Resonances in Positron-Mg Scattering

We present the results for resonances in positron-Mg scattering at low impact energy （0-5.0eV） by using the momentum space coupled-channel optical （CCO） method in momentum space. The S-partial wave resonance at 3.880eV, P-partial wave resonance at 4.020eV, and D-partial wave resonance at 4.267eV are found.     

Limits on the existence of heavy neutrinos in the range 50-1000 eV from the study of the 187Re beta decay

We analyzed the spectrum of the 187Re beta decay, obtained with a cryogenic microcalorimeter, searching for heavy neutrinos in the mass range 50-1000 eV. No evidence has been found for them and the upper limits on the mixing angle with a zero-mass neutrino are reported. Upper limits of 9x10(-3) at 1000 eV/c(2), 1.2x10(-2) at 500 eV/c(2), 4.4x10(-2) at 200 eV/c(2), and 0.116 at 100 eV/c(2) at 95% C.L. have been obtained. These upper limits are a factor of 2 to 4 lower than the current limits reported in the literature.     

Fundamental optical properties of α-RuCl3

The fundamental optical properties in the paramagnetic phase of α-RuCl₃ are studied at different temperatures in the photon energy interval 0.03 to 10 eV. Infrared reflectivity spectra show a transverse optical frequency at 0.038 eV (32 μm) for an E_u mode (E⊥c, in plane atomic displacements). The absorption spectra in the energy range 0.2 to 1 eV reveal three bands (0.29, 0.51, 0.71 eV) attributed to d-d electronic transitions. Reflectance and thermo-reflectance measurements indicate the onset of the charge-transfer transitions at 1.1 eV and show structure at 1.85, 2.55, 3.05, 4.5 eV. The marked reflectivity peak at 5.2 eV is probably related to p(Cl) → s(Ru) band-to-band transitions.     

Formation of transient pairs of F and H centers at next nearest neighbor sites in alkali chloride crystals studied by the time delayed double excitation spectroscopy in picosecond range

Abstract

By making use of the time delayed double excitation spectroscopy, transient absorption spectra just after the excitation of self-trapped excitons (STE) from the lowest triplet state to higher excited states have been observed to find the transient F-H center pair [F-H]. Absorption bands due to transient [F-H] appeared as a doublet peaking at 2.70eV and 2.86eV (NaCl), at 2.25eV and 2.38eV (KCl) and at 1.97eV and 2.09eV (RbCl) at 108ps after the excitation. The ratio of peak heights between bands at low and high energy sides was 2:1 in all crystals. The results were tentatively understood by a recent theory for the STE by Song et al. (1987).     

Photoluminescence of a silver-doped glass

The absorption, emission and excitation spectra of Ag⁺ ions in a soda lime glass doped with two different concentration of silver are investigated. Absorption spectra exhibit a main broad band peaked at about 260 nm (4.77 eV) with a shoulder at about 227 nm (5.46 eV). The relative height of the shoulder depends on silver concentration in the glass. Emission spectra of Ag⁺ are dominated by an ultraviolet broad band at about 330 nm (3.76 eV). The excitation spectra for this emission show a preponderant broad band peaked at about 227 nm (5.46 eV) which coincides with peak position of the shoulder displayed in the optical absorption spectra. A weak broad featureless emission band centred at about 550 nm (2.25 eV) with an excitation peak at about 242 nm (5.12 eV) is tentatively related to an impurity from the host silica glass rather than originated in silver-type centres. Comparison of the luminescence decay curves for both emissions show substantial differences between them. Consequently, the emissions in the time-resolved spectra can easily be discriminated.     

Bestimmung der optischen Konstanten von Lithium und Natrium durch optisch angeregte Oberflächenplasmaschwingungen

The ATR method to excite surface plasmons by light is used to measure optical constants of Lithium and Natrium in the energy region 2–3.8 eV. For energies smaller than 3.5 eV the real part of the dielectric function can be described by the equations valid for the free electron gas. The imaginary part shows an interband transition (~ 2.5 eV at Li, ~ 2.0 eV at Na), which is rather slow (about 1 eV) indicating an indirect transition.     

