Target current spectroscopy of the alkali halides KCl, CsCl and KBr

Thin films of KCl, CsCl and KBr deposited on Si(111) were investigated by means of target current spectroscopy (TCS). From the TCS initial peak position the surface potential changes during deposition were obtained. From the surface potential dependence on the alkali halide film thickness the critical island size in the initial stage of the film growth was estimated: for CsCl, 10 Å; for KCl, 18 Å; for KBr, 25 Å.

In the target current spectra of the alkali halides an intensive structure in the energy region corresponding with the bandgap was discovered. One group of peaks was induced by electron irradiation and vanished at high temperatures. This fine structure was considered to be due to an electronic transition related to the defect states in the bandgap. From the evolution of the TCS maximum intensities the defect concentration changes caused by electron irradiation were estimated.     

Electron transfer and intershell spectroscopic parameters of Pb2+: Changes when lead halide films interact with alkali halide substrates

UV spectral data for Pb²⁺ highly diluted in alkali halide crystals and in lead halide thin films are shown to be consistent in the context of current inorganic spectroscopy. For Pb²⁺ in (i) alkali halides and (ii) lead halides the optical electronegativities are 1.2 and 1.4 respectively, while for the ¹S₀→³P₁ frequency (v_s?p) relationship (v_s?p = v_f?v_fmh where h is the nephelauxetic parameter of the complexing halide ion), the values of v_f are 60 700 cm^-1 and 64 400 cm^-1 and the values of m are 0.20 and 0.22 respectively. The results indicate that the modified Pb²⁺ ion model used for Pb²⁺ ions in alkali halide crystals can be extended to Pb²⁺ halide salts. These rationalizations indicate the usefulness of UV spectroscopic measurements for studying the penetration of lead halide thin films into crystalline alkali halides. Spectral changes occuring when sandwiches of lead halide films between NaCl and KBr substrates are maintained at 300°C indicate substantial evaporation of the lead halide film and preference of Pb²⁺ to diffuse into KBr rather than into NaCl.     

The infrared absorption coefficient of alkali halides

Available data on the absorption coefficient of six alkali halides, LiF, NaF, NaCl, KCl, KBr, and KI, were surveyed, evaluated, and analyzed. For the multiphonon absorption region, an equation was formulated to describe the absorption coefficient as a function of both frequency and temperature. Constants in the equation were determined based on data fitting calculations and empirical correlations. The Urbach Rule is applied to the uv absorption edge of the transparent region, and our equation is considered as its counterpart in the IR absorption edge. Comparing with Deutsch's exponential equation, the present expression includes the temperature as an additional independent variable. The calculated values are in concordance with the experimental data.     

碘化铯薄膜光阴极的研究进展

碘化铯(CsI)薄膜因对X射线及紫外光具有高的光电转换效率而倍受关注。在核物理、高能物理以及天体物理研究的推动下, 研制高量子效率(QE)、性能稳定的CsI薄膜光阴极成为了近年来研究的热点。然而, 目前人们对某些影响其性能的因素还不完全清楚或确定。本文综述了CsI薄膜光阴极的最新研究进展, 总结了影响QE的因素和光阴极的老化机理, 重点关注了一些存在争议的问题, 并对其研究发展方向进行了探讨。     

Quantum efficiency measurement of CsI photocathodes using synchrotron radiation at BSRF

A quantum efficiency(QE) measurement system has been established for CsI photocathodes in the wavelength range of 120-210 nm by using the synchrotron radiation light source at Beijing Synchrotron Radiation Laboratory (BSRF). An AXUV100G photodiode calibrated by Physikalisch-Technische Bundesanstalt (PTB) was used as the transfer detector standard to ensure the accuracy and reliability of the QE measurement. The dependencies of QE measurement on beam energy, vacuum pressure and bias voltage were studied in detail. The influence of photoionization in gas on the QE measurement was observed and is described. The surface morphological characteristics of both substrate and CsI film were analyzed by atomic force microscopy (AFM). The QE results of differently prepared CsI photocathodes were compared, including: the printed circuit board (PCB) of FR-4 (Woven glass and epoxy)+Cu, FR-4+Cu/Ni/Au, and stainless steel substrates; a series of thickness from 60 to 600 nm; and the resistive and electron beam evaporation techniques.     

UV photopolymer materials and technique for making laser glass

A technique and the materials required for making laser glass are described. The technique includes cleaning glass plates, coating UV photopolymer materials onto the glass plates, attaching a master hologram to the plates, using exposure to UV light to cure the coating, screen printing, vacuum film depositing, and covering the plates with a protective coating. The UV photopolymer material is composed of a photoinitiator, a monomer, an oligomer, and other additives. The experimental results show that the laser-glass products have a high resistance to alkali, acid, heating, and UV irradiation.     

Chemical method to increase extreme ultraviolet microchannel-plate quantum efficiency

The quantum detection efficiency of photon-counting microchannel-plate detector channels can be increased by a factor of 2 in the extreme ultraviolet bandpass, from 256 A to 1024 A, by subjecting the input channel plate to a chemical solution treatment. The efficiency increase has proven to be insensitive to the exposure of laboratory atmosphere and is stable over the extracted charge lifetime equivalent of multiyear astrophysical observations. Chemically treated microchannel plates that are overcoated with opaque photocathodes of KBr and CsI show a factor of 2 quantum detection efficiency increase from 834 A to 1100 A in comparison with previous measurements using conventional untreated microchannel plates.     

