Triplet excitons Zeeman splitting

Anodization of Si in HF under an applied magnetic field produces an enhancement of the PL efficiency at RT, accompanied by an enhanced porosity compared to PS samples prepared without an applied field. The degree of polarization of the emitted PL is reduced for field-assisted preparation [Na3]. At low temperatures (4.2 K), the Stokes shift and the decay time of the PL are found to be increased, if compared to PS formed under zero magnetic field. This has been interpreted as Zeeman splitting of the spin-triplet exciton states. It indicates that the ground state of the luminescing silicon crystallite is a triplet state [Kol3]. 

Thus, A(0T)=3. Next, let us consider the case when the Zeeman splitting is much larger than zero-field splitting. In this case, the triplet excitons are described, to a good approximation, by the wavefunctions of 0>, +1>, and -1> where 0, -i-l, and -1 correspond to the standard magnetic sublevel quantum numbers. The nine possible pair states in this case are... 

Thus, A(0T)=6. In the case where the Zeeman splitting is much larger than zero-field splitting, there are six combinations at the collision of a triplet exciton and a doublet quencher... 

Figure 7-3. (a) Zeeman splitting of triplet exciton sublevels as a function of the applied magnetic field, (b) Change in PL intensity at magnetic resonance. 

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. 

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