Showing papers by "École Polytechnique Fédérale de Lausanne published in 1973"

The electronic structure of GaSe

Abstract: The electronic band structure of the layer compound GaSe was calculated with the empirical pseudopotential method. Corrective terms had to be added to the spherical atomic potentials in order to account for the weak van der Waals coupling between layers. It was possible within this approximation to evaluate the effective masses of electrons and holes, both parallel and perpendicular to the layers. The experimentally observed strong dependence of the optical absorption on the polarization of the light is well explained by the present band structure. With the pseudo-wave-functions the charge densities were calculated for several groups of bands. They allow a detailed discussion of the chemical bonding in GaSe.

The band-gap excitons in gallium selenide

Abstract: Absorption, and reflexion spectra of GaSe near the fundamental gap are interpreted in terms of pseudopotential band calculations. Selection rules for the direct optical valence-to-conduction band transitions are derived. Valence band mixing induced by spin-orbit coupling is invoked to explain the low observed probability for transitions in light polarized perpendicular to the crystal c-axis. The spectra of the excitons associated with the direct gap are discussed in the ellipsoidal effectivemass approximation. Corrective terms are added to account for the observed exchange splitting of the exciton ground state. Field-free spectra as well as spectra modified by the presence of magnetic fields parallel and perpendicular to c are considered. The magneto-Stark effect which gives rise to a mixing of the 2s and2py states and thus renders the2py state visible affords determination of the anisotropy parameter. The value of this parameter as well as those of the components parallel and perpendicular to c of the reduced effective masses show that the electronic states in GaSe are nearly isotropic. This is in good agreement with the results of the pseudopotential band calculations which clearly demonstrate the three-dimensional character of valence and conduction bands.

Model studies on mechanics of jointed rock

Abstract: A generalized physical model is tested triaxially in order to relate properties of the intact material and of the discontinuities to the strength and deformability of a jointed rock mass. The behavior of jointed models can be divided into three characteristic zones. At low confining stresses there is either sliding along preexisting joints or brittle failure through intact material and across joints with strength depending on the number of intersected joints. At intermediate confining stressed sliding or failure may occur again, but the failure mode changes from brittle to ductile. At high confining stresses no sliding and only ductile failure occurs. The three zones and the upper and lower bounds of strength are easily obtained by a few confined compression tests on the intact material, by sliding tests along joints, and by a direct tension test. If this characteristic behavior is valid for other materials, as seems likely because of the basically similar underlying mechanism, it may become possible to determine the behavior of a jointed rock mass by relatively simple experiments.

Progress Report on Dolomitization — Hydrology of Abu Dhabi Sabkhas, Arabian Gulf

Abstract: The authors evaluate the dolomitizing potential of three hydrologic systems — “seepage reflux” (Adams & Rhodes, 1960), capillary action (Friedman & Sanders, 1967), and “evaporative pumping” (Hsu & Siegenthaler, 1969). Laboratory experiments demonstrate the efficiency of the latter process as a potential mechanism of water flow necessary for dolomitization, and show that this flow is not due to capillary forces but to reduction in pore pressure due to evaporation of interstitial vapour.

The kinetics of yeast growth on pure hydrocarbons

Abstract: The kinetics of C. tropicalis growth were investigated with pure n-hexadecane as dispersed phase substrate. Two distinct growth phases were found: In the first phase, exponential growth was independent of stirrer speed. The onset of the second phase, one of linear growth, was determined by stirrer speed. By the use of two different fermenter types, it was shown that the drop size of the dispersed phase was not primarily responsible for the observed kinetics. It was considered that the formation of biological flocs determined the observed growth pattern. This was substantiated by the results of continuous cultures in the different fermenter types, with various substrate concentrations.

Systeme de poids sur une algebre de Lie nilpotente

Abstract: Let g be anilpotent Lie algebra (of finite dimensionn over an algebraically closed field of characteristic zero) and let Der(g) be the algebra of derivations of g. Thesystem of weights of g is defined as being that of the standard representation of a “maximal torus” in Der(g) (see l.l). For a fixed integern, it is well-known that there are in general uncountably many isomorphism classes of nilpotent Lie algebra of dimensionn; but we show that there arefinitely many systems of weights, and each of them is explicitely constructed. The class of those Lie algebras having a given (arbitrary) system of weights is also studied.

Vertical eddy diffusion coefficient in Lake Zürich

Abstract: The vertical eddy diffusion coefficient in Lake Zurich is a strong function of both depth and season. In the deep lake, the annual oxygen consumptions by the oxidation of organic carbon is balanced out by the equal amount of oxygen supply from above by diffusion.

