Optical nuclear-spin polarisation

Fig. 7.31 The steady-state value of the optical nuclear-spin polarisation p = pi n a fluorene-dghio crystal doped with acridine, measured at room temperature, as a function of the orientation of the polarising magnetic field whose value is Bp = S mT. Pi is the equilibrium polarisation, and T5 is the nuclear-spin temperature. From [35].

For an understanding of ESR in crystals, a detailed discussion of the molecular fundamentals is necessary. We deal with this primarily in Sections 7.2 and 7.3. There, the spin quantisation in triplet states, magnetic dipole-dipole couphng, zero-field splitting, Zeeman spHtting and fine structure are explained. These fundamentals apply both to isolated molecules and to excitons (Sects. 7.4 and 7.5). In the two later Sects. 7.6 and 7.7 of this chapter, the so called optical spin polarisation in excited triplet states and dynamic nuclear spin polarisation will be treated. 

A well-known and important phenomenon in the area of nuclear-spin resonance (NMR) in gases, liquids, or solid samples is dynamic nuclear-spin polarisation (DNP) (see e.g. [M6]). This term refers to deviations of the nuclear magnetisation from its thermal-equilibrium value, thus a deviation from the Boltzmann distribution of the populations of the nuclear Zeeman terms, which is produced by optical pumping (Kastler [31]), by the Overhauser effect [32], or by the effet solide or solid-state effect [33]. In all these cases, the primary effect is a disturbance of the Boltzmann distribution in the electronic-spin system. In the Overhauser effect and the effet solide, this disturbance is caused for example by saturation of an ESR transition. Owing to the hyperfine coupling, a nuclear polarisation then results from coupled nuclear-electronic spin relaxation processes, whereby the polarisation of the electronic spins is transferred to the nuclear spins. 

Moreover, the manipulation of trapped radioactive ions with laser light opens up possibilities to study questions of the Standard Model. By optical pumping within the hyperfine levels of the ground state, the nuclear spins of radioactive nuclides can be polarised with high efficiency. The detection of the asymmetry of beta decay, for example, will allow one to explore deviations from the vector/axial-... 

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