Transitions that lead luminescence

A simplified schematic diagram of transitions that lead to luminescence in materials containing impurides is shown in Figure 1. In process 1 an electron that has been excited well above the conduction band et e dribbles down, reaching thermal equilibrium with the lattice. This may result in phonon-assisted photon emission or, more likely, the emission of phonons only. Process 2 produces intrinsic luminescence due to direct recombination between an electron in the conduction band... 

Density functional theory (DFT) calculations were also carried out to assign the molecular orbitals involved in the transitions that lead to luminescence, concluding that metal centered (du )1(pu)1 or (da )1 (pa)1 excited states are responsible for the luminescence in the solid state, while in dilute solutions the luminescence arises from ira excited states in the pentafluorophenyl ligands or from ir-MMCT transitions. 

The lowest triplet energy level of the ancillary ligand LX lie well above the energies of LC and MLCT excited states in most of the lr(C N)2(LX)-type complexes, so luminescence of Ir(C N)2(LX) is dominated by LC and MLCT transitions. This leads to similar phosphorescence emission to the/ac-lr(C N)3 complexes with the same cyclometalated ligand [6, 23]. In such cases, density functional theory (DPT) calculations indicate that HOMOs are largely metal-centered, whereas LUMOs are primarily localized on the heterocyclic rings of the cyclometalated ligand. The ancillary is therefore not directly involved in the lowest excited state. 

The preceding results can be compared with the selection rules for all the possible electronic transitions obtained from group theory, to give electronic assignments (term symbols) for each of the luminescence bands. The resulting electronic assignments are A2 and A2u for the HE and LE bands, respectively. A discussion of the concepts of group theory that lead... 

Therefore, there could exist rich defects in Ba3BP30i2, BaBPOs and Ba3BP07 powders. From the point of energy-band theory, these defects will create defect energy levels in the band gap. It can be suggested that the electrons and holes introduced by X-ray excitation in the host might be mobile and lead to transitions within the conduction band, acceptor levels, donor levels and valence band. Consequently, some X-ray-excited luminescence bands may come into being. 

The adsorption of transition metal complexes by minerals is often followed by reactions which change the coordination environment around the metal ion. Thus in the adsorption of hexaamminechromium(III) and tris(ethylenediamine) chromium(III) by chlorite, illite and kaolinite, XPS showed that hydrolysis reactions occurred, leading to the formation of aqua complexes (67). In a similar manner, dehydration of hexaaraminecobalt(III) and chloropentaamminecobalt(III) adsorbed on montmorillonite led to the formation of cobalt(II) hydroxide and ammonium ions (68), the reaction being conveniently followed by the IR absorbance of the ammonium ions. Demetallation of complexes can also occur, as in the case of dehydration of tin tetra(4-pyridyl) porphyrin adsorbed on Na hectorite (69). The reaction, which was observed using UV-visible and luminescence spectroscopy, was reversible indicating that the Sn(IV) cation and porphyrin anion remained close to one another after destruction of the complex. 

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