Photoemission experiments on the valence band of transition metal trichalcogenides

Summary We present a synchrotron radiation photoemission study of the upper-valence-band states of four ZrSe₃-structure compounds: ZrS₃, ZrSe₃, ZrTe₃ and HfSe₃. The sulphide and selenide spectra exhibit a characteristic five-peak structure in agreement with the calculated ZrSe₃ density of states by Myronet al. and by Bullett. The nature of these features is established by an interplay between the rehybridization of paired-anion ligand orbitals and the hybridization of unpaired-anion states with cation states. The spectrum of ZrTe₃ exhibits only three peaks that appear related to three of the sulphide and selenide spectral features. The differences between the sulphide, selenide spectra and the telluride spectra are discussed in terms of the very large distance between pairedanion ligands and neighbour cations in the latter compound.

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Riassunto Si presenta uno studio mediante fotoemissione con luce di sincrotrone degli stati nella banda di valenza superiore di quattro composti con struttura del tipo ZrSe₃: ZrS₃, ZrSe₃, ZrTe₃ e HfSe₃. Gli spettri dei seleniuri e del solfuro presentano una struttura caratteristica a cinque picchi in accordo con le densità degli stati di ZrSe₃ calcolate da Myronet al. e de Bullett. La natura di questi picchi è causata dall’azione combinata della reibridizzazione di orbitali di anioni accoppiati e dell’ibridizzazione di stati di anioni non accoppiati con, stati cationici. Lo spettro di ZrTe₃ presenta solo tre picchi che appaiono collegati a tre dei picchi spettrali, osservati per i seleniuri e per il solfuro. Le differenze fra gli spettri del telloruro e quelli dei seleniuri e del solfuro sono discusse in relazione alla grande distanza fra anioni accoppiati e cationi primi vicini nel telloruro.

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Резюме мы предлагаем результаты исследований фотоэмиссии синхротронного излучения для верхних состояний валентной зоны для четырех соединений со структурой типа ZrSe₃: ZrS₃, ZrSe₃, ZrTe₃ и HfSe₃. Спекты сельфида и селенида обнаруживают структуру с пятью пиками, что согласуется с вычисленной Мироном и др. и Булле плотностью состояний для ZrSe₃. Природа этих особенностей связана с взаимодействием между перегибридизацией орбиталей спаренных анионов и гибридизацией орбиталей не спаренных анионов с катионными состояниями. Спектр ZrTe₃ обнаруживает только три пика, которые, по-видимому, связаны с тремя наблюденными спектральными особенностями сульфида и селенида. Различия между спектрами сульфида и селенида и спектрами теллурида сбсуждаются в терминан очень большого расстояния между спаренными анионами и соседними катионами в теллуриде.

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Photoemission and scanning tunneling experiments with small particles on solid surfaces

The fascination of research with nanometersized objects in contact with a macroscopic surface will be illustrated by two examples: mass-selected supported transition-metal clusters and C₆₀ molecules on metallic single-crystal substrates. The preparation, mass-selection and deposition of the small particles will be described in some detail. The main experimental techniques involved in the characterization of their electronic and structural properties are photoelectron spectroscopy and scanning tunneling methods. For the transition-metal clusters the evolution of the valence band with cluster size reveals a trend to metal formation. When the tip of a Scanning Tunneling Microscope (STM) is placed above individual C₆₀ molecules intense light emission is observed. The diameter of this emission spot is approximately 4 Å. This observation indicates the possibility of an optical spectroscopic analysis on the scale of individual molecules.     

Photoemission on NiO

The interpretation of the photoemission spectra of NiO is considered with respect to the dispersion of thed-bands. The dispersion of thed-bands can not be measured due to the formation of an exitonic state. The electronic nature of the states which are responsible for the optical gap in NiO is discussed.     

Photoemission valence band spectra and electronic density of states in copper oxides and copper based ceramic super-conductors

Photoemission valence band spectra with three different photon energies (21.2 eV, 40.8 eV, 1253.6 eV) for Cu₂O and CuO and a number of copper oxide based superconducting ceramics are investigated, namely La₂CuO₄ (LCO), Y₁Ba₂Cu₃O₇ (YBCO), Bi₂Sr₂CaCu₂O₈ (BSCCO) and Tl₂Ba₂Ca₂Cu₃O₁₀ (TBCCO). From the Cu₂O data it can be infered, that the He-II (40.8 eV) spectra give a fair representation of the density of states (DOS). In addition the agreement between the calculated density of states (DOS) and the He-II photoemission spectrum is almost perfect for Cu₂O. This agreement is worse for CuO and the other materials which all have a Cu groundstate close to a 3d ⁹ configuration indicating a large contribution of thed-d correlation energy to the excitation spectra which is absent in Cu₂O because of the filled 3d-shell. In all cases the experimental DOS atE _F is very small and only the spectra of BSCCO show a well defined Fermi edge. The relevance of these findings with respect to the theoretical local functional density DOS calculations is discussed.     

