Accumulation of intrinsic point defects in temperature-control coatings for space equipment based on ZnO under electron irradiation

The decomposition of the induced absorption of zinc oxide powders and temperature-control coatings (TCC) for spaceflight equipment based on ZnO with organic and inorganic binders after the action of 30 keV electrons is reported for the first time. The regularities in the intensity growth of individual components as a function of the accelerated electron flux was studied. It has been established that for various color centers (CC) and different TCC power and exponential laws with one or two components are characteristic. The free electron accumulation kinetics is characterized by the presence of maximum values which differ for different TCC and agree with previously established flux values at which there is a transition from the formation of CC at preradiation defects to radiolysis of the ilmenite lattice. Tomsk Polytechnical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 79–85, April, 1998.     

Degradation of white anodic-oxide coatings in UV irradiation in vacuum

Photostimulated degradation of the reflectivity of white anodic-oxide coatings on Al alloy (Al-AOCs) that are used as thermal-regulating coatings of space vehicles is investigated by spectroscopy of diffuse reflection in the region of 0.5–6 eV. It is established that irradiation in vaccum when λ≤300 nm leads to the appearance in diffuse-reflection spectra of the absorption bands at 4.0 and ∼4.6 eV that are similar to these induced by vacuum heating for T≥350 K. A growth in the absorption bands is accompanied by gas release from the coatings. These processes have a common excitation spectrum, basic kinetic regularities, and a thermoactivated character. UV irradiation in vacuum is assumed to initiate Al-AOC destruction with the formation of color centers. This process is accompanied by the desorption of molecular products. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 6, pp. 801–806, November–December, 1997.     

阶梯型脉冲电压诱导连续能量质子谱数值仿真

 热控涂层质子辐照的地面模拟研究中采用单一能量质子替代空间能量连续分布的质子,连续能量质子谱是其等效性研究的关键。提出了采用阶梯型脉冲负偏压鞘层加速技术在一个脉冲宽度内获得连续能量质子谱的方法,并利用质点网格法对所获得质子谱的剂量-能量关系进行了数值仿真研究,分析了连续能量质子谱的剂量-能量分布特征及连续能量质子谱的形成过程。结果表明:阶梯型脉冲负偏压鞘层加速能够产生连续能量的质子谱,连续谱是每微秒区间入射到样品的质子叠加而成的,且每个区间所产生质子的能量与该区间电压值相对应,连续谱中,随着质子能量的增加,其剂量总体上呈现下降的趋势。     

A high-stability medium based on CaF<Subscript>2</Subscript>:Na crystals with colloidal color centers: I. Photothermochemical conversion of colloidal color centers in CaF<Subscript>2</Subscript>:Na crystals

Photothermochemical conversion of simple color centers (which include from one to four anionic vacancies) and highly aggregated ones in additively colored crystals of calcium fluoride doped by sodium is studied. The annealing of crystals with a low sodium content in a reducing atmosphere (additive coloration) leads to the predominant formation of simple color centers, which convert into highly aggregated centers under the joint action of heating and irradiation in absorption bands of simple centers. The irradiation of highly aggregated centers into their absorption bands and simultaneous heating causes these centers to convert into simple centers. The additive coloration of crystals with a relatively high sodium content leads to the predominant formation of highly aggregated centers. The heating of these crystals along with the irradiation in absorption bands of highly aggregated centers causes these centers to convert into simple centers. The formation of different color centers in the course of additive coloration of crystals with different impurity content and different results of photothermochemical conversion of centers in these crystals are connected with the dual action of impurities. Anion vacancies, which compensate the charge of the impurity alkali metal, facilitate the aggregation of color centers. At the same time, the alkali impurity stabilizes simple color centers.     

Luminescent patterns based on color centers generated in lithium fluoride by extreme ultraviolet radiation and soft X-rays

Primary and aggregate color centers in lithium fluoride (LiF) crystals and polycrystalline LiF films were produced by an innovative irradiation technique using extreme ultraviolet radiation and soft X-rays generated by a laser-plasma source. This irradiation facility allowed the efficient formation of active color centers on luminescent patterns with submicron spatial resolution on large areas and short exposure times. The method looks promising for the realization of low-dimensionality photonic devices. The optical characterization of the colored structures was performed by means of absorption and photoluminescence measurements on LiF samples colored under different irradiation conditions.     