Experimental determination of the thermal diffusivity of molten alkali halides by the forced Rayleigh scattering method. III. molten NaI,KI, RbI,and CsI

This paper is an addendum to two previous papers which contained data on the thermal diffusivity of molten alkali metal chlorides and bromides. The present salts are alkali metal iodides: NaI, KI, RbI, and CsI. The measurements were performed utilizing the forced Rayleigh scattering method at temperatures up to 1234 K. The accuracy of the reported data is estimated to be ±5 to ±10%. It is again found that our data show one of the smallest values and weakly negative temperature dependencies.     

The characterisation of soft X-ray transmission photocathodes

A simple model of X-ray photoemission from transmission photocathodes is described. Expressions are derived for the pulse (counts per photon) and current (electrons per photon) forms of the photoelectric yield and for the distribution in number of the emitted secondary electrons. The predictions of the model are compared with published measurements on bulk density alkali halides, principally CsI. The use of transmission photocathodes in conjunction with microchannel plate electron multipliers is discussed.     

Europium emission centers in CsI:Eu crystal

Absorption, excitation and luminescence spectra of pure and Eu doped CsI crystals were studied depending on the activator content, excitation energy, heat treatment and X-ray irradiation. Several types of Eu²⁺ related centers were found. It is shown that complex centers are stable at room temperature but their structure and concentration changes at heat treatment and under irradiation. The increased content of oxygen-containing radicals was determined by IR spectroscopy in Eu-containing crystals as opposed to pure one. It is supposed that some of emission centers in CsI:Eu are caused by the presence of intrinsic (vacancy type) and extrinsic (oxygen or hydroxyl ions) defects located in the nearest environment of Eu²⁺ ions.     

Gamma-ray irradiation induced structural and optical constants changes of thermally evaporated neutral red thin films

Neutral red (NR) is polycrystalline in powder form, it transforms to nanocrystallite phase upon thermal deposition. Gamma-ray irradiation with doses 1.25–6 KGy induced partial transformation of nanocrystallite phase to amorphous structure. The changes of optical constants with γ-ray doses were calculated using spectrophotometer measurements of transmittance and reflectance at normal incidence of light over spectral range 200–2500 nm. The complex refractive index of NR film is highly influenced by exposure to γ-ray irradiation, the onset and optical energy gaps decrease with increasing γ-ray doses, and Urbach tail increases linearly with increasing irradiation dose. The type of electronic transition, oscillator, and electric dipole strengths and dispersion parameters were determined before and after irradiation. The spectral behavior of dielectric constant with γ-ray doses was also estimated.     

Refractive-Index Changes of Standard Telecommunication Fiber through Exposure to Femtosecond Laser Pulses at 810 cm

We report on permanent refractive-index changes as great as 6 x 10(-3) in standard fibers on irradiation with tightly focused femtosecond (120-fs) near-IR (lambda = 800-nm) pulses. The refractive-index increase was measured through changes in the transmission spectrum of an UV photowritten Bragg-grating-based Fabry-Perot interferometer. We depict the dependence of the index variation on exposure time. The changes in refractive index can be optimized after a few seconds of irradiation. The measured induced excess loss increases during near-IR exposure.     

Environmental photostability of SF(6)-etched silicon nanocrystals

We report on the long-term environmental stability of the photoluminescent (PL) properties of silicon nanocrystals (SiNCs). We prepared sulfur hexafluoride (SF(6)) etched SiNCs in a two-stage plasma reactor and investigated their PL stability against UV irradiation in air. Unlike SiNCs with hydrogen-passivated surfaces, the SF(6)-etched SiNCs exhibit no photobleaching upon extended UV irradiation despite surface oxidation. Furthermore, the PL quantum yield also remains stable upon heating the SF(6)-etched SiNCs up to 160?°C. The observed thermal and UV stability of SF(6)-etched SiNCs combined with their PL quantum yields of up to ～50% make them attractive candidates for UV downshifting to enhance the efficiency of solar cells. Electron paramagnetic spin resonance indicates that the SF(6)-etched SiNCs have a lowered density of defect states, both as-formed and after room temperature oxidation in air.     

B3LYP investigation of response properties of alkali halides on external static electric fields

Structural, dielectric, and optical properties of six alkali halides were investigated by all-electron B3LYP hybrid density functional calculations. A response of their electronic and structural properties to the external static electric fields was also investigated. While the applied electric field only marginally affects the lattice parameter, the internal relaxation of ions is somewhat more pronounced with the cations displaced along with the field and anions opposite to the field. We further find that in the presence of electric field the valence and conduction bands widens resulting in the reduced band-gap. An explanation for this effect is provided, which is related to a slight delocalization of states in the presence of external electric field. However, this effect is far too small to account for the dielectric breakdown of alkali halides. For this reason, the dielectric breakdown was considered on the basis of von Hippel’s low-energy criterion and Callen’s equation, for which the calculated dielectric constants and optic transverse mode frequency were used. The obtained dielectric strengths are in fair agreement with experiment.     

