Spin-flip process

The role of exchange scattering can be observed most clearly when polarised electrons are scattered from a target of polarised light atoms. If one observes a spin-flip process like... 

Various correlation and internuclear distance measurements are based on zero- or double quantum spin flips. The most efficient correlation methods are based on the adiabatic process, which is relatively insensitive to actual, orientation-dependent coupling strength. A double-quantum spin flip is generated by a sweep of the rf field amplitude, centred at half rotation frequency (the HORROR condition [9]). The increased rotation frequency leads to a crucial improvement in the broad-band character of the DREAM method [10]. The zero-quantum adiabatic spin flip process is considered in the next section. 

It should be noted that this argxunent can be generahzed for an n spin-flip process. Considering the simultaneous flip of two spins, each of them experiences an energy barrier A a = D S. The probabihty to observe this double flip is the square of the probabihty for a single flip. In other words, the activation energy of the Arrhenius law becomes 2 D S. ... 

This approach can be easily generahzed to the n spin-flip process and therefore the temperature dependence of ton is obtained by ... 

At low temperature, the influence of n on ton is dominated by the exponential term. Hence, it is interesting to note that Eq. 22 imphes that the n spin-flip process is slower than the single spin-flip for an infinite chain. 

When 7i becomes very large, the double spin-flips process becomes the fastest pathway of relaxation. In this hmit, the description in terms of effective spin units becomes also relevant for the dynamics and Eq. 33b can be identified to ... 

Finally, n spin flip processes can also be considered. In the case of short chains, a collective reversal of the magnetization has also been discussed [31]. In this latter case, the interaction between magnetic units plays no role and the result is simply, for a segment of n magnetic sites ... 

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. 

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. 

NMR relaxation and Knight shift In an external field nuclear spins exhibit a Larmor precession with a frequency hyperfine coupling to conduction electrons which leads to spin-flip processes as witnessed by the NMR-relaxation rate and the Knight shift of the resonance frequency Scoq- A review of these important effects for HF superconductors is given by Ton et al. (2003). The relaxation rate is determined by the availability of resonant electron-hole excitations. In the normal state this leads to the Kor-ringa law In the superconducting state the presence of A(k) should lead to a faster... 

Fig. 8 Simplified model for a one-center spin-flip process coupled to a change of angular momentum py -> in formaldehyde. The picture of this spin-orbit allowed -> n,n transition is inspired by reference 103. (Reproduced with permission from J. Chem. Phys., 2008, 129, 064117. Copyright 2008 American Institute of Physics).

In our ADMR measurements we found 8N < 0, which can be explained within the distant pair model discussed in Section III.B.2. As explained in that section, the distant pair model can be used only for spin pairs generated with equal probability of parallel (P) or antiparallel (AP) spin configuration. This is certainly not the case for the photogenerated, S . As concluded above, they are thought to be produced in pairs (5-5) from the singlet manifold and hence presumably with preference of an AP spin configuration. To solve this apparent conflict, either a spin-flip mechanism must occur or a spin-spin interaction exists between the solitons within the 5-5 pair. Such a spin-flip process may happen via the native soliton defects, 5/ (spin 1/2 radicals), which exist in... 

The cross section for this light-induced spin-flip process may become very large because the Thomson cross section of an electron with effective mass m in a semiconductor is... 

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