Ground-state properties and optical excitations of a solvated electron in molten alkali halides

Summary Properties of solvated electrons at high dilution in four molten alkali halides are investigated theoretically. A self-consistent evaluation of the bound ground state of the electron and of the surrounding liquid structure, already developed in a previous paper, is compared with the results of NMR hyperfine shifts and magnetic-susceptibility measurements. The absorption bands associated with Franck-Condon 1s→2p and 1s→3p transitions are calculated and compared with the available data of optical absorption, with special emphasis on the detailed analysis by Yuh and Nachtrieb for molten CsCl. An instability of the excited states against ionic relaxation and the origin of a finite lifetime for the ground state are also discussed. Finally, the perturbation induced by the solvated electron in the fundamental absorption of the molten salt is estimated.

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Riassunto Il lavoro presenta uno studio teorico di elettroni solvatati a forte diluizione in alogenuri alcalini fusi. Un calcolo autoconsistente dello stato fondamentale è confrontato con dati NMR d'interazioni iperfini e con misure di suscettività magnetica. Le bande di assorbimento ottico dovute a transizioni 1s→2p e 1s→3p sono calcolate in uno schema di Franck-Condon e i risultati sono confrontati con l'evidenza sperimentale, con attenzione particolare all'analisi dello spettro nel CsCl fuso data da Yuh e Nachtrieb. Si discutono quindi un'instabilità degli stati eccitati sotto rilassamento ionico, la vita media dello stato fondamentale nel liquido e lo spostamento dello spettro eccitonico dovuto all'elettrone solvatato.

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Резюме Теоретически исследуются свойства сольватированных электронов при высокой степени разведения в четырех расплавленных щелочных галоидах. Самосогласованное вычисление основного состояния связанного электрона и окружающей жидкой структуры сравнивается с данными ЯМР для сверхтонких смещений и с измерениями магнитной восприимчивости. Вычисляются зоны поглощения, связанные с переходами 1s→2p и 1s→3p в схеме Франка-Кондона. Полученные результаты сравниваются с имеющимися экспериментальными данными по оптическому поглощению, причем, особое внимание уделяется анализу спектров для расплавленного CsCl. Также обсуждается неустойчивость возбужденных состояний относительно ионной релаксации и конечное время жизни основного состояния в жидкости. В заключение, оценивается смещение экситонного спектра, обусловленное сольватированным электроном.

     

Electronic properties of MgIn2S4 and CdIn2S4 by the self-consistent pseudopotential method

Summary The electronic properties of CdIn₂S₄ and MgIn₂S₄ are studied in the self-consistent local-density pseudopotential scheme based on hard-core atomic potentials. Our self-consistent calculation confirms the assumptions of previous empirical studies and provides for the lowestenergy gap values which agree with experimental data. The electron charge densities indicate that both sulphospinels are highly ionic. The two compounds have similar ground-state properties but different conduction bands. Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

Effect of the exchange potential and of the cut-off radii on the energy bands of α-quartz

Summary An analysis of the effect of different forms of the exchange potential and values of the cut-off radii in the mixed-basis calculation of the band structure of α-quartz is presented. An optimal choice is made and the corresponding bands are computed and discussed.

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Riassunto Si presenta un’analisi dell’effetto che diverse forme del potenziale di scambio e diversi valori dei raggi di cut-off hanno sulla struttura a bande del quarzo-α, calcolata col metodo della base mista. Si fa una scelta ottimale e le corrispondenti bande sono calcolate e discusse.

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Резюме Проводится анализ влияния различных форм обменного потенциала и величин радиуса обрезания на зонную структуру α-кварца. Предлагается оптимальный выбор и вычисляются соответсвующие зоны. Проводится обсуждение полученных результатов.

     

Study of the valence bands of FePS3 and NiPS3 by resonant-photoemission spectroscopy

Summary We have investigated the valence bands of the tiophosphates FEPS₃ and NiPS₃ with the resonant-photoemission technique. Some structures of the energy distribution curves show rapid intensity changes when the photon energy is scanned across the Fe or Ni 3p absorption thresholds. Instead, other structures show smooth excitation spectra. We have related the former to the transition metal ion 3d states (direct photoemission and satellites) and the latter to the (P₂S₆)⁴⁻ states. These results support also a strongly ionic picture of these compounds.

