Delay times in the sputtering of atoms from alkali-halide crystals during low-energy electron bombardment

Delay times in the sputtering of atoms from RbI, RbBr and NaCl by 540 eV electrons were investigated. This was done by simultaneously using the correlation technique and a slotted disc velocity selector. We observed delays in thermally ejected alkali and halogen atoms. These times were found to be independent of target temperature and are ascribed to lifetimes of excitons. Non-thermal halogen atoms are always sputtered without any delay.     

Vibrational spectra of chalkogen hydride molecules in alkali halides

A report is given on the u.v. and visible flourescence of cesium halides and RbBr doped with chalkogen hydrides, where a molecular vibrational structure of the spectra is resolved. Preliminary data for the parameters of an anharmonic oscillator model are given.     

Secondary electron spectra of gold under bombardment by very low-energy positrons

Measurements of the secondary electron energy spectra resulting from very low-energy positron bombardment of a polycrystalline Au surface are presented. The low-energy part of the secondary spectra contain significant contributions from two processes: (1) annihilation-induced Auger electrons that have lost energy before leaving the surface and (2) secondary electrons resulting from direct energy exchange with an incident positron. Our data indicate that the second process (direct energy exchange with the primary positron) is still important at and below 3 eV incident beam energy. Since energy conservation precludes secondary electron generation below an incident beam energy equal to the difference between the electron and positron work functions (∼3 eV), the fact that we still observe significant secondary electron emission at energies at or below this value provides strong evidence that the incident positrons are falling directly into the surface state and transferring all of the energy difference to an outgoing secondary electron. These measurements were also used to obtain the first experimentally determined upper limit on the intensity of the spectrum of Auger-induced secondary electrons.     

Sputtering by light-ion bombardment

The theoretical sputtering yield of heavy targets by low-energy light-ion bombardment is presented. The results, obtained within the framework of a recent unified sputtering theory, are compared with the experimental data of Mo bombarded with H, D, He ions.     

Ion bombardment of alkali halides,II.: Effects observed at high fluence

Color-center effects unique to crystals irradiated to high fluence : 10¹⁶ particles/cm²) are reported for KCl and other alkali halides. These include (1) a minimum in F-center concentration at the end of the proton range and (2) an n-center distribution profile which changes with time in the dark at room temperature after irradiation. The first effect is due to annihilation of F centers by hydrogen atom capture (U-center formation) and the second is due to F-center diffusion and aggregation in a heavily damaged region of the crystal.     

Observation of negative alkali ions from alkali halides during 248-nm laser irradiation

Below laser fluences where a plasma is formed (the so-called plasma or plume formation threshold) a number of fundamental phenomena can occur where particles such as atomic and molecular ions, atoms and molecular neutrals, and electrons can be emitted. An understanding of such processes is necessary to develop predictive models for material removal from laser irradiated surfaces—at the foundation of laser etching, machining, and pulsed laser deposition. We have reported on a number of the mechanisms for such emission processes. Here, due to space limitations, we present a summary of our studies on the formation of negative alkali ions from single crystal KCl during exposure to pulsed 248-nm radiation at fluences well below the threshold for plasma formation. Despite the high electron affinities of the corresponding halogen atoms, negative halogen ions were not detected. Significantly, the positive and negative alkali ion distributions overlap strongly in time and space, consistent with K formation by the sequential attachment of two electrons to K⁺. Negative alkali ions are also observed under comparable conditions from LiF, NaCl, and KBr. In each material, the strong overlap between the positive and negative alkali ion distributions, and the lack of detected negative halogen ions, suggest that negative ion formation involves a similar mechanism.     

Radiolysis of alkali halides

This paper reviews the extensive work pertaining to the creation of colour centres in alkali halides. After a brief historical survey the various mechanisms put forth to account for defect creation are examined in turn and the experimental evidence which supports or belies each of these is presented. After demonstrating from published experimental work that the defects created are of the Frenkel rather than the Schottky type and after consideration of the early stage, extrinsic coloration process, the Valey mechanism and its variants are discussed. Next, the direct displacement model proposed by Balarin is considered and, finally, the recent excitonic mechanism proposed by Pooley and Hersh is examined in detail. The conclusion reached is that, although the excitonic mechanism appears to be much the more preferable model yet proposed, a number of points need careful consideration and experimental study before it can be accepted as having general validity.     

Ground-state properties and optical excitations of a solvated electron in molten alkali halides

Summary Properties of solvated electrons at high dilution in four molten alkali halides are investigated theoretically. A self-consistent evaluation of the bound ground state of the electron and of the surrounding liquid structure, already developed in a previous paper, is compared with the results of NMR hyperfine shifts and magnetic-susceptibility measurements. The absorption bands associated with Franck-Condon 1s→2p and 1s→3p transitions are calculated and compared with the available data of optical absorption, with special emphasis on the detailed analysis by Yuh and Nachtrieb for molten CsCl. An instability of the excited states against ionic relaxation and the origin of a finite lifetime for the ground state are also discussed. Finally, the perturbation induced by the solvated electron in the fundamental absorption of the molten salt is estimated.