Wettability of copper and polytetrafluoroethylene surfaces with water—the influence of environmental conditions

Abstract: Contact angles of water at its boiling point, measured on surfaces of copper and polytetrafluoroethylene, are reported for pressures ranging from 10–770 torr. The results suggest that not only impurities and oxidation have been responsible for the wide range of angles reported in the literature but that inconsistencies may be created because of an adsorbed layer of water vapour on the solid surface.

Asymmetric-selective polymerization of DL-leucine N-carboxy anhydride with optically active amines

Abstract: DL-Leucine N-carboxy anhydride was polymerized in dioxane, benzene, tetrahydrofuran, butanone, and N,N-dimethyl formamide at temperatures between 10 and 55°C using 15 optically active primary and secondary amines as initiators. The optical yield of polymer can be described satisfactorily by an ideal copolymerization equation for monomer conversions between 3 and 94% (R = const). The highest R was found for the initiation with (—)-L-proline methyl ester in dioxane at 25°C (R = 1,30). Ten different models are shown to fit the ideal copolymerization equation depending on whether homopropagation, heteropropagation, homoadsorption, and/or heteroadsorption is taken into account. Most reasonable on the basis of other published evidence seems to be a pure or by far dominating homopropagation (kLL = kDD; kLL[P*L] > kDL[P*D]; kDD[P*D] > kLD[P*L]) with a constant concentration of active chain ends with D- and L-configurations, ([PL*] = const; [PD*] = const′; [PL*]/[PD*] ≠ 1) resp. The present nomenclature in this field is discussed. DL-Leucinanhydrid wurde in Dioxan, Benzol, Tetrahydrofuran, Butanon und Dimethylformamid bei Temperaturen zwischen 10 und 55°C mit 15 optisch aktiven, primaren und sekundaren Aminen als Initiatoren polymerisiert. Die optische Ausbeute der Polymeren kann zufriedenstellend fur Umsatze zwischen 3 und 94% durch eine ideale Copolymerisationsgleichung beschrieben werden (R = const). Das hochste R (=1,30) wurde fur die Initiierung mit (—)-L-Prolinmethylester in Dioxan bei 25°C gefunden. Die ideale Copolymerisationsgleichung kann fur 10 verschiedene Modelle abgeleitet werden, je nach den Annahmen uber Homowachstum, Heterowachstum, Homoadsorption und/oder Heteroadsorption. Am wahrscheinlichsten erscheint nach allen Befunden ein reines oder zumindest stark uberwiegendes Homowachstum (kLL = kDD; kLL; kLL[PL*] > kDL[PD*] und kDD[PD*] > kLD[PL*]) mit einer konstanten Konzentration aktiver Kettenenden in der D- und L-Konfiguration ([PL*] = const; [PD*] = const′; [PL*/PD*] ≠ 1). Die Nomenklaturvorschlage auf diesem Gebiet werden diskutiert.

The effect of applied force on drainage of the film between a liquid drop and horizontal surface

Abstract: The effect of applied force, drop volume, and physical properties on the rate of thinning of the draining films above and below a drop trapped between two horizontal surfaces has been examined theoretically and experimentally. The variation in film thickness with position and time has been obtained photographically and from capacitance measurements. The observed rate of film thinning may be predicted by the simple uniform film model with one immobile surface and, surprisingly, decreases when the applied force and drop volume increases.

High resolution luminescence spectrometer. 1: simultaneous recording of total luminescence and phosphorescence.

Abstract: A luminescence spectrometer is described that has the particular advantage of allowing the simultaneous recording of total luminescence and phosphorescence spectra. The basic design is straightforward and uses commercially available components. The measuring and recording system is completely digital. The technique of single-photon counting is employed, and the data are recorded digitally in the form of listings and punched paper tapes. This output is suitable for further data analysis on a computer. An additional analog recording system allows an instant examination of the quality of the incoming data.

Thermische Zerfallsreaktionen von Nitroverbindungen in Stosswellen. II: Dissoziation von Nitroäthan

Abstract: A shock wave study of the thermal decomposition of nitroethane in excess Ar at temperatures 900 < T < 1350 K and total concentrations of 4,5 · 10−6 < [Ar] < 3 · 10−4 mol cm−3 showed that the CN-bond fission is the primary reaction step. This unimolecular reaction could be observed in its transition region near the high pressure limit. The derived rate constants are k∞ = 1015.9 exp (–57 kcal mol−1/RT) s−1 for the high pressure and k0/[Ar] = 1018.0 exp (-36 kcal mol−1/RT) cm3 mol−1 s−1 (at T ≃ 1100–1200 K) for the low pressure limit. The observed concentration profiles of C2H5NO2 and NO2 permitted to conclude on the subsequent decomposition of the ethyl radical This reaction was found to be in the fall-off range under the applied conditions.