Splitting crystal field-induced

ISOTOPE TRAPPING STICKY SUBSTRATES LIGAND EXCLUSION MODEL LIGAND FIELD SPLITTING CRYSTAL FIELD SPLITTING LIGAND-INDUCED CONFORMATIONAL CHANGE... 

Fig. 33. Crystal field-induced splitting of the spectrum into ortho- and para-hydrogen bands on adsorption on Na4Ca4-A at 90 K the arrow indicates the o-Hj/p-Hj splitting (from [605])...

Instead, we believe the electronic structure changes are a collective effect of several distinct processes. For example, at surfaces the loss of the bulk symmetry will induce electronic states with different DOS compared to bulk. As the particle sizes are decreased, the contribution of these surface related states becomes more prominent. On the other hand, the decrease of the coordination number is expected to diminish the d-d and s-d hybridization and the crystal field splitting, therefore leading to narrowing of the valence d-band. At the same time, bond length contraction (i.e. a kind of reconstruction ), which was observed in small particles [89-92], should increase the overlap of the d-orbitals of the neighboring atoms, partially restoring the width of the d-band. 

Nonradiative transitions can also occur between 4/ rare-earth levels. Orbit-lattice interaction may induce these between two stark-split components of the same 4fN term or between stark-split components of different 4fN terms. One assumes that the crystal field the ion sees is modulated by the vibrations of the surrounding ions. If the spacing between the two 4fN levels is less than the Debye cut-off frequency, there will be acoustical phonons capable of inducing direct transitions between the levels. The theoretical treatment of this problem is quite complex (36). 

The application of an external electric field to the crystal during the NQR experiment is very important. Such experiments have been done by Bloember-gen et al. 62<63>, and by Kushida and Saiki 71). The subject is treated extensively by Bloembergen 8°I an by Dixon 90>102>. The external electric fields usable in such experiments are limited by the electric breakdown in the substance. Electric fields of the order of (104 - 10s) V/cm are the upper limit for most solids. On the other hand, the internal electric fields are of the order of 10 V/cm (the electric field of a point charge e at a distance of one A). In molecular crystals, Bloembergen and Dixon 64>90> studied the electric field induced frequency shifts. The overall crystal field splitting of n(35Cl) in 1,3,5-trichloro-... 

Although the CFT model is successful in explaining many of the properties of transition metal complexes, it fails to account for some important observations. In particular, spectroscopic measurements have been used to define a relative order for ligands in terms of the magnitude of the crystal field splitting they induce. This order is known as the spec-trochemical series ... 

We used crystal field theory to order the energy-level splittings induced in the five d orbitals. The same procedure could be applied to p orbitals. Predict the level splittings (if any) induced in the three p orbitals by octahedral and square-planar crystal fields. 

It has been found that the emission due to Mn doping can be further tuned by changing the crystal field around the Mn + ions. Instead of the S ions, as in the case of ZnS, an oxide host can be used to induce a much larger crystal field splitting of the t2g-eg levels in Mn + ions, for example by doping Mn + ions in ZnO nanocrystals [20], In this case, the Mn d-d emission is observed at 470 nm (2.6 eV), representing a blue shift of 0.5 eV compared to the case of Mn emission in Mn-doped ZnS. 

The 320 nm electronic spectrum of naphthalene was the first for which the theory was developed in detail [67]. In the vapour spectrum there are two interpenetrating band systems. One system with oscillator strength f=0.0002 is very weakly electronically allowed and long-axis polarized. The other stronger (f=0.002) system is short axis polarized, and is induced by vibrational perturbation. In the crystal the effect of the crystal field on these systems is different. We had shown experimentally [66] that the origin transition on the one hand and the vibrationally induced transition on the other, were differently affected by crystal interactions. The origin band is split by 151 cmJl and the vibrational by less than 1 cm-1. 

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