Nuclear spin-flip

It can also be seen from Fig. 6 that if the T+x or T x states mixed with S, this would involve concomitant electron and nuclear spin flipping in order that the total spin angular momentum be conserved, and this would ultimately produce the same polarization in c- and e-products. This point will be discussed further in Section IV. 

In ESR, it is also customary to classify relaxation processes by their effects on electron and nuclear spins. A process that involves an electron spin flip necessarily involves energy transfer to or from the lattice and is therefore a contribution to Tx we call such a process nonsecular. A process that involves no spin flips, but which results in loss of phase coherence, is termed secular. Processes that involve nuclear spin flips but not electron spin flips are, from the point of view of the electron spins, nonsecular, but because the energy transferred is so small (compared with electron spin flips) these processes are termed pseudosecular. 

In the case of NMR, the energy required to induce the nuclear spin flip also depends on the strength of the applied field, H. It is found that... 

A spin-Nuclear spin Fluctuation of A spin quantum state by molecular motion and nuclear spin flip-flop No... 

Second, there is usually no escape channel because the two radical termini cannot separate completely. This usually causes a fundamental difference between CIDNP from radical pairs and CIDNP from biradicals Being nuclear spin sorting as described above, intersystem crossing between IS) and ITq) crucially relies on both exit channels leading to different products, whereas intersystem crossing between IS) and IT i) occurs by simultaneous electron-nuclear spin flips, and so creates net nuclear polarizations without the need of different exit channels. 

In a recent paper by Burland, Carmona, and Cuellar the first OFID measurements on an organic radical, duryl in a durene host ciystal, were reported. At low temperature T2 was found to be 212 78 ns, which is substantial shorter than half the fluorescence lifetime (530 ns). The authors concluded that in this system also the nuclear spin-flip processes determine the OFID decay. We note here that in these multilevel systems the coherence bandwidth of the exciting laser will have a profound effect on the coherent emission. 

Nuclear spin flip (Am/ = 1) lines occur in systems with anisotropic hyperflne couplings of smaller magnitude than the nuclear Zeeman energy. In this case satellite lines appear on each side of an often unresolved doublet of central lines. At X-band the satellites are separated approximately by Bn = 0.5 mT from the... 

The spectral spin diffusion rate caused by nuclear spin flip processes in the bulk nuclear spin system in the sample pentacene-h2di2 in p-terphenyl-dn is slower than in the case of a normal, protonated mixed crystal [251. Nevertheless all three possible configurations of the proton nuclei appear in the spectrum implying that the effective spin diffusion time is still much shorter than the time of 10 minutes needed to record the spectrum. 

The Korringa relationship [21] indicates thatl/Ti oc T, where T is the spin—lattice relaxation time and T is the absolute temperature of the sample. This unique temperature dependence of l/Ti is the NMR fingerprint of a metallic state. It results from the fact that only conduction electrons around the Fermi level can satisfy energy conservation for the electron-nuclear spin flip-flop relaxation process, and the fraction of these electrons is proportional to bT. When all relaxation mechanisms other than the first term in Eq. (1) can be neglected, and there are only x-like electrons at the Fermi level (such as in the alkali metals), the Korringa relationship takes its simplest form ... 

Echo detection of selectively-burned holes in photosynthetic systems led to estimates of distances between 25 and 50 A. If the contributions to spectral diffusion from motion, nuclear spin flip-flops, and instantaneous diffusion are smaller than the contribution from dipolar interaction between unpaired electrons, the spectral diffusion can be used to determine the interspin distance. (97-100). 

Double Resonance Methods. Electron nuclear double resonance (ENDOR) is the effect of applying a radiofrequency that induces nuclear spin flips, in addition to the microwave frequency that induces electron spin flips. In the CW version of the experiment, the ENDOR effect is an increase in the intensity of... 

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