摘要:

The concept of hot holes as energy carriers in irradiated ionic solids, mainly in alkali halides, is reviewed. This concept has been rised to understand the transfer of a considerable portion of radiation-created holes to impurity centers on irradiation of alkali halides at low temperatures where self-trapped holes are practically immobile. It has further been used to handle the nm-to-μm range bulk-to-surface energy transport in alkali halides and metal oxides irradiated by XUV (synchrotron) radiation. This transport may be considered as hole diffusion with the mean free path of about 10 nm, the diffusion length of about 100 nm and the diffusion time of about 1 ps, which is consistent with the model in which hot holes behave as band particles, their motion being limited by holephonon scattering. The holes having reached the crystal surface recombine with conduction electrons which leads to surface recombination luminescence and possibly to creation of surface defects.

ISSN:1042-0150    

DOI:10.1080/10420159408218851

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Desorption of alkali atoms from, and defects formed in, alkali halide crystals stimulated by low-energy electron bombardment were investigated simultaneously by optical absorption-, mass spectroscopy-, and depth-profile surface sputtering-techniques. These techniques not only provide important information about the type, amount, and the spatial distribution of the defects formed in the alkali halide crystals, but also indicate which processes govern the emission rate of neutral alkali atoms during and after bombardment. The results show that at temperatures near and lower than room temperature, F-centers, small F-center clusters, and alkali metal clusters are formed during electron bombardment of the crystals. At high temperatures (up to 400°C), colloid formation (metallic phase in the bulk of the crystals) takes place due to the higher mobility of the F-centers. The experimental results clearly indicate that the thermal stability of the colloids control the delayed alkali atom desorption kinetics at elevated temperatures. Depth profile measurements provide strong evidence that the metallic phase is on the surface of the crystals, whereas the F-center type defects are formed in the bulk and near the surface of the crystals. The surface clearly acts as an efficient trap for defects. The spatial distributions of point defects and metal were investigated down to temperatures as low as −90°C where F-center mobilities are negligible. The results show significant low-temperature metallization on the surfacewhich clearly proves that long-range hot-hole diffusion takes place during electron bombardment.

ISSN:1042-0150    

DOI:10.1080/10420159408218852

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

We have investigated excitation spectra of the In+ion luminescence directly excited by UV-light (6.1 to 6.4 eV) and creation spectra of the photostimulated luminescence, excited by F-band light after a preliminary monochromatic UV irradiation of KBr:In crystals with different In+concentrations. Careful analysis of the excitation spectra allows us to detect a new D absorption band having a Gaussian shape with the following parameters: peak position at E0= 6.375 eV and half-width ΔE = 0.14 eV, effective absorption cross-section at the band peak [sgrave]D, max= 6.9 10−16cm2. The oscillator strength f = 0.12 of the D band and quantum yield η = 0.012 ± 0.001 of In+luminescence excited in the D band are also determined.

ISSN:1042-0150    

DOI:10.1080/10420159408218853

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Energetic halogen atom desorption observed from certain alkali halide crystals under electron or photon stimulation is being investigated. The relaxed structure of the localized one-center (Frenkel-type free exciton) and two-center (self-trapped exciton: STE) electronic excitations are studied. It is shown that the triplet STE state undergoes an instability on and near the surface (the first and second layer below the surface) similar to that in the bulk and results in the expulsion of neutral Br atom in all three bromides studied. The relaxation energy which could be converted to kinetic energy of the ejecta is estimated to be about 0.5–1.0 eV. From the third layer, only in KBr and RbBr such ejection seems possible. The trajectory of the ejected Br atom is originally along the halogen row in <110> axes. However, as the ejecta approaches the (100) surface, it undergoes a deflection resulting in a trajectory outside the crystal which is found to deviate from the vertical by only about ≌20° at a distance of twice the lattice parameter. It is also observed that a localized hole on the surface and below is efficiently converted into a Vkcenter without leading to a halogen atom desorption. These results are presented within a broader context of the STE relaxation in the alkali halide crystals.

