Low-energy secondary-electron spectroscopy of molybdenum

A theoretical interpretation of the fine structure of the secondary-electron emission spectra and the secondary-electron emission coefficient is given for molybdenum. The calculations are carried out with allowance for the energy dependence of the energy-band broadening, the electron-electron and electron-plasmon contributions to the state occupation function, and the isotropic component of the current from surface-scattered electrons. It is shown that the fine structure of the secondary emission spectra and the secondary emission coefficient is mainly attributable to the electronic structure of the final states into which electrons enter and from which electrons are emitted. Fiz. Tverd. Tela (St. Petersburg) 39, 1727–1731 (October 1997)     

Structural and composition changes in superconducting ceramics locally irradiated by electrons

The structural changes and the cation and anion compositions of the surface of superconducting YBa₂Cu₃O_7−x and Tl₂Ba₂CuO_6+x ceramics after local irradiation for several seconds by an electron probe with an accelerating voltage of 25 kV and a current exceeding 10⁻⁷ A are investigated by secondary-electron emission and by cathodoluminescence and x-ray microanalysis in a scanning electron microscope. Morphologically altered regions are detected in the irradiation epicenter, where the structure, chemical composition, and phase composition of the original compound are completely lost. In the intermediate zone between the epicenter and the periphery a distribution of the secondary emission yield is observed with a complex character that differs for the yttrium and thallium ceramics, and anomalies appear in the cathodoluminescence spectra. The experimental data are interpreted on the basis of ideas of oxygen losses under the direct influence of the electron probe and related electronic processes in superconductors. Fiz. Tverd. Tela (St. Petersburg) 39, 452–456 (March 1997)     

Planar and cluster structure of icosahedral quasicrystals

Twofold-, threefold-, and fivefold-symmetry elements are observed in the near-surface region of the quasicrystal Al₇₀Pd₂₀Mn₁₀ using a real-space imaging technique based on secondary-electron emission. The observed icosahedral point-group symmetry implies the presence of atomic clusters within the analyzed region of the solid. The same surface produces perfect low-energy electron diffraction patterns typical for well-defined crystallographic planes. We present a model which reconciles this dual structural nature of quasicrystals. Our model is constructed from an icosahedral seed followed by concentric symmetry-preserving “growth” to form the macroscopic solid. Presented at the VIII-th Symposium on Surface Physics, Třešt’ Castle, Czech Republic, June 28 – July 2, 1999. This work was supported in part by the Schweizerischer Nationalfonds.     

Methodological aspects of the electron-beam investigation of dielectric target charging

The basic characteristics of dielectric target charging with (2–30)-keV electron beams were investigated using the example of a typical insulator, Al₂O₃. A new hypothetical scenario of dielectric charging, based on the reduction of secondary-electron current due to the formation of polarized excitons in the positive layer of a charged target, is proposed on the basis of modeling calculations and the experimental data.     

Study of silicon implanted with zinc and oxygen ions via Rutherford backscattering spectroscopy

The structural features and dopant profiles of a Si surface layer implanted with Zn⁺ and O⁺ ions are studied via Rutherford backscattering spectroscopy based on the analysis of He²⁺-ion spectra with the use of the channeling technique. The doping-impurity redistribution is analyzed upon the formation of zinc-oxide nanoparticles. The sample surface morphology is examined by means of atomic-force microscopy and scanning electron microscopy under secondary-electron emission conditions. X-ray phase analysis of the implanted layers is carried out.     

As(0001) surfaces: Auger,loss, and photoelectron spectroscopic studies

The (0001) surfaces of arsenic single crystals have been characterized by Auger, loss, and X-ray photoelectron spectroscopy. A comparison of the results leads to an unusual suggestion for excitation, conduction, and emission modes in the low-energy region. It is proposed that the dominant 20 eV near-elastic loss is the excitation of N₁(4s) electrons to a conduction level 2 eV above the instrumental vacuum level. It is also proposed that two low-energy “Auger-like” peaks at 12 eV and 1.5 eV are electrons emitted at the surface from permitted bands in the bulk. A strong correlation among Auger and loss signals, integrated secondary-electron emission, elastic peak heights and the Kikuchi display is also reported. A study of the Kikuchi correlation of the 12 eV peak suggests that the surface Debye-Waller effect on As(0001) is a long-range, collective oscillation rather than short-range, individual atomic disorder.     

