A multiwavelength low-pressure radiation source based on argon and xenon chlorides

The characteristics of an excimer radiation source pumped by a subnormal dc glow discharge in an Ar/Xe/Cl₂ mixture with the source operating in a spectral range of 160–310 nm were studied. The emission intensities of the 175, 236, 258, and 308 nm bands due to the transitions ArCl (B-X), XeCl (D-X), Cl₂ (D′-A′), and XeCl (B-X), respectively, were optimized in relation to the pressure, composition, and discharge current of the mixture.     

Investigation of the conditions of efficient I*<Subscript>2</Subscript> (342 nm) luminescence in a barrier discharge in a Kr-I<Subscript>2</Subscript> mixture

The energy and spectral characteristics of a barrier discharge in a mixture of iodine vapor with krypton have been investigated theoretically and experimentally. The emission spectrum consists of the single I*₂ molecular band D′ → A′ peaking at 342 nm, the iodine resonance line at 206.2 nm, and the group of bands corresponding to iodine emission. The highest intensity of the I*₂ (342 nm) band was obtained at a mixture pressure near 450 Torr. A mean output power and an efficiency of 550 mW and 1.6%, respectively, have been achieved. It is shown that, for the I₂ barrier-discharge excilamp, the homogeneous, rather than filamentary form of discharge glow is optimal from the viewpoint of the highest mean output power. The maximal calculated value of the emission efficiency for the 342-nm band was 5%. The main processes determining energy losses in plasma have been found, and ways to increase the efficiency of emission in the D′ → A′ band of the I*₂ molecule have been proposed.     

Spectroscopic diagnostics of barrier-discharge plasma in mixtures of cadmium diiodide vapor with gases

The spectral characteristics of the emission of atmospheric-pressure gas-discharge plasma in mixtures of cadmium diiodide vapor with gases (Ne, Ar, Kr, Xe, and N₂) were investigated along with the time characteristics of the voltage and current. The gas-discharge plasma was produced and excited by a barrier discharge at a repetition rate of sine voltage pulses of up to 140 kHz. The discharge emission was analyzed in the spectral range 200–900 nm with a high resolution (0.05 nm). Radiation from exciplex CdI(B-X) molecules and excimer I₂* molecules was revealed, as well as the atomic lines of cadmium, iodine, and inert gases. In a mixture with xenon, radiation from exciplex molecules XeI(B-X, B-A) was also found. This radiation prevailed in the spectra at mixture temperatures up to 150°C. The further increase of the temperature leads to the prevalence of the CdI(B-X) radiation. It was found that the most intense CdI(B-X) radiation is observed in mixtures CdI₂/Xe(N₂)/Ne. Regularities in the spectral characteristics of the emission of the gas-discharge plasma are discussed.     

Spectral characteristics of a barrier discharge in cadmium dihalide vapor-inert gas mixtures

Spectral characteristics of a barrier-discharge plasma produced in atmospheric-pressure mixtures of cadmium diiodide and cadmium dibromide vapors with neon, krypton, and xenon at a repetition rate of sine voltage pulses up to 130 kHz are studied. The emission from the discharge is studied within the spectral range 200–900 nm with a resolution of 0.05 nm. Emission of exciplex molecules CdI(B → X) and CdBr(C, B → X), and cadmium and inert gas atoms is revealed, as well as emission of exciplex molecules XeI(B → X, A) and XeBr(B → X, A) in xenon-containing mixtures. The emission of xenon halides prevails in the spectra at a mixture temperature up to 200°C. With a further temperature increase, the emission of cadmium halides becomes dominating. It is ascertained that the most intense emissions of CdI(B → X) and CdI₂/CdBr₂/Xe/Kr and CdBr(B → X) molecules are observed, respectively, in CdI₂/CdBr₂/Xe/Kr and CdI₂/CdBr₂/Xe mixtures. The cadmium dihalide-inert gas mixtures are of interest for the use as a radiating gas in a multiwavelength and broadband excilamp emitting in the UV and visible spectral ranges.     

