Calcium Fluoride Crystals Research Articles

Effect of surface quality on transmission performance for (111) CaF 2

With the advent of 193 nm optical lithography, calcium fluoride (CaF 2) has become the material of choice for excimer laser components and for chromatic aberration correction of stepper lens systems. Paralleling the need to maximize transmission performance is the challenge of processing CaF 2 to surface qualities equal to if not superior than for conventional fused silica optics. In this report, (111) calcium fluoride crystals were processed to two different surface qualities and tested for transmittance from 185 to 400 nm. Results show that surface quality in terms of mid- and high-spatial frequency roughness and subsurface damage strongly affect transmittance with a greater increase in transmittance with improved surface quality at lower wavelengths. Specifically for 193 nm, transmittance increased from 90.30% for a 1.4 nm RMS surface to 91.98% for a 0.8 nm RMS surface ( Δ=1.68%).

Far-infrared properties of resonant modes and tunnelling statesin rare-earth-doped calcium fluoride

The low-temperature observation of a broad distribution of two-level systemsin chemically disordered fluorite mixed crystals has heightened interest inthe spectroscopic properties of the corresponding lightly doped crystals. Thelow-temperature far-ir absorption of calcium fluoride crystals doped withY3+, La3+ or Lu3+ in the range less than 1 mol% shows threecompletely different types of defect-induced spectra and the temperature andconcentration dependence are found to vary widely. In addition, the far-irproperties of CaF2 doped with Y3+ or Lu3+ can be dramaticallyaltered by heat treatment, indicating that clustering plays an important role.By comparing the spectroscopic signatures with those previously observed forother defect systems, the lowest-lying excitations for these doped CaF2crystals have been assigned as follows: tunnelling transitions in twodifferent elastic configurations for Y3+, a variety of ground statetunnelling transitions for the La3+ doping and an anharmonic resonantmode for the Lu3+ doped system. The surprising result is that a broaddistribution of two-level systems is clearly evident in two of the threesystems and the optical activity of this distribution appears to be controlledby the optical activity of the dominant defect dynamics.

Manifestation of quantum size effects in optics of ultrathin niobium films

The optical properties of ultrathin (∼nm) niobium films enriched with oxygen are studied by the methods of phase spectroscopy of surface electromagnetic waves (in the far-IR spectral region), IR absorption (in the mid-IR spectral region), and ellipsometry (in the visible spectral region). The films were deposited onto crystal calcium fluoride substrates by the method of high-frequency cathode sputtering. The complex permittivity of the films is calculated from the experimental data. The dependences of both the imaginary and real parts of the permittivity on the film thickness exhibit oscillations with a period of 9 A, the real part sign being changed during oscillations.

Electron Beam-Induced Chemical Reactions Of Single Crystal Calcium Floride By Time - Resolved EELS

Abstract Using parallel beam illumination, electron beam-induced structural transformation has been studied as radiation damages for decades. Recently, however, more attention may need to be paid to beam-induced chemical transformation which happens under illumination by a finely focused beam. Such a small probe is often used in analytical electron microscopy, particularly the core-electron energy loss spectroscopy (EELS). Since the signal for the spectroscopy is very weak long radiation is required and makes the induced chemical reactions happen. Therefore, without attentions to the reactions, the acquired data could lead to misinterpretation of specimen properties. In the present study, using a single crystal calcium fluoride (CaF2), we demonstrated by time-resolved EELS that radiation with a nanometer electron probe could induce chemical reactions between the specimen and the typical residual gases in a microscope column. The thin films of CaF2 were fabricated by vaporizing flakes of single crystal CaF2 with an electron beam and by dissolving the vapors onto a (001) surface of sodium chloride (NaCl) heated at 300°C.