Analysis of the parameters of a barrier discharge in Kr-I2 and Xe-I2 mixtures

The barrier discharges in Kr-I₂ and Xe-I₂ mixtures are investigated as possible light sources at wavelengths λ=185 and 253 nm. The spectral and current-voltage characteristics of the barrier discharges are obtained. The concentrations of mixture components are calculated, and the kinetic diagrams of the reactions are constructed. The emission efficiencies are estimated to be approximately 20% for both wavelengths. The optimum conditions for emission at these wavelengths in the mixtures Xe-I₂ (400 ± 15 Torr) and Kr-I₂ (200 ± 0.1 Torr) are determined.     

Investigation of the luminescence spectrum of UO2MoO4

By time-resolved spectroscopy the intrinsic emission spectrum of UO₂MoO₄ at 4.2 K is obtained. The main progressions in the vibronic structure are identified as couplings with the A_g correlation field components of the symmetric and asymmetric UO₂ stretching modes. The intrinsic zero-phonon line in the emission and excitation spectrum is shown to be split both by the crystal field and correlation field. The steady-state emission spectrum at 4.2 K is dominated by emission from traps. The vibronic structure of the trap emission reveals that all traps are distorted uranyl groups.     

Luminescence of defect centres in Hg2Cl2

Abstract

Luminescence in Hg₂Cl₂, crystals excited with UV light is investigated in the spectral region 0.8–2.25 eV. Measurements are performed on as-grown samples and on samples previously exposed to UV light at RT. Six emission bands are found which depend on the concentration of the photochemical entities produced by irradiation of the crystals at RT. It is concluded that infra-red (IR) luminescence of Hg₂Cl₂ originates from crystal defects. The observed emission bands are tentatively attributed to the emission of (HgCI_xBr_3?x)^? and (HgCl_xBr_4?x)^2? complexes formed with residual Br impurities. Centres responsible for IR Hg₂Cl₂ emissions are excited: (i) via excitons of Hg₂Cl₂, (ii) via excited states of isolated Hg₂Br₂ molecules, and (iii) resonantly through the excitation bands of defect centres.     

Emission spectra of KBr:Sn2+

Crystals of KBr:Sn²⁺ irradiated in the A₁, A₂, B or D absorption bands exhibit strong emission in the region of 500 nm. The dependence of this emission on excitation wavelength in the A absorption band shows the 500 nm emission band to be a doublet. This doublet structure is due to electrostatic perturbation from a nearby cation vacancy. It is not possible from emission spectra alone to decide on the actual symmetry of the A_T1 and A_T2 centres responsible for the emission doublet but the various possibilities are discussed. Quenching experiments show that a small emission band at 700 nm is due to Sn²⁺ dimer centres. A series of weak emission bands on the high-energy side of the A_T band are ascribed to emission from the relaxed excited B and D states.     

Spectroscopic investigation of the plasma of a high-current diffuse discharge in He/Ar/CF2Cl2(CCl4) mixtures