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利用SHML模型计算了密度为ρ=1g·cm-3、温度为150eV、200eV、250eV、300eV、400eV的Sn等离子体的随光子能量变化的辐射不透明度及Rosseland平均不透明度.分析了不透明度随光子能量变化曲线的吸收峰值(不透明度峰值)与能级跃迁的对应关系.还将Sn的Rosseland平均不透明度与DCA/UTA及STA模型计算结果作了比较,吻合较好.     

Hydrogen passivation effect on the yellow-green emission band and bound exciton in n - ZnO

Photoluminescence (PL) measurements performed on as-grown, hydrogenated, and annealed n-type ZnO bulk samples investigated the origins of their yellow (2.10 eV) and green (2.43 eV) emission bands. After hydrogenation, the defect-related peak at 2.10 eV was no longer present in the room temperature PL spectrum, the peak intensity at 2.43 eV was unchanged, and the intensity of the emission peak at 3.27 eV increased significantly. These results indicate that yellow band emission is due to oxygen vacancies, as the emission peak at 2.10 eV disappears when hydrogen atoms passivate these vacancies. The emission peak at 2.43 eV originates from complexes between oxygen vacancies and other crystal defects. We discuss the shallow donor impurities arising due to these hydrogen atoms in the ZnO bulk sample.     

A novel technique for the identification of the surface plasmon in eels

In this work we report on angle-resolved Electron Energy Loss Spectroscopy (EELS) with grazing-emergent electrons, which has been used to enhance considerably the surface sensitivity of EELS and thus distinguish between surface and bulk-related loss features. Surface plasmon losses for Ni (001) are clearly identified at 7 eV and 14 eV. Also, a new interpretation is suggested for a loss peak observed at 10 eV.     

Thermally stimulated current and high temperature resistivity measurements of 4H semi-insulating silicon carbide

High purity semi-insulating 4H SiC single crystals have potential applications for room temperature radiation detectors because of the wide band gap and radiation hardness. To control carrier lifetime, a key parameter for high performance radiation detectors, it is important to understand the nature of the deep traps in this material. For this purpose, we have successfully applied thermally stimulated current (TSC) and high temperature resistivity measurements to investigate deep level centers in semi-insulating 4H SiC samples grown by physical vapor transport. High temperature resistivity measurements showed that the resistivity at elevated temperatures is controlled by the deep level with an activation energy of 1.56 eV. The dominant traps revealed by TSC measurements were at 1.1-1.2 eV. The deep trap levels in 4H-SiC samples, the impurity and point defect nature of TSC traps peaked at ∼106 K (0.23 eV), ∼126 K (0.32 eV), ∼370 K (0.95 eV), ∼456 K (1.1-1.2 eV) are discussed.     

Reflection of slow electrons from the (110) face of a tungsten crystal

Secondary-electron emission from the (110) face of tungsten was studied at low primaryelectron energies (1–30 eV). The electrons reflected from the target surface were distinguished by analyzing the retardation curves of the secondary particles found in retardation in a spherical capacitor. The reflection coefficient R for an electron energy of about 1 eV is 0.08; it increases with increasing energy, reaching a maximum (0.34) at 3.3 eV; then it decreases; passing through a low maximum near 13 eV. Contamination of the target surface shifts the maximum by 0.7 eV and reduces the maximum reflection coefficient to 0.22. The total secondary-emission yield at the clean surface also reaches a maximum at 3.3 eV. Above 13 eV, increases, reaching a value of 0.5 at 30 eV. The value for a contaminated target is lower than that for a clean target. On a clean target the yield corresponding to true secondaries and inelastically reflected electrons remains at 0.1 for primary energies below 5 eV (down to 1 eV), while the value of this yield for a contaminated surface is nearly 0 for a primary energy of 1 eV.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 88–94, November, 1970.The authors thank Prof essor G. N. Shuppe for his interest in this study, A. Kashetov for assistance in the experiments, and master glassblower G. S. Rubets for constructing the apparatus.     

