Crystal field charge penetration

The intrinsic parameters discussed in the previous section open a practicable possibility to analyze the fundamental interactions occurring between the f-electrons and the ligands. In an ab initio calculation for the system Pr3+-Cl , Newman and Ng (1986) took into account five different mechanisms contributing to the intrinsic crystal-field parameters at various Pr3+-Cl distances. These were point charges, charge penetration, exchange, p and, v overlap, and p and covalency. 

A considerable contribution to ab initio calculations is due to Newman and coworkers. More than twenty years ago, they evaluated the contribution of a variety of mechanisms to the one-ligand one-particle crystal field, namely overlap and exchange effects (Ellis and Newman 1967), exchange charge effects (Bishton et al. 1967), charge penetration (Ellis and Newman 1968), and ligand-ligand overlap effects (Curtis and Newman 1969). 

Boron implant with laser anneal. Boron atoms are accelerated into the backside of the CCD, replacing about 1 of 10,000 silicon atoms with a boron atom. The boron atoms create a net negative charge that push photoelectrons to the front surface. However, the boron implant creates defects in the lattice structure, so a laser is used to melt a thin layer (100 nm) of the silicon. As the silicon resolidihes, the crystal structure returns with some boron atoms in place of silicon atoms. This works well, except for blue/UV photons whose penetration depth is shorter than the depth of the boron implant. Variations in implant depth cause spatial QE variations, which can be seen in narrow bandpass, blue/UV, flat fields. This process is used by E2V, MIT/LL and Samoff. 

The reduction of the free-ion parameters has been ascribed to different mechanisms, where in general two types of models can be distinguished. On the one hand, one has the most often used wavefunction renormalisation or covalency models, which consider an expansion of the open-shell orbitals in the crystal (Jprgcnscn and Reisfeld, 1977). This expansion follows either from a covalent admixture with ligand orbitals (symmetry-restricted covalency mechanism) or from a modification of the effective nuclear charge Z, due to the penetration of the ligand electron clouds into the metal ion (central-field covalency mechanism). 

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