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Riassunto Le bande di valenza dei tiofosfati FePS₃ e NiPS₃ sono state studiate utilizzando la tecnica della fotoemissione risonante. Brusche variazioni dell'intensità di alcune strutture degli spettri di distribuzione dell'energia dei fotoelettroni sono state osservate quando l'energia dei fotoni era variata intorno alle soglie di assorbimento 3p del Fe e del Ni. Invece altre strutture mostravano degli spettri di eccitazione lisci. Le prime sono state associate agli stati 3d del metallo di transizione (fotoemissione diretta e satelliti), le seconde agli stati dei gruppi (P₂S₆)⁴⁻. Infine, i risultati ottenuti confermano il modello fortemente ionico di questi composti.

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Резюме Мы исследовали валентные зоны тифосфатов FePS₃ и NiPS₃ с помощью техники резонансной фотоэмиссии. Некоторые структуры кривых энергетичеких распределений обнаруживают быстрые изменения интенсивности, когда энергия фотона проходит через пороги 3p поглощения Fe или Ni. Напротив, другие структуры обнаруживают плавные спектры возбуждения. Первые структуры связаны с переходом ионов металла в состоянии 3d (прямая фотоэмиссии и сателлиты), вторые структуры связаны с (P₂S₆)⁴⁻ состояниями. Полученные результаты подтверждают сильно ионную модель этих соединений.

     

Pseudostructural disorder in metallic solid solutions: A study of Ag−Cd and Ag−Mg random alloys

Summary We have investigated some Ag−Cd and Ag−Mg alloys within the KKR-CPA framework in order to give insights on their electronic properties. Notwithstanding Cd and Mg have different electronic configurations it appears that their effect on the pure Ag is quite the same. In fact measurements on their optical properties and Fermi surface indicate that most of the spectra and Fermi surface parameters are comparable. In the meantime it also appears that they cannot be classified as rigid band alloys. From the results we obtain it is possible to give sensible explanations to this peculiar behaviour in terms of the angularmomentum channels of the coherent scattering amplitude obtained within the KKR-CPA theory, the main conclusion being thats- andp-like states remain essentially free-electron-like, whiled-like states are very much perturbed by the alloying. The authors of this paper have agreed to not receive the proofs for correction. Work sponsored also by CRRN/SM.     

Charge screening in valence band of solids and in the Fermi sea. Origin of weak-boson masses

Summary An attempt is made to explain the masses of Z and W^± bosons by means of a screening mechanism analogous to charge screening in metals. It considers the effect of the electroweak force on the distribution of negativekinetic-energy neutrinos, which fills the whole universe (neutrino Fermi sea). The large weak-boson masses appear to be a direct consequence of the vanishing neutrino mass.     

Effect of CDW and magnetic interactions on the electrons of the manganite systems

We address a model study which includes the co-existence of the charge density wave (CDW) and ferromagnetic interactions in order to explain the colossal magnetoresistance (CMR) in manganites. The Hamiltonian consists of the ferromagnetic Hund's rule exchange interaction between e_g and t_2g spins, Heisenberg core spin interactions and the CDW interaction present in the e_g band electrons. The core electron magnetization, induced e_g electron magnetization and the CDW gap are calculated using Zubarev's Green's function technique and determined self-consistently. The effect of core electron magnetization and the CDW interaction on the induced magnetization as well as on the occupation number in the different spin states of the e_g band electrons are investigated by varying the model parameters of the system like the CDW coupling, the exchange coupling, the Heisenberg coupling and the external field. It is observed that the induced magnetization exhibits re-entrant behaviour and exists within a narrow temperature range just below the Curie temperature. This unusual behaviour of the e_g band electrons will throw some new insights on the physical properties of the manganite systems.     

Response function of coarsening systems

We calculate analytically the conditions that establish the number of bound states in finite superlattices as a function of the depth, width and separation of the wells. We consider a lattice of -wells and a set of rectangular wells. For this latter case, we show how for finite systems the energy levels already group together in bands separated by gaps. Received 24 April 1998     