Thermoluminescence properties of KCl(1-X)KBrX:Pb2+ mixed crystals

A study is presented of the thermoluminescence (TL) of phosphors based on potassium halides doped with divalent lead, such as KCl:Pb2+, KBr:Pb2+ and the crystalline series KCl(1-X)Br(X):Pb2+. The defects in the crystals generated by irradiation have been investigated as well as the trapping and room temperature recombination mechanisms. The samples were gamma irradiated to a dose of 10 kGy. The TL spectral response during the recombination stage was obtained through simultaneous measurements of temperature, emitted light intensity and emission wavelength by using an automated thermoluminescence system with optical fibre couplings and a diode array as a detection device. The TL emission is a broad band in the 350-600 nm range with a maximum strongly dependent upon the mixed composition.     

Studies on the setting of polyelectrolyte cements: Part VI The effect of halide salts on the mechanical properties and water balance of zinc polycarboxylate and glass–ionomer dental cements

A study is reported in which a zinc polycarboxylate and a glass polyalkenoate dental cement, respectively, were prepared from aqueous solutions of NaCl, KCl, KBr and KI, all at 1 mol dm3 concentration, as well as from pure water. For the zinc polycarboxylate, setting as determined by oscillating rheometry was speeded up and water uptake was enhanced by the presence of the salts. Conversely, compressive strength at 24 h was unaffected. On the other hand, for the glass polyalkenoate, the setting reaction was slowed down, water uptake inhibited and compressive strength at 24 h reduced (from 94.3 MPa with pure water to 59.8 MPa with NaCl, 65.8 MPa for KCl, 67.0 MPa for KBr and 81.1 MPa for KI). Previous work with polyelectrolytes in aqueous solution suggests that the halides probably enhance the rate of the neutralization process. For the zinc polycarboxylate, this leads to a more rapid setting reaction. By contrast, for the glass polyalkenoate, it results in slower setting and weaker cements. This result is attributed to inhibition of the secondary setting reaction, involving the formation of the silicate/phosphate network, by enhanced neutralization, a process which is consequently concluded to occur earlier in the overall setting of these cements than had been assumed previously. © 1998 Chapman & Hall     

Influence of thermal annealing and ultraviolet light irradiation on LaF3 thin films at 193 nm

Lanthanum fluoride (LaF3) thin films were prepared by resistive heating evaporation and electron-beam gun evaporation under the same deposition rate, deposition substrate temperature, and vacuum pressure. The coated LaF3 films were then treated by heat annealing and UV light irradiation. The optical properties, microstructures, stress, and laser-induced damage threshold (LIDT) at a wavelength of 193 nm were investigated. The surface roughness, optical loss, stress, and LIDT of the films were improved after the annealing. The films had better properties when irradiated by UV light as compared with heat annealing.     

Oxidative properties and chemical stability of fluoronanotubes in matrixes of binary inorganic compounds

The chemical stability of fluoronanotubes in selected solid inorganic matrixes has been studied by initially mixing and mechanically grinding the components and subsequently heating them at temperatures ranging from 35 to 600 degrees C. The inorganic compounds selected for matrixes included halides (KBr, KI, Lil, LiBr, LiCl, NaCl, Znl2), oxides (Li2O, Fe2O3, PbO, MnO), lithium peroxide (Li2O2), potassium superoxide (KO2), sulfides (Li2S and ZnS), zinc selenide (ZnSe), lithium nitride (Li3N), and aluminum phosphide (AIP). Solid products, resulting from the proceeding chemical reactions, were analyzed by X-ray diffraction, Raman spectroscopy, and SEM/EDX elemental analysis. Gaseous and volatile products were identified with the help of the TGA/MS technique. Experimental data presented in this paper provide clear evidence that fluoronanotubes are not chemically inert toward the solid matrixes studied and exhibit significant oxidative properties in the redox reactions occurring under various temperatures, depending on the nature of the inorganic compound.     

Coloration caused by mechanical stress on rare earth oxysulfides

Oxysulfide powders of yttrium, gadolinium, terbium and lanthanum display distinctive colors when they are either heavily ball-milled or pressed in a steel die. These colors are due to optical absorption bands in the visible region, peak positions of which obey an empirical rule like the Mollowo-Ivey relation of color centers in alkali halides. The c-axis shrinks in the colored samples. The colors are bleached and the cell constants are restored to the original values by heat treatment up to 450°C. Some distortions, however, still remain in the annealed samples.     

On the surface morphology of thin alkali halide photocathode films

Thin alkali halide films are currently used as transmissive UV-photocathodes and as protecting layers for visible photocathodes. The surface morphology of 20 and 75 nm thick evaporated CsI, NaI and CsBr films was investigated by means of a scanning electron microscope, to which the samples were transferred, under vacuum, with practically no contact with air. It is shown that the film continuity, in particular that of NaI, is strongly affected by short exposure to moisture. CsI, which is the less hygroscopic material among the three, exhibits the most continuous structure.