Optically forbidden transition

Figure Bl.6.8 Energy-loss spectra of 200 eV electrons scattered from chlorine at scattering angles of 3° and 9° [10]. Optically forbidden transitions are responsible for the intensity in the 9° spectrum that does not appear in the 3 ° spectrum.

All the neutral single donors without d or f electrons have spin 1/2 while the double donors and acceptors have spin 0 in the ground state, but in some excited states, they have spin 1 and optically forbidden transitions between the singlet and triplet states have been observed. The spins of the neutral acceptors in the ground state depend on the electronic degeneracy of the VB at its maximum. For silicon, the threefold degeneracy of the valence band results in a quasi spin 3/2 of the acceptor ground state. 

In this study, we have employed momentum-transfer (q) resolved EELS to probe the momentum-transfer dispersion of the valence band excitations in BaBiOs. We report an optically forbidden transition ( 4 eV) and the effective mass of the optical gap ( 2 eV), both of which exhibit crystalline anisotropy. 

Another important feature is the optically forbidden transition located at... 

For two-atomic molecules without a dipole moment (H2, O2, N2, CI2 and so forth) the rotational states lie close as well, but the pky disappear for transitions between these states. The series expansion (3) therefore starts for these (optically forbidden) transitions only with the third term, which in general yields only a small contribution. Since we can in general restrict ourselves to the dipole terms, for these molecules we only have to consider those states in the summation (1) which result from a transition out of the ground state to a neighbouring state by an electronic transition. In such conditions we have to conceive of a broad resonance in the rotational terms as well. 

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