S 7 andŠ 7

We investigate the seven-sphere as a group-like manifold and its extension to a Kac-Moody-like algebra. Covariance properties and tensorial composition of spinors underS ⁷ are defined. The relation to Malcev algebras is established. The consequences for octonionic projective spaces are examined. Current algebras are formulated and their anomalies are derived, and shown to be unique (even regarding numerical coefficients) up to redefinitions of the currents. Nilpotency of the BRST operator is consistent with one particular expression in the class of (field-dependent) anomalies. A Sugawara construction is given.     

Structure of Si(111)−7 × 7

A model of the Si(111)?7 × 7 surface atomic arrangement is put forward on the basis of results already established for Si(111) and Si(100) surfaces. The unit mesh contains a triangular double-layer island with 21 first-layer atoms. The island is laterally expanded and is bounded by [1̄1̄2] steps with second-layer edge atoms forming asymmetric dimers. It is shown that salient features of low energy electron diffraction (LEED) patterns for Si(111)?7 × 7 can be explained by the model. The LEED patterns are interpreted qualitatively by a double-diffraction mechanism involving forward diffraction in the selvedge. It is shown that the patterns contain characteristic formations of fractional-order spots attributable to the dimers at the island boundaries. The best agreement with observed patterns is obtained with the following parameter values: dimer bond length 2.5 ± 0.2 Å, island lateral expansion 3 ± 2%. Some of the implications of the model for the chemical reactivity and electronic properties of the Si(111)?7 × 7 surface are discussed.     

STM study of low pressure adsorption of silane on Si(111)7 × 7

We report a study of silane adsorption on the Si(111)7 × 7 surface. We have been interested in the first stages of chemisorption at room temperature. Reactive sites of the unit cell have been clearly identified on Scanning Tunneling Microscopy (STM) images: the reaction involves the rest atom and the adjacent adatom of the DAS structure with preferential adsorption on the center adatom. We propose an original chemisorption mechanism which leads to the formation of two SiH₂ species by chemisorption and involves the breaking of Si---Si backbonds of the adatom.     

Nucleation behaviour during silicon UHV-CVD on Si(111)7×7

We report a UHV-CVD homoepitaxy study on the Si(111)7×7 surface investigated with Scanning Tunnelling Microscopy (STM). We have investigated the two-dimensional island density in the temperature range from 450 to 550°C and silane pressure range from 2 to 8×10⁻⁴ Torr. Contrary to experiments using molecular beam epitaxy, we find that the two-dimensional island density in UHV-CVD cannot be directly described by the standard nucleation theory. We discuss this point and show that the variation of the steady-state hydrogen coverage on the surface during pressure- or temperature-dependent experiments can explain the observed two-dimensional island density behaviour.     

7-氨基喹唑啉的合成

7-氨基喹唑啉是合成杂环药物的重要中间体。为了寻找经济的合成7-氨基喹唑啉的方法,本文以Pd/C和Zn粉共同作用还原4-氯-7-硝基喹唑啉制备7-氨基喹唑啉,产率达到52%。比文献报道的产率略高,最关键的是与文献报道的直接用Pd/C还原的方法相比较,此法更经济。     

Contents of volume 7

Molecular Diversity -     

Estimating the lifetime of R7T7 glass in various media

Fission product containment glasses were the first matrices for which long-term behavior studies were undertaken. A substantial body of results has been obtained over the last twenty years, international scientific cooperation programs have been developed, and a strict methodology has progressively been defined to predict the behavior of these materials on a time scale of several tens of thousands of years. The excellent self-irradiation resistance of the French ‘R7T7’ glass has been demonstrated. Models of aqueous alteration under optimized but realistic geological disposal conditions suggest a glass package lifetime of several million years. To cite this article: É.Y. Vernaz, C. R. Physique 3 (2002) 813–825.     

Investigation of7Be(n,p)7Li reaction

The⁷Be(n, p)⁷Li nuclear reaction with thermal and 2 keV neutrons was studied. For thermal neutrons the total cross section of (46 800± 4 000) b was obtained and the proton branching ratio to the first excited and ground states of⁷Li was determined to be ₁/₀ = (2±1) %. For 2 keV neutrons the upper limit of the cross section of 540 b was estimated.Dedicated to Academician Václav Votruba on the occasion of his eightieth birthday.The autors thank Mr. A. Dvoák and Mrs. B. Michalcová for their manufacturing the⁶Li standards, Dr. J. Frána for his help in determining the number of⁷Be atoms and Ing. J. Hoffmann, Mr. J. Novotný and Mr. J. Badura for their careful preparation of electronics.     

The ν7 vibrational states of C3O2: A least-squares fit of the rigid-bender model to the v7ν7l7 energies and rotational constants

The rigid-bender model is used to treat the large-amplitude, low-frequency, bending vibration ν₇ of C₃O₂. Different parameterizations of the bending potential function are considered, and a simple two-term power series is found to give the best fit. With this parameterization, using a least-squares fit to energies and B values, the ν₇ potential function is determined for the ground state as well as for the states in which ν₂, ν₃, ν₄, ν₆, 2ν₆, ν₁ + ν₃, ν₁ + ν₄, ν₂ + ν₃, and 2ν₂ + ν₄ are excited. The excitation of other vibrations has in some cases a drastic effect on the ν₇ potential. In the ground state the potential has a 29 cm^?1 barrier at the linear position, in ν₁ + ν₃ the barrier increases to 79 cm^?1, while in 2ν₂ + ν₄ the barrier vanishes. An equilibrium potential is determined by correcting the ground state potential for the effects of zero-point motion of the normal vibrations ν₁, …, ν₆. This potential has a 35.6-cm^?1 barrier with a minimum at α = 11.14°, where 2α is the angular deviation from linearity. The model accurately predicts the quartic and sextic centrifugal distortion terms for the low-lying v₇ν₇^l₇ states. Second-order l-type coupling is included in the calculations of the quartic terms. The effects of this coupling, which are most pronounced for the ν₇ ≥ 2 states, adequately explain the negative D term recently reported for the ν₂ + 4ν₇⁰ state.     

Symmetrization of the trimer model for Si(111) 7×7

A symmetrized extension of the trimer model for the Si(111) 7×7 structure is presented. It exhibits new features such as sixfold coordinated Si atoms which are not unusual in Si chemistry but have not been considered in previous structural models. Atomic coordinates are optimized by a Keating-type strain energy minimization and the structure factor for transmission electron diffraction is calculated.