Laws governing the creation of electronic color centers in LiF crystals acted on by pulsed irradiation

The processes of creating and transforming electronic color centers in an LiF crystal irradiated with a nanosecond electron pulse are investigated using pulse spectrometry with nanosecond resolution for times in the range 10⁻⁸ to 10⁵ sec. It is shown that the thermally activated mechanism of forming Frenkel pairs in the 12–200 K range consists of successively creating exciton states, as the temperature rises, having different degrees of spatial separation of the electron and hole components. It is concluded that the structure of self-trapped excitons evolves as a function of temperature and time, and that this evolution commences for any alkali halide crystal with the creation of self-trapped excitons ofD _2h point symmetry at 4 K. It is established that the interaction of electronic excitations with electronic color centers changes the properties of both the electronic excitations themselves and the color centers. In a crystal containing neutral electronic centers there is a fall in the yield of self-trapped excitons and Frenkel pairs and an increase in the contribution of the radiative channel for loss of the irradiation energy by the color centers. A mechanism is proposed for exciting luminescence of electronic color centers. It is established that short-lived irradiation-induced states exist, in particular a change in the spin state or in just the energy state of a center in the irradiation field, and that the appearance of these states changes the efficiency and directivity of the charge evolution of the electronic color centers. State Architectural Building Academy, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 57–75, November, 1996.     

Photoluminescence properties of SHI induced F2 and F3 color centers in nano-granular LiF thin films

Photoluminescence (PL) properties of swift heavy ions-induced F₂ and F₃⁺ color centers in nano-granular lithium fluoride (LiF) thin film were studied. LiF films were deposited on glass and silica substrates and irradiated with various ion species (Ag, Ni and Au) at different irradiation temperatures. The role of ion species, their fluence and the irradiation temperature on the PL intensity of color centers induced in LiF thin films is discussed.     

Elastic atomic displacements and color center creation in LiF crystals irradiated with 3-, 9- and 12-MeV Au ions

Creation of color centers in LiF under irradiation with 3–12-MeV Au ions was studied. Comparison of experimental data of color center creation with computer simulation of the energy deposition and elastic atomic displacements reveals the role of elastic collisions in defect creation by these ions, which have comparable magnitudes of electronic and elastic stopping. The experimentally measured efficiency of color center creation and that predicted by the simulation of elastic displacements have a similar dependence on the projectile energy. Thus, the color center creation is mainly associated with the elastic collisions, despite the relatively large values of the electronic stopping power for these ions.     

Thin-film aggregate color centers as media for frequency domain optical storage

Thin films of aggregate color centers have been produced by electron and ion irradiation of bulk alkali halide crystals. The transverse spatial distribution of the centers was controlled on a submicron scale using electron lithography. Photochemical hole burning has been accomplished for the first time in a thin film of color centers, using the 6070 Å zero-phononN ₁ line produced by ion and electron irradiation of NaF crystals.     

Accumulation of intrinsic point defects in powdered zinc oxide and reflective coatings based on it under the action of electromagnetic radiation imitating the solar spectrum

Diffuse reflection spectra of powdered ZnO and coatings based on it irradiated with photons whose spectrum is close to the solar one have been decomposed into individual components. The influence of the binder type has been demonstrated on the pattern of the formation of color centers in coatings irradiated by a DKSR-3000 M lamp that imitates the solar spectrum. It has been found that the band intensity growth for electron and hole color centers and the accumulation of free electrons are described by power-law dependences. Tomsk Polytechnic University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 70–75, May, 1999.     

Optical properties of UV-induced color centers in a KY3F10:Ce3+ crystal

The evolution of color centers induced in a KY₃F₁₀:Ce³⁺ crystal by UV radiation has been observed and interpreted. It has been revealed that, initially, the UV irradiation of the KY₃F₁₀:Ce³⁺ crystal induces the formation of color centers predominantly of the F-type, which, in a short time period of about ten minutes, are transformed into complex color centers of the F ₂-type, as well as into impurity color centers. Based on the data obtained, a diagram of energy states of the crystal, dopant, and color centers has been constructed, on which most probable processes that are caused by electronic transitions occurring in the KYF:Ce³⁺ crystal after its UV irradiation have been indicated.     

Thermal decay of photochromic color centers in CaF2, SrF2, and BaF2 crystals doped by La and Y impurities

The absorption spectra of photochromic centers in CaF₂, SrF₂, and BaF₂ crystals doped by La and Y impurities and thermal decay of the centers in the temperature range 80–600 K are investigated. Under low-temperature x-ray irradiation, ionized photochromic color (PC⁺) centers are generated in La- and Y-doped CaF₂ crystals and in a La-doped SrF₂ crystal. It is revealed that, upon heating of the CaF₂-LaF₃ crystal, PC⁺ centers are transformed into photochromic color (PC) centers. In the SrF₂-YF₃ crystal irradiated at room temperature, photochromic color centers are generated as well. All color centers decay at a temperature of approximately 600 K. After irradiation of the BaF₂-YF₃ crystal at a temperature of 80 K, absorption bands are observed at energies of 2.25 and 3.60 eV, which are related to neither PC centers nor PC⁺ centers.     

