Acceptor spin-orbit splitting

The energy separation between 1T8+ and 1 7+ acceptor levels is called the acceptor spin-orbit splitting Aa and it depends on the chemical nature of the acceptor and the host crystal. The nP1/2 and nP3/2 states associated with the T7+ VB give T5 and T states distinct from those of the Ts+ VB, and are resonant with the VB. The acceptor transitions between the 1T8+ state and the odd-parity states associated with the p3/2 VB r8+ are responsible for the so-called p3/2 spectrum. Logically, transitions between the 1T7+ state and the odd-parity resonant states associated with the p1/2 VB should also produce a distinct spectrum. No such transitions have been observed for Si, as the 1T7+ state, which lies in the band gap, is depopulated up to RT, but for diamond, due to the smaller value of Asoa in this material, weak transitions between 1T7+ and the odd-parity states associated with the T7+ VB have been observed near 80 K [27]. As no selection rule forbids them, transitions between the 1T8+ state and the p7/2 resonant acceptor states have indeed been observed in silicon, reported first in [66], and they are known as the pi/2 spectrum. 

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... 

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