Optical transitions of the H centers in alkali halides

Theoretical modeling of charge trapping in crystalline and amorphous Al2O3

The characteristics of intrinsic electron and hole trapping in crystalline and amorphous Al₂O₃ have been studied using density functional theory (DFT). Special attention was paid to enforcing the piece-wise linearity of the total energy with respect to electron number through the use of a range separated, hybrid functional PBE0-TC-LRC (Guidon et al 2009 J. Chem. Theory Comput. 5 3010) in order to accurately model the behaviour of localized states. The tuned functional is shown to reproduce the geometric and electronic structures of the perfect crystal as well as the spectroscopic characteristics of Mg_Al hole centre in corundum α-Al₂O₃. An ensemble of ten amorphous Al₂O₃ structures was generated using classical molecular dynamics and a melt and quench method and their structural characteristics compared with the experimental data. The electronic structure of amorphous systems was characterized using the inverse participation ratio method. Electrons and holes were then introduced into both crystalline and amorphous alumina structures and their properties calculated. Holes are shown to trap spontaneously in both crystalline and amorphous alumina. In the crystalline phase they localize on single O ion with the trapping energy of 0.38 eV. In amorphous phase, holes localize on two nearest neighbour oxygen sites with an average trapping energy of 1.26 eV, with hole trapping sites separated on average by about 8.0 Å. No electron trapping is observed in the material. Our results suggest that trapping of positive charge can be much more severe and stable in amorphous alumina rather than in crystalline samples.

First-principles calculations of oxygen interstitials in corundum: a site symmetry approach

Site symmetry analysis allows one to define four possible spatial configurations for inserting O atoms into the α-Al₂O₃ crystalline lattice.

Structure and properties of electronic and hole centers in CsBr from theoretical calculations

The electronic structure, geometry, diffusion barriers and optical properties of fundamental defects of CsBr are calculated using hybrid functional DFT and TD-DFT methods. The B3LYP functional with a modified exchange contribution has been used in an embedded cluster scheme to model the structure and spectroscopic properties of the self-trapped triplet exciton, interstitial Br atoms and ions, self-trapped holes and Br vacancies. The calculated migration barriers and positions of maxima of optical absorption bands are in good agreement with experiment, justifying the obtained defect geometries. The off-center triplet exciton luminescence energy is also accurately calculated.

Laser Control of Desorption through Selective Surface Excitation

We review recent developments in controlling photoinduced desorption processes of alkali halides. We focus primarily on hyperthermal desorption of halogen atoms and show that the yield, electronic state, and velocity distributions of desorbed atoms can be selected using tunable laser excitation. We demonstrate that the observed control is due to preferential excitation of surface excitons. This approach takes advantage of energetic differences between surface and bulk exciton states and probes the surface exciton directly. We demonstrate that desorption of these materials leads to controlled modification of their surface geometric and electronic structures. We then extend the exciton mechanism of desorption, developed for alkali halides, to metal oxide surfaces, in particular magnesium oxide. In addition, these results demonstrate that laser desorption can serve as a solid-state source of halogen and oxygen atoms, in well-defined electronic and velocity states, for studying chemical processes in the gas phase and at surfaces.