Epitaxial growth of lithium fluoride on the (1 1 1) surface of CaF 2

Growth of lithium fluoride by molecular beam epitaxy on the (1 1 1) surface of calcium fluoride crystals was studied by TEM and LEED for crystal temperatures from 400 to 773 K and impinging lithium fluoride fluxes from 3×10 11 to 3×10 14 cm −2 s −1. Growth starts, usually, at the 〈1 1 0〉 steps on the (1 1 1) surface of CaF 2. For larger step distances and at later growth stages also growth on the terraces between the steps is found. Preferably, longish, roof-like crystallites are formed, which can be interpreted by growth of LiF(2 0 1 ̄ )[0 1 0] parallel to CaF 2(1 1 1)[ 1 ̄ 0 1]. To a lesser extent square crystallites, i.e. growth with LiF(0 0 1), and, rarely, three-folded pyramidal crystallites, i.e. growth with LiF(1 1 1) parallel to CaF 2(1 1 1), are observed. While the pyramidal crystallites show strict epitaxial orientation with LiF[ 1 ̄ 0 1]‖CaF 2[ 1 ̄ 0 1] and LiF[ 1 ̄ 0 1]‖CaF 2[1 2 ̄ 1], only about 80% of the square crystallites exhibit an epitaxial alignment, where LiF[1 0 0]‖CaF 2[ 1 ̄ 0 1] is preferred to LiF[1 1 0]‖CaF 2[ 1 ̄ 0 1]. The epitaxial relationships are discussed on the basis of theoretically calculated adsorption positions of the lithium fluoride monomer and dimer on the terrace and at the steps of the CaF 2(1 1 1) surface.

Creating primary defects in calcium fluoride crystals with various prehistories, using pulsed electron irradiation

Optical spectroscopy with nanosecond resolution has been used to study how the prehistory (the presence of impurities, heat treatment) of CaF2 crystals affects the processes of creating autolocalized excitons (AEs) under the influence of a pulse of accelerated electrons. The breakdown of the dispersivity condition for the creation of AEs is detected in a nominally pure, nonheat-treated crystal. It is shown that the initial structural defects play a defining role in creating the initial defect density. A model of the radiation disordering of an actual fluorite crystal is constructed. An analogy is traced to the processes of impurity and thermal disordering.

In situ study of ion-beam induced lattice damage in calcium fluoride crystals

Lattice damage in calcium fluoride induced by 360 keV xenon ion irradiation was measured in situ using Rutherford backscattering and channeling (RBS/C) techniques. Calcium fluoride single crystals of (100) orientation were irradiated at − 115 and 35°C by Xe ion beams along a random direction 15° off the (100) axis. A 2 MeV He ion-beam from a 3 MV tandem accelerator was incident on the CaF 2 crystal along the (110) axis to determine the radiation damage. No amorphized layer was observed at both irradiation temperatures up to an irradiation dose of 2 × 10 16 Xe/cm 2. RBS/C measurements showed that lattice disorder in the irradiated layer saturated at 70% of the random level. Analysis of cross-sectional transmission electron microscopy (XTEM) revealed a damaged but single-crystalline CaF 2 layer. This study demonstrates the radiation tolerance of CaF 2 under displacive irradiation at low to ambient temperatures.

Inorganic Photochromic Systems

The excited states of many organic molecules are sufficiently long lived so that excitation gives rise to a change in the absorption spectrum that persists for seconds or even longer. The author is unaware of any perfect crystal for which this is true. Vacancies are essential to the appearance of photchromism in alkali halides and sodalite crystals. Changes in absorption spectrum have accompanied charge transfer reactions between two impurities in titanate and calcium fluoride crystals. Photochromism has been observed in suspensions in glass of crystals too small to scatter light as well as in homogeneous glasses.

Ho:CaF_2 solid-state saturable-absorber Q switch for the 2-μm Tm,Cr:Y_3Al_5O_12 laser

The holmium-doped calcium fluoride (Ho:CaF(2)) crystal was shown to be an effective solid-state saturable-absorber Q-switch for a flash-lamp-pumped Tm,Cr:Y(3)Al(5)O(12) laser at 2.017 µm. With a 1-cm-thick Ho(0.5%),Er(5%):CaP(2) saturable absorber and a 6.3% output coupler, a single Q-switched laser pulse of 51 mJ in energy and 60 ns in duration was obtained at a flash-lamp input energy of 85 J. With a 14.6% output coupler, a typical Q-switched laser pulse of 84 mJ and 82 ns was observed.