Survey emission spectra in the region of 190–600 nm and time and service-life characteristics of a transverse nanosecond discharge in He/Ar/CF₂Cl₂(CCl₄) mixtures at a pressure of 10–100 kPa are investigated. In the emission spectra, excited products of the decomposition of freons—C₂(A−X), CN(B−X), Cl ₂ ^* , C^*, Cl^*, and Cl^+*— and the emission of ArF at λ=193 nm are revealed. The emissions of Cl ₂ ^* at λ=258 nm and ArF at λ=193 nm were the most intense. The discharge in the He/Ar/CF₂Cl₂ mixture is a multiwave emission source with λ=258 nm Cl ₂ ^* 193 nm ArF, and probably, 175 nm Arcl. It is of interest for applications in UV-VUV-range pulse photometry. The duration of the emission on Cl ₂ ^* , ArF, ArI, ClI, and ClII transitions in the discharge in the Ar/CF₂Cl₂ mixture (P=10–20 kPa) was 200–300 nsec. With adding He and increasing pressure to 100 kPa the duration of the emission decreased by a factor of 1.5–2. The basic mechanisms of the formation of Cl₂, ArF, and CN(B) molecules in the transverse-discharge plasma are considered. Uzhgorod State University, 46, Pidgirna Str., Uzhgorod, 294000, Ukraine. Translated from Zhurnal. Prikladnoi Spektroskopii, Vol. 66, No. 2, pp. 241–246, March–April, 1999.     

Pressure and temperature dependence of the emission in BaF2:Eu and SrF2:Eu

The 4f⁶5d¹(e)→4f⁷ emission (normal) of Eu²⁺ in SrF₂ and normal and anomalous emissions of Eu²⁺ in BaF₂ are studied as a function of pressure and temperature. Although in BaF₂ both emissions (anomalous and normal) shift with pressure in the same spectral direction (a red shift), the anomalous emission converts to the normal emission when the pressure increases to 33 kbar. Considering the dependence of BaF₂:Eu²⁺ luminescence on the pressure and temperature we have found that spectral transformation takes place due to a smooth pressure-induced level crossing at approximately the same pressure as the phase transition.     

Study of the luminescence of H2O and D2O ices induced by charged-particle bombardment

We measured the optical emission of H₂O and D₂O ices in visible region (300-500 nm) induced by energetic hydrogen ions (H⁺, H₂⁺, and H₃⁺) irradiation. Our analysis of the data of ion-stimulated luminescence (ISL) shows that all spectra of ISL emission are identical, independent of projectile. We show that all lines in the ISL emission spectrum may be assigned to decays from excited particles and/or fragments of H, H₂, OH, and H₂O. From the independence of emission spectrum on projectile we conclude that the final process causing the emissions may be attributed to the interaction between H⁺ (and/or H) and the water molecules.     

Synthesis and photoluminescence study of rare earth activated phosphor Na2La2B2O7

The photoluminescence properties in UV and N-UV excitable range for the phosphors of Na₂La₂B₂O₇: RE (RE=Eu, Tb, Ce, Sm, Gd) are investigated. The solution combustion synthesis technique was employed for the synthesis of the phosphors Na₂La₂B₂O₇: RE. The photoluminescence measurements of the phosphors were carried out on a HITACHI F7000 Fluorescence Spectrophotometer. The PL and PL excitation (PLE) spectra indicate that the main emission wavelength of Na₂La₂B₂O₇: Eu are 591 and 615 nm, Na₂La₂B₂O₇: Ce shows dominating emission peak at 387 nm and Na₂La₂B₂O₇: Tb displays green emission at 493, 544, 593 and 620 nm at 254 nm excitation, while Na₂La₂B₂O₇: Sm shows the main emission peak wavelengths 566 and 604 nm at 405 nm excitation and Na₂La₂B₂O₇: Gd shows dominating emission peak at 312 nm at 274 nm excitation. These phosphors may provide a new kind of luminescent materials under ultraviolet and near ultraviolet excitation for various applications.     

Photoluminescence properties of blue-emitting Li4SrCa(SiO4)2:Eu2+ phosphor for solid-state lighting

Blue-emitting Li₄SrCa(SiO₄)₂:Eu²⁺ phosphors have been synthesized by solid-state reaction. The photoluminescence (PL) excitation spectrum shows broad-band absorption and matches well with the emission of a near-UV (n-UV) chip. The PL emission spectrum exhibits a broad-band emission peaking at 430 nm, which is the characteristic emission of the f–d transition of the Eu²⁺ ion. The diffuse reflection spectra, temperature-dependent emission spectra, fluorescence decay, and mechanism of concentration quenching are also studied in detail. Li₄SrCa(SiO₄)₂:Eu²⁺ is a candidate blue phosphor for n-UV excited solid-state lighting.     