Photoemission on highT c superconductors

Ultrathin-layer (AlAs) _m (GaAs) _m superlattices withm = 1, 2, and 3 were grown by molecular beam epitaxy and characterized by x-ray diffraction and photoluminescence measurements. The appearance of distinct satellite peaks around the Bragg reflections demonstrate the formation of high-quality layered crystals. The observed luminescence shows a maximum at 2.033 eV form = 3, and the emission energy decreases form = 2 andm = 1 as well as for them = 4 superlattice. This result for the monolayer superlattice is in good agreement with recent theoretical calculations, and it shows that the (AlAs)₁(GaAs)₁ superlattice represents a new artificial semiconductor material with novel electronic properties.On leave from Mitsubishi Electric Corp., Central Research Laboratory, Amagasaki, Hyogo 661, JapanOn leave from Institute of Semiconductors, Academia Sinica, Beijing, PR China     

Photoemission on a Cu-Zr glassy metal

In this paper we present a study of XPS, UPS and magnetic susceptibility on Cu₆₀·Zr₄₀ glassy metal. Our results can be understood in a band structure where the zirconium “d” levels lie close to the Fermi level meanwhile copper “d” levels are at higher binding energies; the density of states at the Fermi level is comparable to that of pure zirconium.     

脉冲光电子枪中光电子束性能研究

利用新设计的脉冲光电子枪,研究了光电子束的能量分布,光数密度与激光强度的关系和光电子在电离区内的滞留时间。大部分电子的能量为光发射电子的剩余能量,但量由于电子的空间电荷效应,电子能量分布具有加宽现象。每个激光脉冲发射到电离区内的光电子数密度在１０＾９／ｃｍ＾３以上。     

Photoemission of molecular adsorbates

In photoemission the observed changes in band intensities as a function of light polarization, electron emission direction, and photon energy can be used to deduce information, for example, about geometry, two-dimensional band structures, intermolecular interactions and chemical reactivity of molecular adsorbates. To characterize the ion states of molecular adsorbates in the so-called inner valence electron region we discuss results from autoionization spectroscopy as a method that yields complementary information to photoemission in this spectral region. We use results on CO, N₂, and CO₂ adsorbates to discuss the various aspects of photoemission of molecular adsorbates.     

Photoemission and coincidence studies on gas-phase molecules

This article describes Young’s double-slit experiment using high-energy core-level photoemission from N₂ molecules and experimental identification of interatomic Coulombic decay in Ar₂ dimers after Auger decay using k-resolved electron–ion–ion coincidence spectroscopy, aiming to illustrate the leading edge of gas-phase experiments using synchrotron radiation.     

Photoemission from Cerium systems

The analysis of the valence band photoemission spectra of Ce compounds is inspected. It is found that the data and the models agree in that they give a 4f hole state at a binding energy of 2 eV and a 4f band near (within 100 meV) the Fermi energy. The implications of photoemission data for the electronic structures of heavy fermion systems is discussed.     

Photoemission study of oxygen chemisorption on tin

Photoemission measurements have been made at photon energies from 5–12 eV and at 21.2 eV on evaporated Sn films and the same films with varying room temperature exposure to oxygen. For hν ? 9 eV the quantum yield for Sn with exposures of as much as 4000 L oxygen (1 L = 1 Langmuir = 10^?6 Torr sec) differs only slightly from the clean metal. For hν ? 9 eV no change in yield is observed with oxygen exposure. The energy distribution of photoemitted electron (EDC's) from Sn with increasing exposure to oxygen above ? 20 L are characterized by the growth of two peaks which were not present in the EDC's for the clean metal, located 2.9 ± 0.1 eV and 4.8 ± 0.1 eV below the Fermi level. We associate this structure with the presence of SnO₂. No sharp resonance which could be associated with adsorbed oxygen was seen. Uniformly reduced emission from metallic Sn states and a Fermi level as sharp as for the clean metal is observed in the EDC's at all oxygen exposures. In addition, no change in work function with oxygen exposure was detected. The effects of oxygen saturate for exposures ? 4000 L. We suggest that under the conditions used in this experiment, the oxygen penetrates beneath the surface forming SnO₂ and leaving metallic Sn on the surface.     