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Riassunto Il lavoro presenta uno studio teorico di elettroni solvatati a forte diluizione in alogenuri alcalini fusi. Un calcolo autoconsistente dello stato fondamentale è confrontato con dati NMR d'interazioni iperfini e con misure di suscettività magnetica. Le bande di assorbimento ottico dovute a transizioni 1s→2p e 1s→3p sono calcolate in uno schema di Franck-Condon e i risultati sono confrontati con l'evidenza sperimentale, con attenzione particolare all'analisi dello spettro nel CsCl fuso data da Yuh e Nachtrieb. Si discutono quindi un'instabilità degli stati eccitati sotto rilassamento ionico, la vita media dello stato fondamentale nel liquido e lo spostamento dello spettro eccitonico dovuto all'elettrone solvatato.

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Резюме Теоретически исследуются свойства сольватированных электронов при высокой степени разведения в четырех расплавленных щелочных галоидах. Самосогласованное вычисление основного состояния связанного электрона и окружающей жидкой структуры сравнивается с данными ЯМР для сверхтонких смещений и с измерениями магнитной восприимчивости. Вычисляются зоны поглощения, связанные с переходами 1s→2p и 1s→3p в схеме Франка-Кондона. Полученные результаты сравниваются с имеющимися экспериментальными данными по оптическому поглощению, причем, особое внимание уделяется анализу спектров для расплавленного CsCl. Также обсуждается неустойчивость возбужденных состояний относительно ионной релаксации и конечное время жизни основного состояния в жидкости. В заключение, оценивается смещение экситонного спектра, обусловленное сольватированным электроном.

     

Cathodo-luminescence spectra of alkali halides

Cathodo-luminescence spectra of fourteen alkali halides and of some mixed crystals of NaCl and KCl have been studied. The spectra show broad structureless bands. In some ten halides it is possible to find a strong band whose peak position shifts towards longer wavelengths as the inter-atomic distance increases. For pottasium halides the relationship λ_max=1380d, where “d” is the inter-atomic distance, holds approximately but is not generally true. A mechanism of cathodo-luminescence in alkali halides has been postulated, and an interpretation of the above band and of some NaCl bands has been made on that basis.     

Depth distribution profiles of colour centers produced in alkali halides by electron irradiation

Depth distributions of colour centers produced in 〈100〉 KCl crystals by irradiation with 1.0 and 2.0 MeV electrons were measured. A distinct anisotropy in the depth distributions of the optical colouration density as well in the F- and M-center density was observed.     

Raman spectra of alkali halides

In this article, we present a comparative study of the Raman spectra of alkali halides in relation to the lattice dynamics ofBorn andRaman. It is shown that the experimentally observed limit of the second-order spectra in almost all the cases can be explained well by the Lyddane-Sachs-Teller relation. It is also seen, while, an explanation of the second-order Raman spectrum of a crystal of diamond or zinc blende structure requires the frequencies from the critical points,W, Γ, X andL inBorn's analysis, the frequencies fromΓ, X andL alone are sufficient and necessary for an interpretation of the same onRaman's model. Some similarities in the determination of the long wave properties of crystals like elastic constants and limiting frequencies of the lattice vibrations in the symmetry directions in both the models are pointed out.     

Dislocation damping in alkali halides

In the amplitude independent region the dislocation damping is attributed to either phonon-drag (Granato-Lücke theory) or to the compensating charge-cloud surrounding electrically charged dislocations (Robinson-Birnbaum theory). The experimental results for the dependence of the damping on temperature, frequency and dislocation charge are compared with the two theories. Since it is found that in some cases it is necessary to include both forms of damping, a more complete theory is developed which includes both terms.

In the amplitude dependent region the dislocation damping was thought to be due to the dislocations breaking away from pinning points or breaking through the compensating charge-cloud. Using the piezoelectric defect results for electrically charged dislocation in KCl the force-displacement hysteresis loop for the moving dislocations is determined together with the force-displacement curves for dislocations assuming phonon and charge-cloud damping. These results are found to be inconsistent with the “break away” models for amplitude dependence but instead to be consistent with the restoring force due to an elliptical compensation charge cloud, with a size proportional to the square root of the dislocation charge.     

Exciton states in alkali halides

Recent experiments based on modulation spectroscopy have shown that it is possible to detect exciton levels in alkali halides up to n = 4. Therefore we worked out numerical calculations in order to predict the whole exciton series in KI and RbI. In our calculations the deep exciton levels are treated by considering the actual hole-electron interaction, whereas the effective mass approximation is used for the shallow exciton levels. The direct and exchange terms of the hole-electron interaction have been evaluated by performing three and four center integrals, the Wannier wave functions appearing in such integrals being approximated by suitable gaussian expansions of atomic orbitals.It is shown that by allowing the exciton state to extend up to 42 shells of neighbors it is possible to predict the exciton levels up to n = 2, the n = 3, 4 excitons being accounted for by the effective mass approximation. Similar computations performed for excitons in solid rare gases were found in excellent agreement with the experimental data and confirmed the reliability of our method.     

Muon radiolysis in alkali halides

In order to shed new light on the initial loss of muon spin polarization, or socalled missing fraction, which is commonly observed in non-metallic solids, we have studied muon-induced excitation in various alkali halides by measuring the luminescences associated with the radiative decay of the self-trapped excitons (STE). The result strongly suggests that the spin-exchange interaction between muonium and muon radiolysis products including STE's causes fast muon depolarization in those materials.