Resonant photoacoustic measurements of optical absorption in solids

For glasses and crystals the optical absorption near the infrared is much lower than that in the visible. While the previous noncontact resonant photoacoustic (PA) measurement is several orders of magnitude more sensitive than that of conventional PA in the visible for a piezoelectric sample, it was further developed to apply to nonpiezoelectric materials in the near infrared. The noncontact interdigital capacitor transducers were replaced by lithium niobate disks and PVDF films. By supporting the sample between transducers only at its corners, the quality factor has been improved and is comparable to that of a noncontact system, while the separate supports and the precise positioning of the sample are avoided. An optical absorption coefficient on the order of 10−7 cm−1 at 1.06 μm has been measured with a 20 signal-to-noise ratio. Recent results on single crystals of quartz and calcium fluoride, using different piezoelectric transducers, will be discussed. [Work supported by NSF Grant No. DMR-9000549 and by the Office of Naval Research.]

Formation of phosphate-containing calcium fluoride at the expense of enamel, hydroxyapatite and fluorapatite.

During the caries process complex reactions involving calcium, phosphate, hydrogen and fluoride ions as main species take place. In this study the precipitation and dissolution reactions occurring in suspensions of enamel, hydroxyapatite (HAP) and fluorapatite (FAP) on addition of fluoride were investigated under well-defined conditions. pH and pF were monitored; calcium and phosphate concentrations were measured at selected times; the solid phases were examined by infra-red, X-ray diffraction and transmission electron microscopy. Precipitation of phosphate-containing calcium fluoride crystals, CaF2(P), can cause severe reduction in the calcium ion concentration and release of hydrogen ions from the precipitated phosphate. These reactions result in considerable dissolution of enamel, HAP and even of FAP. More of the added mineral dissolves with 50 mmol/l fluoride than with 10 mmol/l fluoride, mainly due to the greater reduction in calcium ion concentration. This work shows that phosphate-containing calcium fluoride is most likely an important compound to be considered in the caries process.

Dark matter search with calcium fluoride crystals

A first result on dark matter direct search with calcium fluoride scintillators is presented. The low and high energy spectra are discussed together with the measurements of the fluorine and calcium recoil scintillation efficiencies. Exclusion plots for axial vector coupled WIMPs are derived and compared with previous measurements with NaI and Ge detectors.

The kinetics of dissolution of calcium fluoride in the presence of some metal ions

The kinetics of dissolution of calcium fluoride crystals has been studied in sodium chloride solutions at 25°C in the presence of zinc, magnesium, lead and aluminium ions, using a constant-composition method, in which the undersaturation and ionic strength were maintained constant. With the exception of zinc, the addition of metal ions, even at relatively low concentration (5X10 -5 mol dm -3), markedly retards the rates of dissolution of calcium fluoride crystals. Moreover, the effect was enhanced as the relative undersaturation decreased. The subsequent blocking of dissolution sites by the adsorbed ions can be interpreted in terms of a Langmuir isotherm. Direct adsorption experiments have also been made by using magnesium ions. Comparison of the kinetically derived equilibrium constants with the directly measured adsorption equilibrium constant showed increasing kinetic equilibrium constants with decreasing relative undersaturation.

STATUS OF STUDY OF COLOUR CENTERS IN CALCIUM FLUORIDE

A review is given on studies of different types of colour centres in calcium fluoride crystals. Production, analysis and application of some colour centres like F and F aggregate centres, M A centre, oxygen centre, colloidal centre, and photochromic centre are discussed. The results of ESR, ENDOR, Magnetic Circular Dichroism (MCD). Raman spectroscopy, positron annihilation spectroscopy are reviewed. Irradiation effects on the colourability of pure and doped calcium fluoride crystals are also discussed.

Resonant photoacoustic measurements of very low optical absorption in piezoelectric and dielectric crystals

The photoacoustic effect is one of the most sensitive methods for measurements of the low optical absorption in glasses and crystals. Traditional methods using high‐powered pulse lasers and attached piezoelectric transducers are limited by noise and scattered light at the transducer. Recently, a new resonant photoacoustic technique has been developed for highly transparent solids, where a cw laser modulated at the acoustic resonant frequency of the sample generates an acoustic signal amplified by the quality factor (Q) of the resonance. This technique is several orders of magnitude more sensitive than conventional pulse techniques. Noncontact capacitive transducers eliminate the problem of scattered light. Two different transduction mechanisms are important in the detection of the acoustic wave at the sample surface. First, the displacement of the surface of the dielectric sample in the electric field of the capacitor results in small changes in the capacitance. Second, through the piezoelectric effect, the strain generates a small electric field at the sample surface; here, the transducer acts as an rf receiver, detecting the changes in the electric field intensity. Based on recent measurments on single crystals of calcium fluoride and quartz, the relative contributions of these two mechanisms will be discussed. [Work supported by the Office of Naval Research.]

