Donor spin-orbit splitting

Fig. 17-E-3 has a representation of the potential wells for the HS and LS configuration relative to the Fe—donor atom distance. Crossover occurs when the vertical separation of the zero-point energies of the wells (AEHl) is small (of the order of kT) that is, when the spin-pairing energy and the magnitude of the orbital splitting (Ao) are about equal.

Radiative transfer plays a role essentially when the absorption band of the acceptor ion is allowed. A photon emitted by an ion is absorbed by an other ion before escaping from the material. This requires overlap of the emission spectrum of the donor with the absorption spectrum of the acceptor. Radiative transfer between identical ions causes a modification of the spectral distribution. This is the case for the Ce + emission when the Stokes shift is small. The cerium emission originates from the lowest 5d state and consists of two bands because the ground state 4f is split by spin-orbit coupling into the states Fs/2 and F7/2 (Figme 5), the shorter-wavelength component 5d 4f( Fs/2) having the... 

The Ch-related donor spectra differ on that point as several parity-forbidden transitions are observed. They start with symmetry-allowed transitions from the Is ground state to the valley-orbit split Is excited states, and are supplemented with 2s (T2) and 3s (T2) lines and Fano resonances within the photoionization spectrum. This is shown in Fig. 6.13 for Se°. Compared to group-V donors, this extends the energy span of the Ch°-related spectra to the ionization energy of the Is (T2) level (35-40 meV in isolated chalcogens) and it can even increase to 40-48 meV when singlet-triplet spin-forbidden transitions are observed. 

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