The radar system for the exploration of Titan

Summary In the field of the exploration of the Solar System NASA and ESA have jointly planned two cornerstone missions whose aim is the close observation of two largely unknown planetary bodies: Titan (Cassini mission) and comet nuclei (Rosetta mission). Our attention in this paper will be devoted to the first one, however some results of this study can be applied also to the second one. Titan is one of Saturn's moons and is characterized by the presence of a dense atmosphere, opaque for optical sensors. Therefore radar data are the only ones at least theoretically capable of giving relatively detailed informations about the geomorphologic structure of Titan. As a matter of fact the radar cross-section depends on both the dielectric constant of the target by means of the Fresnel reflectivity and on its surface roughness. Moreover if the attenuation is sufficiently low, the radar in principle could be able to detect subsurface discontinuities as well. The above considerations hold true only if a well-suited analytical model accounting for surface backscattering is known. All throughout this paper the radar cross-section of planetary bodies will be evaluated in the case of high-resolution radars. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

Planetary Fourier spectrometer: An interferometer for atmospheric studies on board Mars 94 mission

Summary PFS is a two-channel Fourier spectrometer operating in the infra-red wavelengths between 1.25 and 45 μm. The instrument will be used mainly in the study of the Martian atmosphere. The principal goals are the measurements of the atmospheric temperature and pressure, atmospheric constituents, aerosol and clouds, ground pressure for surface topography, optical and thermophysics properties of the Martian soil. PFS will fly on the Mars 94 spacecraft which should be launched in 1994 and reach the planet in 1995. It is essentially constituted by two different interferometers located in the same box which is divided into two parts. A dichroic placed on the PFS entrance is used to separate the spectral range into two parts, a division needed by the different optical materials which have to be used in each spectral range. The optical layout of the experiment is very compact. Each channel uses two cubic mirrors mounted on an L-structure pivoted on a motor. The motor moves the mechanics and permits the optical-path difference between the arms to be varied. Each interformeter operates in a different spectral range, respectively, between (1.25÷4.8) μm (8000÷2083cm⁻¹) and (6÷45)μm (1666÷220)cm⁻¹). The spectral resolution is 2 cm⁻¹. The entrance aperture area is 30 cm² per channel and the field of view is 2 and 4 degrees. Every measurement lasts about 4 s. The time and, therefore, the relative optical-path difference for the measurement of every point of the interferogram is given by the zero crossings of the interferogram of a reference monochromatic channel at 1.2 μm which uses a laser diode as source. The two interferograms are double-sided and will have 16384 and 4096 points, respectively, corresponding to spectra of 6250 and 1823 useful points. Paper presented at the 6th Cosmic Physics National Conference, Palermo, 3–7 November 1992.     

Disorder induced granularity in an amorphous superconductor

We present experimental evidence that shows that the transport mechanism on the insulating side of the superconductor to insulator transition may be modified by the presence of small “droplets” of non-percolating superconducting regions. From the conductivity point of view, the system behaves as a granular system despite the fact that the underlying structure is essentially amorphous.     

The cooperation between NASA and ASI on the Cassini mission

Summary The updated status of the cooperation between NASA and ASI on the Cassini mission is here presented. This outstanding scientific mission is devoted to study the Saturnian system with a NASA orbiter and a ESA probe. The Italian involvement, managed by the Agenzia Spaziale Italiana (ASI), concerns the design and development of important Cassini Orbiter subsystems. Promotion of the Italian science community's interests and of the national industry's technological capabilities are the rationales for the cooperation. This is an updated version of the contribution presented at San Miniato.     

Renormalization group theory for fluid and plasma turbulence

For the last several decades, renormalization group (RG, or RNG) methods have been applied to a wide variety of problems of turbulence in hydrodynamics and plasma physics. A comprehensive review of this work will be presented, covering RG methods in hydrodynamic turbulence and in turbulent systems with coupled fluctuating fields like magnetohydrodynamic (MHD) turbulence. This review will attempt to specifically consider several questions about RG: (1) Does RG provide an improvement over previous analytical theories like the direct interaction approximation, or is RG a useful simplification of those theories? (2) How are nonlocal, or ‘sweeping’ effects treated in RG formalisms, or are they ignored entirely? (3) Can RG theories treat both local and nonlocal interactions in turbulence?     

tetracene 分子在Ru(1010)表面的吸附结构及其电子态研究

利用紫外光电子能谱(UPS)、角分辨紫外光电子能谱(ARUPS)和扫描隧道显微镜(STM)等方法研究了tetracene分子在Ru(1010)表面上吸附的电子态，吸附位置和吸附取向.UPS实验显示，与tetracene分子有关的光电子谱峰在费米能级以下2.1, 3.5, 4.8, 6.0, 7.1和9.2 eV处；ARUPS 结果表明，tetracene分子的分子平面基本平行于衬底表面；从STM图像中可以看到tetracene分子的长轴沿［0001］和［1210］两个晶向.基于密度泛函理论的从头算计算证实了上述结论.当分子长轴沿［0001］晶向时，分子中心位置在衬底表面的“短桥位”上，当分子长轴沿［1210］晶向时，分子中心位置在衬底表面的“四原子中心空位”上. 关键词： tetracene分子 Ru(1010)表面 吸附结构 吸附电子态     