Emission characteristics of a glow discharge in a mixture of heavy inert gases with iodine vapor

The results of a systematic investigation of the emission characteristics of a low-pressure UV excimer-halogen lamp pumped by a longitudinal dc glow discharge are presented. The discharge was initiated in mixtures of heavy inert gases with iodine vapor at a total pressure of 100–2000 Pa and a power deposited into the plasma of 10–100 W. Current-voltage characteristics of the glow discharge and emission spectra of the plasma in the region of 190–360 nm are studied. The radiation intensity at the resonance line of the iodine atom (206.2 nm) and the intensity at the peaks of the XeI(B-X) (253 nm) and I₂(B-X) (342 nm) emission bands are analyzed as functions of the pressure and partial composition of the mixtures of Ar, Kr, and Xe with iodine vapor, as well as the electric power of the glow discharge. The most efficient gas mixtures are determined for an electric-discharge UV iodine vapor lamp with continuous-wave emission and a long service life before a change of the mixture is required.     

Effective production of Xe<Subscript>2</Subscript>I excimer molecules by high-energy charged particles in Xe containing a small amount of C<Subscript>3</Subscript>F<Subscript>7</Subscript>I

An anomalously high efficiency of generating Xe₂I* excimer molecules in dense Xe–C₃F₇I gaseous mixtures with a small amount of C₃F₇I that are excited by a pulsed beam of fast electrons is discovered. The electron energy is 150 keV, and the beam current amplitude and duration are, respectively, 5 A and 5 ns. The temporal–spectral characteristics of spontaneous radiation from XeI* and Xe2I* excimer molecules are measured. Also, the luminescence times of the upper level for the B–X transition in the XeI* molecule (λ_max = 253 nm) and the upper level for the 4²Γ–1²Γ transition in the Xe₂I* molecule (λ = 352 nm), as well as the rate constants of quenching these levels by the gaseous mixture components, are determined. Based on the characteristics of the track structure of a high-energy plasma produced by charged particles in the dense gaseous medium, a model of plasma-chemical processes leading to the formation of XeI* (λ_max = 253 nm) and Xe₂I* (λ = 352 nmnm) excimer molecules in a Xe–C₃F₇I mixture with a small amount of an iodine atom donor is suggested.     

Photoionization of the Xe atom and Xe@C<Subscript>60</Subscript> molecule

Photoionization of the Xe atom and Xe@C₆₀ molecule have been studied usingthe random phase approximation with exchange (RPAE) method. The Xe atom was described byrelaxed orbitals including overlap integrals. The C₆₀ fullerene has beenrepresented by an attractive short range spherical well with potentialV(r), given byV(r) =  ?V ₀ forr _i  < r < r _o ,otherwise V(r) = 0 wherer _i andr _o are respectively, the inner and outerradii of the spherical shell. The time independent Schrödinger equation was solved usingboth regular and irregular solutions and the continuous boundary conditions atr _i andr _o . The results demonstrate improvementto previous calculations for both the Xe atom and Xe@C₆₀ molecule and comparevery well with the recent experimental data.     

Pressure-induced antiferromagnetism in La<Subscript>0.75</Subscript>Ca<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript> manganite

The crystal and magnetic structures of La_0.75Ca_0.25MnO₃ manganite are studied under high pressures up to 4.5 GPa in the temperature range 12–300 K by the neutron diffraction method. At normal pressure and temperature T _C = 240 K, a ferromagnetic state is formed in La_0.75Ca_0.25MnO₃. At high pressures P ≥ 1.5 GPa and at temperatures T < T _N ≈ 150 K, a new A-type antiferromagnetic state appears. A further increase in pressure leads to an increase in the volume fraction of the antiferromagnetic phase, which coexists with the initial ferromagnetic phase. The effect of high pressure causes a considerable increase in T _C with the slope dT _C /dP ≈ 12 K/GPa. Calculations performed in the framework of the double exchange model with allowance for the electron-phonon interaction make it possible to explain this pressure dependence of T _C on the basis of experimental data.     

Piezo-optic effect in Cu<Subscript>6</Subscript>PS<Subscript>5</Subscript>Br crystals

The effect of hydrostatic (P=10–400 MPa) and uniaxial σ=0–5.8 MPa) pressures on birefringence Δn of Cu₆PS₅Br single crystals at the wavelength λ=0.6328 μm has been studied below the temperature of the ferroelastic phase transition (T<268 K). It is found that Δn linearly depends on pressure. The obtained data are analyzed.     

