ALBERT

Notes: The thermal lensing characteristics of several silicate, germanate, phosphate, and borate glasses were studied using a laser with a 7 ns pulse at 457 nm in a tight focus geometry. A geometric model was developed to describe the quadratic radial profile of the refractive index resulting from the laser-induced temperature profile. This model was utilized to interpret the effects of some of the relevant experimental parameters on the fluence transmission experiments. The influence of material properties such as different types of network former and modifier ions on the nonlinear optical properties of these materials were also studied. It was found that: (i) the greatest influence of the network modifier ions was due to their effect on the absorption coefficient of the glasses; (ii) in lead glasses, the thermo-optic coefficients dn/dT of the germanates and silicates with random network structures were greater than those of the borate and phosphate glasses with ring and chain structures; and (iii) the main contribution to the thermo-optic coefficient comes from the thermally induced changes in the electronic polarizability of the glass components. In these glasses, the oxygen polarizability provides the dominant contribution and is affected by the variations in the polarizing power (charge to radius-squared ratio) of the network former ions.

Notes: In this paper we investigate the properties of near-critical mixtures as solvent media, with particular regard to the critical azeotrope. The results from Part I are used in conjunction with integral equation theory calculations to demonstrate that the critical azeotrope has a number of unique properties as a solvent medium. Volumetric and thermal effects related to the solvation of dilute solutes are greatly enhanced and these can be related to higher order divergences of these properties near a critical azeotrope. Finally, we consider synergistic solvent effects near a mixture critical point, showing that solvent mixtures exhibit cooperative behavior that affect solvation thermodynamics in interesting ways that have been little discussed to date.

Notes: In this series of papers we provide new results related to critical phenomena in binary mixtures particularly with regard to the phenomenon of critical azeotropy. In Part I we carefully develop a set of necessary and sufficient conditions guaranteeing the existence of critical azeotropy in these mixtures. The sufficiency conditions are particularly useful for determining whether or not a critical azeotrope exists in a system, without having to actually find this point itself, typically a difficult experimental problem. Classical critical exponents of various thermodynamic properties along various paths are determined for the critical azeotrope. These are compared to the analogous situation near a pure fluid critical point in a dilute binary mixture. Both situations display analogies as well as differences that are discussed at some length. In Part II we concentrate upon describing interesting synergistic solvation effects in these systems using many of the results presented in Part I.

Notes: The properties of permanent, laser induced refractive index gratings in rare-earth doped glasses have been characterized in a variety of materials including lithium borates, lead and magnesium silicates, lead borate-germanates and lead germanates. This work is combined with the previous results obtained on phosphate and silicate glasses to develop an understanding of the effects of glass composition (both network former and modifier ions) on the ability of a material to produce gratings with high scattering efficiencies. The grating efficiencies and the writing/erasing dynamics were studied, and the two-level-system-model developed previously was employed to explain the results. The results show the importance of having flexible glass networks with low polarizing power of the network former ions, and a high content of nonbridging oxygen ions. In addition, it is important to have light, highly polarizable modifier ions with weak chemical bond strengths to the oxygen ions. This type of glass combined with rare-earth ions that like to exist in more than one local coordination environment and that couple strongly to high energy vibrational modes of the host, are found to be very effective in producing local structural changes that lead to room-temperature-stable refractive index gratings.

Notes: In this short note we describe a novel method, based upon the Kirkwood equation, for doing free energy simulations in asymmetric molecular systems typical of those encountered with supercritical fluid mixtures involving large organic species solvated in a lighter supercritical gas. The rationale behind this modified Kirkwood approach has been to develop simulation procedures that are both accurate and stable, by minimizing the perturbation to the system that is implicit to all perturbation-based simulation methods. A good example of an algorithm in this latter class of methods is the free energy perturbation method (FEP) which has been used in this work for comparative purposes with our proposed technique which we refer to as the modified Kirkwood coupling method (MKC). The MKC approach described here effectively makes the perturbation process in the simulation process vanishingly small and appears to result in a stable, accurate algorithm for free energy calculations in chemical mixtures. Numerical results supporting these conclusions are provided in two Lennard-Jones model systems at supercritical conditions.