Molecular orbital study of co chemisorption and oxidation on a Pt(111) surface

A molecular orbital study was made, using an atom superposition and electron delocalization (ASED) technique, of the structures and energy levels of CO on Pt(111) surface. CO is predicted to be preferentially adsorbed at a height of 2.05 Å from the surface at a 1-fold position with the carbon end down. The calculated binding energy (1.7 eV) is in good agreement with the recent experimental result (1.5 eV) of Campbell et al. Calculated binding energies for bridging (1.3 eV) and high coordinate (1.1 eV) sites are predicted to be smaller in magnitude. Calculated results are used to discuss the ordering of energy levels of adsorbed CO. The interaction between CO (adsorbed) and O (adsorbed) has been studied to estimate the energy of activation for the oxidation of CO on Pt(111) surface. The calculated activation energy (1.6 eV) is in reasonable agreement with the recent experimental result (1 eV) of Campbell et al. The Langmuir-Hinshelwood mechanism is found to be favored. We predict CO₂ bonds vertically.     

电子与锂和铍原子碰撞角分布的理论研究

用模型势方法研究电子与锂和铍原子弹性散射，计算了能量从０．１到１．０ｅＶ散射电子的角分布（微分散射截面），从理论计算中，看到在０．１－１．０ｅＶ能区内，随入射电子能量增加，电子被Ｌｉ和Ｂｅ原子散射的微分截面有相似变化，即小角微分散射截面越来越大     

Photoluminescence band at 4.4 eV in oxygen-deficient silica: temperature effects

We report experimental results on the spectral properties and time behaviour of the 4.4 eV photoluminescence (PL) band in oxygen-deficient silica [Formula: see text]. Our measurements, performed both at T = 300 K and T = 10 K, show that at room temperature the PL features are independent of the particular excitation energy (5.0 eV, 6.8 eV and 7.6 eV) whereas at low temperature, upon excitation at 7.6 eV, the decay of the PL emission is faster than for lower excitation energies. This shortening of the PL lifetime is consistent with previously reported data, which were explained by hypothesizing an interconversion mechanism between two structural configurations of the same oxygen defect. Nevertheless, our results do not support the proposed mechanism and we tentatively suggest a different interpretation of the experimental data.     

Electron energy-loss spectra of clean and hydrogen-covered Cr(110) surfaces

Electron energy-loss spectra of clean and hydrogen-covered Cr(110) surfaces have been investigated in the spectral range of 0–80 eV for primary energies of 60–500 eV. The observed peaks for the clean surface are at the loss energies of 2, 3.5, 5.5, 9.6, 23, 35, 42, 48 and 55 eV. The 3p-excitation spectra for high primary energies (> 250 eV) are in good agreement with the corresponding optical spectrum. The edge at & 42 eV observed for low primary energies is tentatively attributed to the onset of the transition from the 3p subshell to the local 4s-band in the vicinity of the core hole. A characteristic energy-loss at ≈ 15.5 eV is observed after hydrogen adsorption. The 3p-spectrum is not influenced by hydrogen adsorption, indicating that the excitation is of a localized character.     