Effects induced on the transverse-spatial-impulse response of an inhomogeneous photoreceptor with a nonsymmetric refractive index profile and arbitrarily located origin

Summary In a few earlier communications, spatial-transverse-impulse response (STIR) associated with the initial field transversely distributed across the entrance pupil of an inhomogeneous optical wave guide (photoreceptor) has been investigated. In the present work, effects of indetermination in the location of the origin on an arbitrarily defined refractive index profile, representing the degree of inhomogeity at the aperture pupil of a single receptor, have been studied. Some consequences related to a few possible technical applications have been discussed. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Density of states and extent of wave function: two crucial factors for small polaron hopping conductivity in 1D

Abstract

We introduce a theoretical model to scrutinize the conductivity of small polarons in 1D disordered systems, focusing on two crucial – as will be demonstrated – factors: the density of states and the spatial extent of the electronic wave function. The investigation is performed for any temperature up to 300 K and under electric field of arbitrary strength up to the polaron dissociation limit. To accomplish this task, we combine analytical work with numerical calculations.     

Renormalization and resummation in the O(N) model

In the O(N) model for the large N expansion one needs resummation which makes the renormalization of the model difficult. In the paper it is discussed, how can one perform a consistent perturbation theory at zero as well as at finite temperature with the help of momentum dependent renormalization schemes.     

Electronic and magnetic structure of the undoped two and three-leg ladder cuprates in an itinerant electrons model

We study the electronic and magnetic structure of the undoped ideal two and three-leg ladder cuprates by assuming a moderate on site coulombic repulsion. This analysis is an extension of the Fermi liquids studies proposed for the CuO₂ plane in view to explain the high T_c superconductivity and the competition with the antiferromagnetic phase. At zero doping, the quasi-one-dimensionality of the structure results in SDW correlations with different (commensurate) vectors according to the number of legs, which contrasts with the predictions made from the Heisenberg model. At mean field, and for n = 3 (Sr₂Cu₃O₅), we predict a magnetic ordered state, detected by μSr and NMR measurements with critical temperatures consistent with our assumptions on the physical parameters, the modulation vector being .The presence of several bands at the Fermi level explains why there is no observable gap in the static susceptibility measurements. For n = 2, we predict a gap consistent with experimental Curie susceptibility. But the expected magnetic instability is detected only in La₂Cu₂O₅, where the interladder coupling is stronger. In every case the one-dimensional van Hove singularities are far from the Fermi level, making difficult the obtaining of high T_c superconductivity. Received 3 June 1998     

Critical properties of the metal-insulator transition in anisotropic systems

We study the three-dimensional Anderson model of localization with anisotropic hopping, i.e., weakly coupled chains and weakly coupled planes. In our extensive numerical study we identify and characterize the metal-insulator transition by means of the transfer-matrix method. The values of the critical disorder obtained are consistent with results of previous studies, including multifractal analysis of the wave functions and energy-level statistics. decreases from its isotropic value with a power law as a function of anisotropy. Using high accuracy data for large system sizes we estimate the critical exponent as . This is in agreement with its value in the isotropic case and in other models of the orthogonal universality class. Received 25 October 1999     

c(2×2)O吸附Cu(001)表面结构、电子态与STM图像的研究

用投影子缀加波和CP分子动力学方法研究了贵金属Cu(001)面的表面结构、弛豫以及O原子的c(2×2)吸附状态. 研究结果得出在这种吸附结构中，O原子与衬底Cu原子之间的垂 直距离约为0069nm，Cu—O键长为0.194nm，功函数约为5.29 eV；吸附O原子形成金属性能带结构，由于Cu—O的杂化作用,在费米能以下约6.7 eV处出现了局域的表面态.用Tersoff-Hamann途径计算了该表面的扫描隧道显微镜图像，并讨论了与实验结果之 间的关系. 关键词： Cu(001)-c(2×2)/O 电子态 STM图像