Optimization of the irradiation power in chemical exchange dependent saturation transfer experiments

In chemical exchange dependent saturation transfer imaging experiments, exchangeable solute protons are saturated and the transfer of saturation to water is subsequently detected. When the applied irradiation power is comparable to the resonance frequency difference between the water protons and saturated solute protons, the proton transfer (PT) efficiency is reduced due to concomitant direct saturation effects. In this study, the PT process is modeled using a two-pool system. An empirical general proton transfer ratio (PTR) equation for arbitrary RF irradiation power is derived, and its optimal power to maximize the PTR is analyzed. The results are confirmed experimentally on 4.7 T using a poly-L-lysine solution. The theory provides a useful tool for optimizing the irradiation power of the PT sequences in the presence of direct saturation effects.     

Recording holograms on color centers in CaF2:Na crystals by light with different wavelengths

The absorption spectra and holographic characteristics of a CaF₂:Na crystal sample are measured at different wavelengths within the absorption band of colloidal centers, along with the same characteristics of the samples with holograms after their irradiation by incoherent UV light. Results of the study confirm the diffusion-and-drift mechanism of recording holograms on color centers in ionic crystals.     

Degradation of the reflectivity of white anodic-oxide coatings in thermal vacuum treatment

Using the spectroscopic method of diffuse reflection in the region between 0.5 and 6 eV, we investigate the degradation of reflectivity in thermal vacuum treatment of white anodic-oxide coatings on an Al alloy (Al–AOC) that are used as thermoregulating coatings of space vehicles. It is established that at T≥350 K absorption bands at 4.05 and ∼4.7 eV resembling those induced by UV irradiation in vacuum appear in the diffuse reflection spectra of Al–AOC. The kinetics of the increase of absorption in heating Al–AOC with a constant rate correlates with the rate of gas liberation from specimens. We assume that a thermally activated reaction with the formation of color centers identical to those produced by the photoeffect occur in Al–AOC in vacuum at T≥350 K. This process is accompanied by desorption of molecular products. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 5, pp. 664–667, September–October, 1997.     

Deep UV laser induced luminescence in oxide thin films

Time-resolved luminescence experiments have been set up in order to study the interaction of 193-nm laser radiation with dielectric thin films. At room temperature, Al₂O₃ coatings show photoluminescence upon ArF excimer laser irradiation, with significant intensity contributions besides the known substrate emission. Time- and energy-resolved measurements indicate the presence of oxygen-defect centers in Al₂O₃ coatings, which suggests a strong single-photon interaction at 193 nm by F⁺ and F center absorption. Measurements on highly reflective thin-film stacks, consisting of quarter-wave Al₂O₃ and SiO₂ layers, indicate similar UV excitations, mainly from color centers of Al₂O₃. Received: 20 February 2002 / Accepted: 11 April 2002 / Published online: 5 July 2002     

Formation of Color Centers and Molecular Complexes with a Weak Hydrogen Bond in Lithium Fluoride Crystals Depending on the Form of OH<Superscript>¯</Superscript> Inclusion

The form of oxygen-containing inclusions in the LiF crystal was shown to be determined by the laws of isomorphic substitution and impurity accumulation in a melt during crystal growth in open air. Color centers and hydrogen-bonded complexes of different types appear in the LiF crystal after irradiation depending on the form of OH^– and Mg²⁺ inclusions. The radiative decay of OH^– ions and the properties of molecular complexes (MCs) with a weak hydrogen bond are responsible for the decreased efficiency of formation of the F- and F₂-color centers and increased concentration of the positively charged F ₂ ⁺ - and F ₃ ⁺ -centers. Radiochemical reactions involving hydrogen and fluorine atoms and ions that compete with the formation of color centers were presented.     

Refractive Index Change and Color Center Formation in LiYF4 Crystal Induced by a Femtosecond Laser

The refractive index change and color centers formation in LiYF4 crystal at room temperature are induced by a femtosecond laser irradiation. A mechanism for refractive index change and color centers formation is proposed.     

Optical Degradation of a Fluoroplastic Coating Placed in a Geostationary Orbit

The results of ground-based tests of coatings under separate irradiation by electrons and protons with energies of several tens of kiloelectronvolts and under simultaneous irradiation by electromagnetic radiation with spectrum close to the solar one and by electrons, which imitate field conditions, are presented. Changes in the total solar radiation absorption coefficient have been predicted for a 30-year period of operation of the reflecting heat-regulating coating prepared from F4MB-grade fluoroplastic with deposited aluminum and placed in a geostationary orbit. The prediction is made under a special program which allows an optimal solution to be chosen from a set.     

Ionoluminescence and formation of color centers in α-Al2O3 single crystals under proton irradiation

This paper describes results of experimental studies on radiation defects in nominally pure single crystals of corundum in two initial states: α-Al₂O₃ with an unperturbed lattice and α-Al₂O₃:C with a high concentration of anion vacancies. Defects were identified from optical absorption spectra, ionoluminescence, pulsed cathodoluminescence and photoluminescence spectra. It is shown that mostly color centers of the F- and F⁺-types are formed in the α-Al₂O₃ lattice under irradiation with 5,7 MeV protons.