光发射谱测量发现准晶能带结构

固体能带论将电子在晶格中的运动表述为布洛赫波 (振幅受晶体周期调制的一系列不同波长ｋ的平面波 ) ,它是近代固体物理学的范式之一 .1984年 ,由于准晶的发现 ,能带论的适用性受到了挑战 .今天 ,随着准晶研究的不断深入 ,人们终于在能带论和准晶之间找到了新的契合点 .晶体的平移周期性是指将一种基块无空隙、不重叠地堆砌 ,即可构建出整个晶格 .对于准晶来说 ,情况却大相径庭 .研究表明 ,为了构成 5次对称或10次对称的二维准晶 ,或者至少需要两种基础拼块 ,或者 (在使用一种拼块的条件下 )应允许拼块的复叠 (ｃｏｖｅｒａｇｅ) .复叠概念…     

Photoemission properties of metals

Photoemission has emerged over the last decade as a powerful method for studying electronic band structures. The applicabhty of the method to metals was established by Berglund and Spicer¹ in their landmark experiments on Cu and Ag. It had been believed, largely on the basis of optical absorption data, that the upper edge of the d bands in Cu and Ag lay at 2 and 4 eV below the Fermi level, respectively. Berglund and Spicer's measurements of the photoelectron energy spectra revealed these edges in a spectacular way and also uncovered some of the substructure of the d bands. Since then the technique has undergone some refinement and has been applied to a large number of materials. Photoemission measurements share the advantage of optical absorption measurements in that they are able to probe electronic states well removed from the Ferrni level. They have the added advantage that they give the initial and final energies as well as the frequencies of prominent optical transitions.     

ICF物理实验用纳米Cu块体靶材的制备研究

 采用自悬浮定向流法制备了金属纳米粉体并采用真空手套箱专利技术和冷压法在高压（1.5 GPa）作用下保压40 min后，成功制备出了相对密度达97％和显微硬度达1.85 GPa的金属Cu纳米晶材料。经XRD分析，其晶粒大小为20 nm。正电子湮没(PAS)实验结果表明，其空隙大小和数量与采用惰性气体冷凝法原位压制(IGC)的样品相比，空位簇数量较多，微空隙的大小和数量基本相当。激光惯性约束聚变（ICF）模拟实验表明：采用该方法制备的纳米Cu块体材料靶的激光转换效率比常规Cu材料靶高5倍。     

Photoemission of solid argon

Photoemission yield spectra of solid argon using synchrotron radiation for excitation in the photon energy range 11.7–16 eV show peaks closely following the known excitonic transitions, further peaks at 11.96 and 13.1 eV and also give an indication for band-gap photoconductivity.     

多碱阴极光电发射机理研究

通过比较有Cs-Sb表面层多碱阴极和未有Cs-Sb表面层多碱阴极的荧光谱,发现有Cs-Sb表面层多碱阴极的荧光谱峰值波长向短波方向“蓝移”以及荧光峰增强的现象.这一现象表明在多碱阴极Na2 KSb基层上制作Cs-Sb表面层之后,不仅多碱阴极的逸出功降低,而且Na2 KSb基层的结构也发生了变化.这意味着在相同功率和相同...     

Metal Nanoparticles with Effective Photoemission

Journal of Russian Laser Research - We find the maximum size of metal nanoparticles, when the surface photoemission (SPE) becomes important, and the size, when SPE exceeds the volume photoemission...     

Photoemission study of Nb-Ni glasses

The photoemission spectra for two Nb-Ni glasses are compared with those for the pure elements Nb and Ni. A decrease in the density of states at the Fermi level is found for glasses as compared with that of either pure element. The significance of this result as related to the formation of the glass is discussed.     

Photoemission studies of TCNQ compounds

The densities of occupied states of a series of TNCQ compounds have been determined by u.v. photoemission. A clear difference between TCNQ weak charge transfer complexes and anion radical salts including TTF-TCNQ is found. Due to strong electron-molecule coupling narrow delocalized electron bands are not observable.     

Photoemission and energy loss spectroscopy on semiconductor surfaces

The application of multiple experimental techniques to silicon surface studies is discussed. Two specific cases are considered: the chemisorption of oxygen on silicon at coverages up to one monolayer and the intrinsic surface states on cleaved and on annealed Si (111) surfaces. It is shown that by combining electron energy loss spectroscopy and ultraviolet photoemission spectroscopy, one can obtain an approximate energy level model for both occupied and unoccupied states