ISSN:1042-0150    

DOI:10.1080/10420159408218854

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Kinetic energy distributions of halogen atoms emitted due to electron-stimulated desorption (ESD) were measured for a (100) surface of NaCl-type alkali-halide crystals (NaCl, NaI, NaF, KCl, KBr, KI, RbCI, RbBr, RbI) under the same experimental conditions. It was found that energy spectra of halogen atoms emitted from potassium and rubidium chlorides, bromides, and iodides, kept at temperatures below 250°C, consist of two components: the thermal (Maxwellian) peak and the higher energy peak (at about 0.25 eV) whose energy is temperature-independent. From the energy spectra and complementary total yield measurements relative desorption rates for both thermal and nonthermal components were determined for all investigated samples. The results are compared with predictions of the Rabin-Klick criterion and the self-trapped exciton relaxation energy calculations by Songet al. Finally, it is shown that the hyperthermal halogen emission could be well correlated with a measure of the excess space between the neighboring halogen ions in the lattice. This correlation is consistent with the recently proposed concepts of ESD from alkali halide surfaces.

ISSN:1042-0150    

DOI:10.1080/10420159408218855

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The interaction of low energy electrons with the surface of alkaline-earth halides results in a variety of microscopical physical phenomena commonly described in terms of defect formation and diffusion, surface metallization and desorption of neutral- and charged particles. These processes are accompanied by local changes in the electronic and geometrical structure of bulk and surface and result in a variation of macroscopically measurable parameters like modulated optical reflectance and a deformation of the crystal lattice.

ISSN:1042-0150    

DOI:10.1080/10420159408218856

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Optical transmission and laser-induced desorption (LID) of neutral particles and positive ions from opticalgrade CaF2has been studied, using nanosecond laser pulses of 532 nm. Two different emission characteristics were found: one for which the breakdown of transmission coincides with the onset of ablation; and another for which the transmission breakdown occurs at about 20–30% lower fluences compared to the ablation threshold. In the latter case the transmission breakdown is accompanied by delayed (tenths of seconds) particle emission. We propose that the particle emission below the ablation threshold is defect-assisted LID, in the sense that light absorption by preexisting defects leads to local heating and thermal expansion, causing microcracks from which particles are ejected.

ISSN:1042-0150    

DOI:10.1080/10420159408218857

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Ultraviolet laser radiation (Ti: sapphire 4th harmonic) tuneable in the exciton absorption region of KI was used to produce near-surface excitation of potassium iodide samples on the stage of an atomic force microscope. Surface images with lateral resolution of ∼50 nm and vertical resolution of ∼3 nm have been recordedversusexposure to 5.9-eV light. Ultraviolet exposures in air were made both during AFM scanning and with the tip disengaged. Circular pits of a size dependent on uv intensity are formed quickly, followed by appearance of islands ascribed to potassium (oxidized), and finally formation of a rough eroded surface, which continues to develop after termination of the uv irradiation. Thallium doping slows the rate of erosion.

ISSN:1042-0150    

DOI:10.1080/10420159408218858

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Photon stimulated desorption of KI has been measured at several temperatures between 300 K and 485 K using photons in the energy region around the fundamental absorption threshold of KI. The yield spectra vs photon energy of the emission of potassium and iodine atoms are very similar to each other and show a weak structure near threshold, in correspondence of the main exciton absorption peak, followed by a smooth exponential increase, unrelated with the higher energy absorption features of KI. The time dependence of the desorption signal of potassium atoms, measured after turning either on or off the radiation beam, shows the existence of two simultaneous processes with completely different time constants: a ‘fast’ process with a time constant shorter than our time resolution of 0.5 s, and a ‘slow’ process with a time constant of several seconds. The intensity of the slow process is strong at low temperatures and in the region of the main exciton absorption peak, and becomes negligeable at high temperatures or in the region of the interband transitions.

ISSN:1042-0150    

DOI:10.1080/10420159408218859

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Desorption of positive ions in alkali halides resulting from the repulsive environment created by core-hole Auger decay has been previously found not likely due to lattice rearrangement and trapping of the ion. We revisit the problem by studying ion trajectories using classical molecular dynamics in the crystalline (rather than cluster) geometry with careful account of the Madelung energy. We find that the previous findings remain unchanged. In contrast to previous works, we also assume that the positive ion gained substantial amount of kinetic energy at the onset of simulations, crudely mimicking ion-stimulated desorption. Then the ejection of the formed positive halogen ion occurs for initial kinetic energies of the order 2 eV forNaFand 0.65 eV forLiF. Implications for viability of the Knotek-Feibelman mechanism are discussed.

ISSN:1042-0150    

DOI:10.1080/10420159408218860

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor