Spin polarized field emission

Figure 6.1. Jablonski-type diagram for pyrazine. The zero-field splittings (between tx, tV) t2) are not drawn to scale. Spin polarization ( x x x) resulting from the most probable intersystem crossing routes and part of the emission spectrum where different vibronic bands (v = /,/, k) have different zf origins are schematically indicated. (After El-Sayed.(17))...

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). 

A final noteworthy feature of these spectra is the lack of RPM polarization, which for these radicals would appear as low-field emissive, high-field absorptive transitions. It is curious that such polarization never develops at any delay time, even out to 20 xs where we have observed only TM polarization. The creation of RPM polarization requires reencounters of radicals on a suitable timescale and modulation of the exchange interaction between the unpaired electrons. This is normally accomplished by diffusion of the radicals between weak and strong exchange regions. That it never develops indicates that either these radicals do not make a significant number of reencounters, or perhaps it is due to the fast spin relaxation in the oxo-acyl radical. It may also be that the TM is simply so dominant that the RPM intensity is always much weaker and is never observed. At lower temperatures (cf. Fig. 14.1, top spectrum at 25°C), there does appear to be a slight superposition of an EIA pattern on top of the emissive TM polarization, but it has a very small effect. 

The most fascinating development in this field of CIDNP within the last years has been the observation, by Zysmilich and McDermott [146], of nuclear spin polarized (solid state) 15NNMR spectra from photosynthetic reaction centers in which the forward electron transfer from the primary charge-separated state to the accepting quinone was blocked. The all-emissive polarizations were proposed to be due to a radical pair mechanism, though many of the details are still not very clear. The reaction scheme is virtually identical to that of Chart VIII (Section V.A.2), the donor D being the special pair and the acceptor A the pheophytin. As in that example, the polarizations from the triplet exit channel are hidden in the triplet product 3D for the lifetime of the latter. This feature, in combination with the fact that nuclear spin relaxation in the molecular triplet localized on the special pair is relatively fast, serves to avoid the cancellation of CIDNP that would occur otherwise because the products from both exit channels are identical. 

As mentioned in section 2.3.1, the energy bands of Gd are polarized when the 4f moments are ferromagnetically ordered. There are a number of experiments on the spin polarization effects. These studies have more in common with each other than with the band structure and Fermi surface studies using the same methods. We will review here a positron annihilation experiment (Hohenemser et al., 1968), two field emission experiments (Hofmann et al., 1967 Chrobok et al., 1968), and a UPS experiment (Busch et al, 1969). 

Clearly, if a situation were achieved such that exceeded Np, the excess energy could be absorbed by the rf field and this would appear as an emission signal in the n.m.r. spectrum. On the other hand, if Np could be made to exceed by more than the Boltzmann factor, then enhanced absorption would be observed. N.m.r. spectra showing such effects are referred to as polarized spectra because they arise from polarization of nuclear spins. The effects are transient because, once the perturbing influence which gives rise to the non-Boltzmann distribution (and which can be either physical or chemical) ceases, the thermal equilibrium distribution of nuclear spin states is re-established within a few seconds. 

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