Luminescence of Thin Bi2W2O9 Films

The luminescence spectra of thin Bi₂W₂O₉ films have been investigated. The spectra were separated into elementary components by the Alentsev–Fock method. The radiation band with a maximum at 2.43 eV in the luminescence spectrum of Bi₂W₂O₉ has been assigned to the Frenkel autolocalized excitons. The luminescence bands with maxima at 2.10 and 1.90 eV have been assigned to the emission of the centers whose energy levels are located in the forbidden band. The luminescence of the Bi₂W₂O₉ films is due to the emission of the WO₆ complex.     

SrAl_2B_2O_7:Dy~(3+)材料的制备及其发光性能

采用高温固相法制备了SrAl2B2O7:Dy3+发光材料.在350nm紫外光激发下,测得SrAl2B2O7:Dy3+材料的发射光谱为一个多峰宽谱,主峰分别为480,573和678nm;分别和Dy3+的4F9/2→6H15/2,4F9/2→6H13/2,4F9/2→6H11/2的跃迁发射相对应;监测573nm的发射峰,得到材料的激发光谱为一个多峰宽谱,主峰分别为295,325,350,365,400nm.研究了Dy3+掺杂浓度对SrAl2B2O7:Dy3+材料发射光谱的影响,随着Dy3+掺杂浓度的增大,SrAl2B2O7:Dy3+材料的Iy/Ib逐渐增大,根据Judd-Ofelt理论解释了其原因.随着Dy3+掺杂浓度的增大,Dy3+的4F9/2→6H13/2跃迁产生的573nm发射峰强度先增大,在4%时达到最大值,之后减小,其自身的浓度猝灭机理为电偶极-电偶极相互作用.不同的电荷补偿剂Li+,Na+,K+的引入均使发光强度得到提高,尤其以Li+最佳,发光强度提高了大约33%.     

Luminescence of Ni doped CaF2

Luminescence measurements of CaF₂ : Ni are reported. Before X-irradiation an emission band at 680 nm is observed with the corresponding excitation band at 255 nm. After RT X-irradiation the emission spectrum consists of three peaks at 290, 375 and 680 nm, all of them with an excitation band at 255 nm. The same emission spectrum together with the electron-hole recombination band is obtained by X-ray excitation.A comparison with previous EPR and optical absorption measurements indicates that the emission bands are due to different kinds of Ni²⁺ centers. The emission processes are similar to those found in CaF₂ : Co and CaF₂ : Mn.     

Ni2+ emission in SrF2

Luminescence measurements of X-irradiated SrF₂:Ni are reported. After X-irradiation two emission bands have been found. One of them peaked at 293 nm and has an excitation band at 267 nm. The other one at about 770 nm, which is much weaker, has an excitation band at 274 nm. Both emission bands are also observed under X-ray excitation. A comparison with some previous studies of the absorption and thermoluminescence properties of X-irradiated SrF₂:Ni indicates that the emission bands are due to two different kinds of Ni²⁺ centers. The proposed emission mechanisms are similar to those found in CaF₂:Ni.     

Optical characteristics of the plasma of a transverse volume discharge in Cl2 and He/Cl2 mixtures

Results are presented from a study of UV and VUV emission from the plasma of a transverse volume discharge in chlorine and a He/Cl₂ mixture. In the wavelength range Δλ=140–300 nm, the Cl₂(D′-A′) band with an edge at 258 nm and the Cl ₂ ^* band with edge at λ=195 nm are found to be dominant. It is shown that, in the pressure range [Cl₂]=0.1–2.0 kPa, the intensity of emission with λ≤195 nm is higher than the intensity of the Cl₂(D′-A′) band. At [Cl₂]≥2 kPa, emission in the 258-nm band is dominant.