Kinetics of dissolution of calcium fluoride crystals in sodium chloride solutions: influence of additives

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTKinetics of dissolution of calcium fluoride crystals in sodium chloride solutions: influence of additivesSalem M. Hamza and Samia K. HamdonaCite this: J. Phys. Chem. 1991, 95, 8, 3149–3152Publication Date (Print):April 1, 1991Publication History Published online1 May 2002Published inissue 1 April 1991https://doi.org/10.1021/j100161a036RIGHTS & PERMISSIONSArticle Views261Altmetric-Citations21LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (423 KB) Get e-Alerts

Kinetics of crystallization and flocculation of calcium fluoride

The kinetics of growth of calcium fluoride crystals from supersaturated solutions has been investigated using the constant composition method. During the reactions, a fluoride specific ion electrode was used to control the addition of titrants containing calcium and fluoride ions in order to maintain constant supersaturation. The quadratic dependence of crystal growth rate upon the relative supersaturation suggests a spiral growth mechanism. Particle size measurement, both during the growth kinetics experiments and at equilibrium in saturated solutions of calcium fluoride, showed that considerable flocculation takes place. In the former case agglomeration rather than aggregation occurred, following the DLVO theory as developed by von Smoluchowski. The calculated Hamaker constant was 6.0·10 −21 J.

A study of the relationship between the chemical structure of some carboxylic acids and their capacity to inhibit the crystal growth of calcium fluoride

The influence of oxalic acid, malonic acid, malic acid, tartaric acid, succinic acid and glycolic acid on the growth of calcium fluoride has been studied and interpreted in terms of Langmuir-type adsorption isotherms. Oxalic acid was the most active inhibitor of calcium fluoride growth, followed by tartaric, malic and malonic acids. Succinic acid caused a very slight effect and mono-carboxylic acids caused no inhibitory effects under the conditions studied. An attempt to relate the chemical structure of the studied carboxylic acids with their inhibitory capacity using molecular models and MNDO calculation has been described. The presence of a sequence constituted by a carboxylic group and an hydroxy group in the α position, in addition to another carboxylic group demonstrated a special efficacy to inhibit crystal growth of calcium fluoride crystals due to structural features. Interactions between inhibitors and the crystal surface were found to be mainly electrostatic in nature.

Determination of citric acid based on inhibition of the crystal growth of calcium fluoride

Inhibition of the growth of calcium fluoride crystals in the presence of citrate was followed using a kinetic-potentiometric technique and a calcium ion-selective electrode, and as a consequence, a method for the determination of citrate in the range 0.5-2.4 micrograms ml-1 has been developed. The method was successfully applied to the determination of citrate contained in pharmaceutical products and urine. Urine analysis requires prior separation of phosphate, sulphate and magnesium(II). Elimination of these interferences was studied and accomplished using precipitation processes. Magnesium and phosphate were jointly eliminated in basic media by the addition of ammonium ions. Phosphate and sulphate were eliminated with barium(II). Phosphate was also eliminated as a lithium salt.

Preparation of a Tissue Model for Quantitative FT-IR Microspectroscopic Functional Group Imaging

There is much interest in the biological and pharmacological sciences in knowing the specific location, concentration, and chemical structure of biologically active molecules in tissue. Infrared spectroscopy can be a suitable method for addressing these considerations; it can give quantitative and qualitative information without destroying the sample. IR spectra of approximately 20-μm-thick slices of animal tissue, mounted on barium or calcium fluoride crystals, can be obtained in the transmittance mode. Also, 3–5-μm-thick tissue slices can be mounted on gold-coated glass slides, and IR spectra can be obtained in the reflection-absorption mode.