Optical characteristics of plasma of I*<Subscript>2</Subscript>, Cl*<Subscript>2</Subscript>, Br*<Subscript>2</Subscript> halogen dimer barrier-discharge excilamps

Barrier-discharge excilamps operating in homonuclear chlorine, bromine, and iodine and their mixtures with inert gases have been studied. The spectral and energy characteristics of the barrier-discharge plasma have been obtained. The conditions have been determined at which the band D′ → A′ predominates in the spectra of molecules I*₂ (342 nm), Cl*₂ (257.8 nm), and Br*₂ (291 nm). The efficiencies of I₂, Cl₂, and Br₂ excilamps were found to be 1.6, 2, and 3.8%.     

Refractometry of mechanically compressed (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>SO<Subscript>4</Subscript> crystals

The effect of uniaxial mechanical pressure σ _m ≤ 150 bar on the spectral (300–800 nm) dependence of the birefringerence Δn _i and refractive indices n _i of (NH₄)₂SO₄ crystals has been investigated. It is shown that the dispersion of n _i (λ) and Δn _i (λ) is normal and sharply increases with approach to the absorption edge. It is established that uniaxial pressure does not change the character of the dispersions dn _i /dλ and dΔn _i /dλ and only affects their magnitudes. It is shown that the increase in the refractive indices under uniaxial stress is mainly due to the increase in the refraction caused by the increase in the band gap and long-wavelength shift of the UV absorption band maximum.     

Metal-insulator transition in whiskers of the NbS<Subscript>3</Subscript> quasi-one-dimensional conductor under pressure

The effect of pressure on the conduction of the NbS₃ quasi-one-dimensional conductor is studied. A pressure-induced insulator-metal transition is observed. The transition is accompanied by an increase in conductivity by six orders of magnitude at room temperature. Under pressures of 3–4 GPa, an additional phase transition appears in the temperature dependences of resistance. This transition manifests itself in an increase in the local conduction activation energy. The quantity dln(R)/d(1/T) reaches its maximum under pressures of 4–5 GPa, and the temperature position of the maximum of dln(R)/d(1/T) depends on the pressure as T* ≈ 7.5P + 202 K.     

Synthesis and properties of high-pressure phase [Er<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript>3</Subscript>](V<Subscript>4</Subscript>)O<Subscript>12</Subscript>

A new perovskite-like compound Er_0.73Cu₃V₄O₁₂ (space group Im \(\bar 3\), Z = 2, a = 7.266 Å) has been synthesized barothermally (P = 8.0 GPa, t = 1000°C). Its electrical and magnetic properties have been studied. It is found that the temperature dependence of the electrical conductivity (in the range 78–300 K) has of semiconductor type. The behavior of the impedance and admittance has been analyzed at 290 K and frequencies of 200 Hz to 200 kHz under atmospheric pressure and at high (15–42 GPa) pressures.     

Recombination population of excited states of the hydrogen atom in an He-H<Subscript>2</Subscript> plasma

The population of excited states of the hydrogen atom in an afterglow plasma produced by a pulsed discharge in helium (40 Torr) with a small admixture of hydrogen ([H₂] ≈ 10¹² cm^?3) has been studied spectroscopically. The contribution from electron-ion recombination Γ ₃ ^rec to the production rate of atoms H(n = 3) has been separated. On the basis of an experiment in which the response of the spectral line intensities to the perturbation of the electron temperature in the afterglow phase was observed, the dependence Γ ₃ ^rec (T _e ～ T _e ^?(0.9–1.0) has been obtained in the region kT _e = 0.026–0.064 eV.     

Pressure-induced structural transformations,orbital order and antiferromagnetism in La<Subscript>0.75</Subscript>Ca<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript>

High pressure evolution of structural, vibrational and magnetic properties of La_0.75Ca_0.25MnO₃ was studied by means of X-ray diffraction and Raman spectroscopy up to 39 GPa, and neutron diffraction up to 7.5 GPa. The stability of different magnetic ground states, orbital configurations and structural modifications were investigated by LDA + U electronic structure calculations. A change of octahedral tilts corresponding to the transformation of orthorhombic crystal structure from the Pnma symmetry to the Immaone occurs above P ~ 6 GPa. At the same time, the evolution of the orthorhombic lattice distortion evidences an appearance of the e _g d _{x² ? z²} orbital polarization at high pressures. The magnetic order in La_0.75Ca_0.25MnO₃ undergoes a continuous transition from the ferromagnetic 3D metallic (FM) ground state to the A-type antiferromagnetic (AFM) state of assumedly 2D pseudo-metallic character under pressure, that starts at about 1 GPa and extends possibly to 20–30 GPa.     

