3) and (15)-(17), as these reactions have the effect of incoherently changing the spin states of the correlated pairs the latter is due to the enhancement of the singlet-triplet intersystem crossing of S in reactions (13)-( 15), which depletes S and the frequency of reaction (6). Hence this results in a subsequent decrease in the yields of D and D as well. This analysis was also found to be true by Borovkov [36]. It can also be seen from Fig. 8.13 that the contribution of the annihilation of triplet solutes to the recombination fluorescence intensity becomes comparable with reaction (4) at 30ns time range at the luminophor concentration of about 20 mM. 

Muns ENDOR mvolves observation of the stimulated echo intensity as a fimction of the frequency of an RE Ti-pulse applied between tlie second and third MW pulse. In contrast to the Davies ENDOR experiment, the Mims-ENDOR sequence does not require selective MW pulses. For a detailed description of the polarization transfer in a Mims-type experiment the reader is referred to the literature [43]. Just as with three-pulse ESEEM, blind spots can occur in ENDOR spectra measured using Muns method. To avoid the possibility of missing lines it is therefore essential to repeat the experiment with different values of the pulse spacing Detection of the echo intensity as a fimction of the RE frequency and x yields a real two-dimensional experiment. An FT of the x-domain will yield cross-peaks in the 2D-FT-ENDOR spectrum which correlate different ENDOR transitions belonging to the same nucleus. One advantage of Mims ENDOR over Davies ENDOR is its larger echo intensity because more spins due to the nonselective excitation are involved in the fomiation of the echo. 

Here we comment on the shape of certain spin-forbidden bands. Though not strictly part of the intensity story being discussed in this chapter, an understanding of so-called spin-flip transitions depends upon a perusal of correlation diagrams as did our discussion of two-electron jumps. A typical example of a spin-flip transition is shown inFig. 4-7. Unless totally obscured by a spin-allowed band, the spectra of octahedral nickel (ii) complexes display a relatively sharp spike around 13,000 cmThe spike corresponds to a spin-forbidden transition and, on comparing band areas, is not of unusual intensity for such a transition. It is so noticeable because it is so narrow - say 100 cm wide. It is broad compared with the 1-2 cm of free-ion line spectra but very narrow compared with the 2000-3000 cm of spin-allowed crystal-field bands. 

In addition to sample rotation, a particular solid state NMR experiment is further characterized by the pulse sequence used. As in solution NMR, a multitude of such sequences exist for solids many exploit through-space dipolar couplings for either signal enhancement, spectral assignment, interauclear distance determination or full correlation of the spectra of different nuclei. The most commonly applied solid state NMR experiments are concerned with the measurement of spectra in which intensities relate to the numbers of spins in different environments and the resonance frequencies are dominated by isotropic chemical shifts, much like NMR spectra of solutions. Even so, there is considerable room for useful elaboration the observed signal may be obtained by direct excitation, cross polarization from other nuclei or other means, and irradiation may be applied during observation or in echo periods prior to... 

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