Theoretical and experimental aspects of excited-state energy transfer between chiral donors and chiral acceptors

Department of Chemistry, Michigan Technological University, Houghton, MI 49931. Published in Zhurnal Prikladnoi Spektroskopii, Vol. 62, No. 5, pp. 87–93, September–October, 1995.     

Hall-Effekt an Protonen in Eis

Zusammenfassung Eine Hall-Effekt-Messung an Eis gelingt nur, wenn der Einflu? der Oberfl?chenleitung berücksichtigt wird. Leitf?higkeitsmessungen mit Schutzring-Elektroden erlaubten eine getrennte Bestimmung von Oberfl?chen- und Volumen-Strom. Die Temperaturabh?ngigkeit der Volumenleitung entspricht einer Aktivierungsenergie von ∼ 8,5 kcal/mol. Die bisher ver?ffentlichten, durchwegs h?heren Werte lassen sich aus der st?rkeren Temperaturabh?ngigkeit des Oberfl?chenstroms erkl?ren, der oberhalb −20°C die Messung beeinflu?t. Unter Berücksichtigung der Oberfl?chenleitung ist es nun gelungen, den Hall-Effekt an Eis festzustellen und zu messen. Es wurde ein positives Vorzeichen des Hall-Effekts gefunden und damit gezeigt, da? die Protonen-Beweglichkeit wesentlich h?her als jene der OH⁻-Ionen ist. Die Beweglichkeit w?chst mit fallender Temperatur und betr?gt 1,4 cm²V⁻¹sec⁻¹ bei −8°C. Auf dieses Verhalten der Beweglichkeit kann unabh?ngig davon auch aus der Temperaturabh?ngigkeit der Leitf?higkeit geschlossen werden. Der relativ hohe Absolutwert der Beweglichkeit deutet an, da? man das Proton nicht als quasifreien Ladungstr?ger betrachten kann. Ein einfaches Modell beschreibt den Leitungsmechanismus mit den Parametern Sprungzeit eines korrelierten Protonensprungs über mehrereH-Brücken, Wartezeit zwischen aufeinander folgenden Sprüngen und effektiver Masse.

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Surface conductivity has to be taken into account for a successful Hall-Effect measurement in ice. Bulk and surface currents have been determined separately by guard-electrode techniques. The apparent thermal activation energy of the bulk conductivity is ∼ 8,5 kcal/mole. Published values are invariably higher. This can be explained by the strong temperature dependence of surface currents. A contribution due to surface currents has to be considered above −20°C. By taking into account the influence of surface conductivity it has now proved possible to detect and to measure the Hall-Effect in ice. A positive sign of the Hall coefficient was found. This result provides evidence for a protonic mobility considerably higher than the mobility of OH⁻-ions. The mobility decreases with increasing temperature. The value is 1,4 cm²V⁻¹sec⁻¹ at −8°C. The relatively high value of the mobility seems to indicate that the proton cannot be considered as a quasi-free charge carrier. A simple model characterizes the conductivity mechanism in terms of the time of correlated proton jumps across severalH-bonds, the time between consecutive jumps, and the effective mass.

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Résumé Une mesure de l'effet Hall dans la glace n'est possible que si l'on tient compte de l'influence de la conduction de surface. Des mesures de le conductibilité effectuées avec des électrodes de garde ont permis l'évaluation séparée des courants de surface et de volume. La variation en température de la conduction de volume correspond à une énergie d'activation de 8,5 kcal/mole environ. Les valeurs publiées jusqu'à présent sont toujours plus grandes, ce qui peut s'expliquer par l'influence prédominante de la température sur le courant de surface entre −20 et 0°C. En tenant compte de la conduction de surface on a réussi à observer et à mesurer l'effet Hall dans la glace. En trouvant un signe positif de l'effet on a montré que la mobilité protonique est considérablement plus grande que celle des ions OH⁻. La mobilité augmente lorsqu'on abaisse la température; sa valeur à −8°C est de 1,4 cm²V⁻¹sec⁻¹. Cette valeur relativement grande suggère que le proton ne peut pas être considéré comme un porteur de charge quasi libre. Un modèle simple décrit le mécanisme de conduction à l'aide des paramètres suivants: 1. Durée du saut corrélé d'un proton le long de plusieurs liaisons hydrogène. 2. Temps d'attente entre sauts successifs. 3. Masse effective.

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Auszug aus der von der Fakult?t für Allgemeine Wissenschaften der Technischen Hochschule München genchmigten Dissertation über „Hall-Effekt-Messung an protonenleitenden Eis-Einkristallen” des Dipl.-Phys.Bernhard Bullemer.     

Two connected inverse spectral transforms

We establish a transformation which connects the potentials of the one-dimensional Dirac and Klein-Gordon operators. This transformation links the solutions of the nonlinear evolution equations solvable by means of the two inverse spectral transforms which use the Dirac and Klein-Gordon direct and inverse spectral problems.     

Facile separation of enantiomers,geometrical isomers,and routine compounds on stable cyclodextrin LC bonded phases

The effectiveness of employing stationary phases composed of chemically bonded cyclodextrin molecules in the high performance liquid chromatographic separation of a variety of different types of compounds is summarized. Over one hundred compounds, including optical, geometrical, and structural isomers, diastereomers, and epimers were successfully separated from each other via use of beta- or gamma-cyclodextrin bonded phases and aqueous methanolic mobile phases. The mechanism of separation is based upon inclusion complex formation between the compounds being separated and the cyclodextrin molecules bonded to the stationary phase. The effects of temperature, mobile phase composition and flow rate upon the chromatographic selectivity and resolution are described. The results indicate that the cyclodextrin columns may be more versatile, flexible, and effective compared to the conventional normal or reversed phase columns.     

The spectral function for Sturm-Liouville problems where the potential is of Wigner-von Neumann type or slowly decaying

We consider the linear, second-order, differential equation

(∗)     

Pulse duration and wavelength effects on the optical limiting behavior of carbon nanotube suspensions

We investigate pulse duration and wavelength effects on the optical limiting behavior of single-wall carbon nanotubes suspended either in chloroform or in water. The principal optical limiting effect in carbon nanotube suspensions is nonlinear scattering that is due to heat transfer from particles to solvent, leading to solvent-bubble formation and to sublimation of carbon nanotubes. We report on nonlinear transmittance measurements for pulse durations ranging from 3 to 100 ns and for wavelengths from 430 to 1064 nm. The dependence of optical limiting behavior on pulse duration and wavelength is analyzed and discussed in terms of nonlinear mechanisms.     

Die Züchtung von KCl-Einkristallen mit bekanntem Tl-Gehalt und die Bestimmung des Verteilungskoeffizienten

A modifiedCzochralski method for the growth of single crystals is described. This technique permits controlled doping of the crystal with easily volatile additives. The impurity distribution within the crystal is calculated. Analysis of crystal and remaining melt permits determination of the distribution coefficient. — Pure single crystals of KCl quite homogeneously doped with Tl in a range of molar concentration from 4.10^?8?1.10^?4 were grown by this method. Their Tl content was determined radiochemically to within 3%. The distribution coefficient was found to bek=0.31±10% throughout the range of concentration under investigation. It is shown that the effect of evaporation may not be neglected.