Electronic Structure,Optical Properties,and Pressure Behavior of the CdB<Subscript>4</Subscript>O<Subscript>7</Subscript> and HgB<Subscript>4</Subscript>O<Subscript>7</Subscript> Compounds

Ab initio calculations of the structural, electronic, and optical properties of the CdB₄O₇ and HgB₄O₇ tetraborate compounds in three structural modifications with the Pbca, Cmcm, and Pmn2₁ symmetry have been performed in the framework of the density functional theory using the VASP package. The calculations of the electronic band structure showed that these compounds in all the investigated modifications are dielectrics with a band gap of 2–4 eV. The calculation of the structural properties of the tetraborates under pressure showed that the phase transition between the Pbca and Pmn2₁ structures in cadmium and mercury tetraborates occurs under pressures of 4.8 and 4.7 GPa, respectively.     

X-ray study of [N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>]<Subscript>2</Subscript>ZnCl<Subscript>4</Subscript> at low temperatures

The unit cell parameters a, b, and c of [N(CH₃)₄]₂ZnCl₄ have been measured by x-ray diffraction in the temperature range 80–293 K. Temperature dependences of the thermal expansion coefficients α_a, α_b, and α_c along the principal crystallographic axes and of the unit cell thermal expansion coefficient α_V were determined. It is shown that the a=f(T), b=f(T), and c=f(T) curves exhibit anomalies in the form of jumps at phase transition temperatures T₁=161 K and T₂=181 K and that the phase transition occurring at T₃=276 K manifests itself in the a=f(T) and b=f(T) curves as a break. A slight anisotropy in the coefficient of thermal expansion of the crystal was revealed. The phase transitions occurring at T₁=161 K and T₂=181 K in [N(CH₃)₄]₂ZnCl₄ were established to be first-order.     

Spectroscopic diagnostics of barrier discharge plasmas in mixtures of zinc diiodide with inert gases

The spectral characteristics of the emission of gas discharge atmospheric pressure plasmas in mixtures of zinc diiodide vapor with inert gases (He, Ne, Ar, Kr, and Xe) are investigated. The formation of a gas discharge plasma and the excitation of the components of a working mixture were performed in a high-frequency (with a repetition frequency of sinusoidal voltage pulses of 100 kHz) barrier discharge. The gas discharge emission was analyzed in the spectral range 200–900 nm with a resolution of 0.05 nm. Emission bands of ZnI(B-X) exciplex molecules and I₂* excimer molecules, lines of inert gases, and emission bands of XeI* exciplex molecules (in Xe-containing mixtures) were revealed. It is ascertained that the strongest emission of ZnI* molecules is observed in ZnI₂/He(Ne) mixtures. The regularities in the spectral characteristics of the gas discharge plasma emission are considered.     

Superposition of M<Subscript>5</Subscript>X<Subscript>5</Subscript> superstructures and X-ray diffraction in TiO<Subscript>1.0</Subscript> titanium monoxide

The interpretation of diffraction spectra of ordered high-temperature phases of solid solutions and strongly nonstoichiometric compounds is discussed. It has been shown that variations of the intensities of superstructure reflections, which cannot be explained within simple ordering models, can be due to the superposition of superstructures with different symmetries in the matrix of the basis crystal structure. Using an example of atom–vacancy ordering in TiO_1.0 titanium monoxide, a model of the order–order transition state formed by the superposition of low-temperature monoclinic (space group A2/m (C2/m)) and high-temperature cubic (space group Pm3?m) M₅X₅ superstructures has been proposed. It has been shown that the transition state is thermodynamically equilibrium and should be implemented instead of the M₅X₅ cubic superstructure. The transition state model can be considered as an M_(5–i)X_(5–i) superstructure (i = 1, 14/18, 11/18) with the monoclinic symmetry (space group P1m1).     

Dielectric constant anomalies of an improper Hg<Subscript>2</Subscript>Cl<Subscript>2</Subscript> ferroelastic near the phase-transition temperature

The temperature dependence of the permittivity of a Hg₂Cl₂ crystal has been investigated within the Landau phenomenological theory. At T < T _C, the linear permittivity has a singularity in the form ～(T _C?T)^1/2; however, this anomaly may disappear in a multidomain sample. The nonlinear permittivity also has an anomaly near T _C but of stronger type: ～(T _C ? T)^?1/2.