Notes: Field studies were conducted in 1988 and 1989 to determine the phenology and growth of broomrape in sunflower as affected by crop sowing dates. The parasitic weed attachment was observed in sunflower plants with 6–7 leaves; later it was found throughout the vegetative and flowering period of the crop. O. cernua seedlings with shoot development extensive but subterranean were mainly observed at the early heading stage of sunflower. At this stage they were beginning to reduce sunflower plant growth as compared with non–parasitized plants. The emergence of broomrape began at the early flowering stage of the crop and was particularly abundant throughout this and the achene–filling stage. Generally, broomrape infection was much less at the first than at the second or third sunflower sowings, and caused less reduction in yield.The duration of the broomrape emergence period increased as sowing date was delayed. Since sunflower yield was higher at earlier sowing dates, early sowing may be recommended for sunflower crops in broomrape–infested areas under mild mediterranean climates. Effet de la date de semis sur les infections d'orobanche (Orobanche cernua Loefl.) en culture de tournesol (Helianthus annuus L.)Des études de plein champ ont été conduites en 1988 et 1989 pour déterminer comment la phénologie et la croissance des orobanches en culture de tournesol étaient affectées par les dates de semis de la culture. Le rattachement de 1'adventice parasite a été observeé chez les plantes de tournesol avec 6–7 feuilles; plus tard il était observé tout au long des périodes végétatives et de floraison de la culture.Des plantules d'O. cernua avec un développement de la tige important mais souterrain ont été principalement observées au stage bourgeon précoce du tournesol. A ce stade elles ont commencé & réquire la croissance du tournesol en comparaison avec des plantes non parasitées. La levée de 1'orobanche a commencé au début du stade de la floraison de la culture et a été particuliérement abondant pendant celle–ci et le stade remplissement des akénes. Généralement, l'infection d'orobanche a été beaucoup moins forte au premier qu'au second et troisiéme semis et a causé des baisses de rendement plus faibles.La durée de la période de levée des orobanches a augmenté quand le semis a été retardé. Puisque le rendement en tournesol a été plus élevé pour les dates de semis précoces, un semis précoce peut être recommandé pour le tournesol en zones infestées par les orobanches sous les climats méditerranéens. Die Bedeutung des Saattermins der Sonnenblume (Helianthus annuus L.) für den Befall mil der Sommerwurz (Orobanche cernua Loefl.) In Feldversuchen wurden 1988 und 1989 die PhÄnologie und das Wachstum der Sommerwurz in Sonnenblumenkulturen untersucht. Der Befall der Sonnenblumenpflanzen wurde im 6- bis 7-Blatt-Stadium festgestellt, aber auch später in der gesamten vegetativen und generativen Phase der Kulturpflanze. Keimpflanzen der Sommerwurz mit einer kräftigen, aber unterirdischen Sproßentwicklung wurden überwiegend im frühen Stadium der Blütenentwicklung der Sonnenblume beobachtet. In diesem Stadium begann die Wachstumshemmung der Sonnenblume. Zu Beginn der Blüte der Sonnenblume lief die Sommerwurz auf und war während dieses Entwicklungsstadiums und der folgenden Samenbildung besonders häufig anzutreffen. In der ersten Aussaat der Sonnenblume war der Befall allgemein geringer als in der zweiten oder dritten. Mit späterem Saattermin verlängerte sich die Keimungsperiode der Sommerwurz. Nachdem die Sonnenblume bei früherer Saat einem höheren Ertrag brachte, wird für mild-mediterrane Gebiete mit Befall von Sommerwurz eine frühe Aussaat empfohlen.