Optical spectra of CaF2(Tb) crystals

The luminescence and absorption spectra of fluorite crystals, alloyed with terbium fluorides, oxyfluorides, and oxides at concentrations of 0.01 to 5 mol.% were studied. The change in the optical spectra was studied as a function of the chemical composition of the alloy addition and also as a function of the concentration. It is proposed that the change with the concentration in the spectra of the crystals takes place as a consequence of mutual interactions between the ions with formation of cation pairs. It was observed that the ratio of the relative luminescence intensities of the crystals to the absorption coefficient changes nonlinearly with the concentration of the alloying addition. The problem of an interaction between an isomorphic admixture and crystal defects is discussed.In conclusion, the authors express their gratitude to E. V. Sobolev and M. V. Konovalova for help in the work.     

Raman spectra of undoped and uranium doped CaF2 single crystals

Raman scattering experiments for nominally pure and uranium doped CaF2 single crystals were presented.In all crystals, the Raman active T2g vibration mode of CaF2 was observed, whose frequency shift and fullwidth at half-maximum (FWHM) broadening correspond well with defects and impurities in CaF2 lattice.Additional Raman peaks develop in nominally pure CaF2 with high etch pits density and U6+:CaF2crystals. Part of additional Raman peaks in the experimental results, which are assumed due to vibration modes from F- interstitials and vacancies, are in well agreement with the theoretical predications by employing the Green-function formulation.     

Study of Nd2+ absorption in x-irradiated CaF2, SrF2, BaF2 crystals

The absorption spectra of x-irradiated alkaline-earth fluoride (CaF₂, SrF₂, BaF₂) crystals doped with Nd³⁺ ions have been investigated. X-irradiation results in creating the absorption bands of inter-configuration 4fⁿ–4fⁿ⁻¹–5 d¹ transitions of Nd²⁺. The charge reduction of the neodymium by irradiation is not temperature-stable and the ions reoxidation (Nd²⁺ → Nd³⁺) occurs under heating to 570 K in CaF₂, 520 K in SrF₂ and 470 K in BaF₂.     

Additive colouring of CaF2:Yb crystals: determination of Yb2+ concentration in CaF2:Yb crystals and ceramics

When growing CaF₂ crystal doped with rare-earth ions, most of these ions are present in a trivalent state. However, due to contact with graphite crucible, a small proportion of a number of ions (Eu, Sm, Yb and Tm) are reduced to a bivalent state. A similar situation takes place during fabrication of CaF₂ ceramics doped with rare-earth metals. This fact is of particular importance for laser CaF₂:Yb crystals (ceramics), a promising material for short-pulse, high-power, high-energy diode-pumped solid state lasers since the presence of bivalent Yb ions can be a source of thermal losses. To date, there has been no technique to determine Yb²⁺ concentration in as-grown crystals. The proposed technique is based on a total reduction of Yb³⁺ ions via the heating of as-grown CaF₂ crystals with known concentration of Yb in the reducing atmosphere of metal vapour and determining the cross section of absorption bands of Yb²⁺ ions. The knowledge of these parameters allows estimation of the Yb²⁺ content in CaF₂:Yb crystals or ceramics by analysing their absorption spectra. Examples of using this technique are given. The technology of CdF₂ crystals reduction (an “additive colouring”) and features of colouring of crystals doped with rare-earth ions are considered.     

Thermoluminescence of proton- and electron-irradiated CaF2 crystals

Russian Physics Journal -     

Additive coloring of CaF2:Sm crystals

The study of the mechanism of additive coloring of calcium fluoride crystals, both pure and samarium-doped, demonstrates dynamical nature of processes that, in the course of coloring, tend to maintain equilibrium between Sm³⁺ and Sm²⁺ ions.     

Thermal emission spectra of CaF2 thin films

Thermal emission spectra of CaF₂ thin films evaporated on Pt plates are measured at elevated temperatures. P-polarized spectra observed at the emission angle of 25° show sharp peaks around phonon frequencies. Considerable changes in the spectra are found for films prepared under different conditions. The spectra of a film heat-treated at 680°C are analysed using the response function based on the virtual-mode theory for ionic slabs, in order to estimate the temperature dependence of the lattice dynamical quantities such as phonon frequency and damping. The temperature dependence is discussed in terms of a theory of phonon anharmonicity.     

Absorption of CaF2 crystals containing lead and oxygen

The absorption in the vacuum and near ultraviolet region of CaF₂ crystals containing lead and oxygen has been measured. Two main absorption bands at 155 and 202 nm are presumably due to lead centres and the band at 206 nm to oxygen. The nature of the weak band at 166 nm remains obscure. The absorption below 145 nm is probably due to excitons.The authors would like to express their gratitude to Ing. E.Mariani, CSc., for his interest and helpful discussions during the course of this work.     

Giant pulses from CaF2: Dy2+crystals

These crystals produce 30 nsec pulses of peak power 1.4 MW in the single-pulse mode, or 40 nsec and 1 MW in the repetition mode (500 Hz repetition frequency). The beam produces a bright plasma when focused on the surface of a metal.     

Polarized absorption spectra of Co2+ in Na2Cd3Cl8 single crystals

Summary The polarized optical absorption spectra of Na₂Cd₃Cl₈: Co²⁺ in the range of 15 000 to 40 000 cm⁻¹ down to 15 K are reported. The Co²⁺ ion is found to occupy the Cd²⁺ sites in octahedral geometry and the spectra are interpreted satisfactorily in terms of a cubic ligand field model including spin-orbit coupling. The observed crystal field spectra are well reproduced withB=745 cm⁻¹,C=3410 cm⁻¹,Dq=700 cm⁻¹ and ζ (spin-orbit interaction) =520 cm⁻¹. No spectral evidence for tetragonal distortion is observed.     

ESR of X-ray induced centre in CaF2:Gd crystals

An ESR centre with orthorhombic symmetry was induced in CaF₂:Gd crystals by X-ray irradiation after heat treatment at 900°C. The observed ESR spectrum has resolved shfs in particular crystal directions. Spin Hamiltonian analysis indicates that the ESR centre is a hole trapped at an oxygen substituted for a fluorine site and interacts mainly with two nearby fluorine ions located along the crystal [001] direction. The determined spin Hamiltonian parameters are g_xx = 2.0036, g_yy = 2.0159, g_zz = 2.0306, A_xx = 22.5, A_yy = 5.6 and A_zz = 5.6 (G), respectively. A probable model is discussed by considering both the angular dependence of the ESR lines and the local charge neutrality around the ESR centre.