Synthesis and optical properties of nanocrystalline ZnO powders prepared by a direct thermal decomposition route

This paper reports the synthesis and optical properties of nanocrystalline ZnO powders with crystallite sizes of 32.5 (±1.4)–43.4 (±0.4) nm prepared by a direct thermal decomposition of zinc acetate at the temperatures of 400, 500, 600, and 700°C for 4 h. The structure of the prepared samples was studied by XRD and FTIR spectroscopy, confirming the formation of wurtzite structure. The morphology of the samples revealed by SEM was affected by the thermal decomposition temperature, causing the formations of both nanoparticles and nanorods with different size and shape in the samples. The synthesized powders exhibited the UV absorption below 400 nm (3.10 eV) with a well defined absorption peak at around 285 nm (4.35 eV). The estimated direct bandgaps were obtained to be 3.19, 3.16, 3.14, and 3.13 eV for the ZnO samples thermally decomposed at 400, 500, 600, and 700°C, respectively. All the samples exhibited room-temperature photoluminescence (PL) showing a strong UV emission band at ∼395 nm (3.14 eV), a weak blue band at ∼420 nm (2.95 eV), a blue–green band at ∼485 nm (2.56 eV), and a very weak green band at ∼529 nm (2.35 eV). The mechanisms responsible for photoluminescence of the samples are discussed.     

Observation of a quantum-confinement effect with GaN nanoparticles synthesized through a new gas reaction route

Spherical wurtzite GaN nanoparticles with diameters of 5-8 nm were synthesized through a new gas reaction route. The concave-surface-induced nucleation process based on the surface roughness of the substrate played an important role in the formation of quantum dots. Photoluminescence spectrum revealed the quantum-confined excited states in the nanoparticles with features at 3.73 eV (332 nm) and 3.85 eV (322 nm), which show significant blueshift compared with the band-gap emission from bulk GaN crystals. Four blue emission bands with low intensity, centered at 2.85 eV (435 nm), 2.75 eV (451 nm), 2.67 eV (464 nm), 2.62 eV (473 nm) respectively, were also observed. These blue bands may originate from surface states.     

UV and vacuum-UV properties of ge related centers in gamma irradiated silica

Photochemical inhomogeneity in the reduction process of the optical activity related to Ge oxygen deficient point defects in silica, characterized by an absorption band centered at 5.15 v eV and two emission bands centered at 3.2 v eV and 4.3 v eV, have been investigated. We have made a comparative study of the stationary and time dependent photoluminescence under excitation in the UV (5 v eV) and in the vacuum-UV (7.4 v eV) ranges in natural silica samples with native and with n -irradiation bleached optical activity. Our measurements evidence that the same spectral features are observed in the native and in the irradiated samples, but for an intensity reduction in the irradiated ones. Moreover, the time decay of the photoluminescence at 4.3 v eV is the same independently from the irradiation of the sample. On the basis of these results it is suggested that the inhomogeneous distribution of defects is not changed by the irradiation.     

氮气在100 eV以下的绝对振子强度密度

在入射电子能量为2500 eV、能量分辨为100 meV的条件下，得到了氮气在100 eV以下的绝对光学振子强度密度和广义振子强度密度；得到了23 eV和31.4 eV两个超激发态的绝对广义振子强度、并讨论了它们的动量转移依赖特性。     

闪烁晶体碘化铯在紫外光和X射线激发下的发光特性

室温下掺铊碘化铯(CsI∶Tl)晶体的吸收谱在230~320 nm范围内有3个特征峰:310 nm(4 eV)、270nm(4.6 eV)和245 nm(5.1 eV)。采用这3种不同激发能量(对应不同激发机制)的近紫外(UV)光激发得到的荧光(PL)光谱相同。这些PL谱与钨(W)靶X射线激发的辐照致荧光(RL)谱也类似。经分峰计算,PL和RL均含有4种熟知的3.1 eV(400 nm)、2.55 eV(486 nm)、2.25 eV(550 nm)和2.1 eV(590 nm)发光组分,但RL中2.1 eV组分高于PL,同时2.55 eV组分又低于PL。分析认为,这一差异来自于X射线对晶体的辐照损伤Tl+Va+、Tl0Va+,相关的2.1 eV吸收峰与2.55 eV发光带重叠。结果表明:X射线比紫外光更易产生损伤从而影响晶体CsI∶Tl的发光特性。