Single-pulse collision-insensitive picosecond planar laser-induced fluorescence of OHA 2 Σ + (v′ = 2) in atmospheric-pressure flames

Single-pulse two-dimensional picosecond Laser-Induced Fluorescence (LIF) imaging of the OH density in a single quantum state was performed for the first time, using a premixed methane-oxygen flame at atmospheric pressure. A picosecond, excimer-Raman-laser system (268 nm, 470 ps FWHM) was used for excitation of OH. The fluorescence from the laser sheet was imaged onto a fast gated intensified camera with a 400 ps gate width. The short laser pulse minimizes the collisional redistribution of population in the ground state during excitation, while the short camera gate avoids significant quenching of the excited-state fluorescence. The fluorescence signal obtained in this way is a direct measure of the population in a selected quantum state. In contrast to common nanosecond LIF signals no corrections on variations of the collisional environment are necessary. This collision-insensitive approach to two-dimensional LIF yields an OH detection limit of 10 ppm in a cube of 330 µm per side with a single 1 mJ laser pulse. A rate-equation model is used to estimate the effects on the observed signal of fluctuations in pulse energy and duration, laser-camera timing jitter, and spatial variations in the collisional environment.     

Selective guest encapsulation by a cobalt-assembled cage molecule

Metal-assembled resorcinarene-based cages enclose space and entrap organic molecules from water. Addition of cobalt(II) ions to a neutral, aqueous solution of a resorcinarene that has iminodiacetic acids attached to its upper rim results in the formation of cages. These cages not only entrap organic molecules, but they do so in a selective manner. Guests with optimum size, shape, and polarity are preferentially entrapped. For example, selection of p-xylene is twenty thousand times more favorable than that of m-xylene. The enthalpy of resorcinarene deprotonation and cage formation was calculated by performing calorimetry studies and ranged from -305 to -348 kJ mol(-1). The change in enthalpy of guest encapsulation varied by as much as 43 kJ mol(-1). The differences in change in free energy of guest encapsulation varied by -16 kJ mol(-1). The changes in enthalpy and free energy of guest encapsulation were used to calculate the changes in entropy, which ranged from -97 to +37 J mol(-1) K(-1). An enthalpy-entropy compensation of guest encapsulation was observed.     

Perturbation theory for excited states of molecules. I

The problem of perturbations of excited states is discussed and three methods are developed. The first of these uses a zero order wave-function made up of a linear sum of singly excited SCF configurations, whereas the second uses just one of these configurations. The third method is restricted to small -systems, the zero order wave-function being a linear sum of all possible determinants formed from the basis set used. The perturbations considered here are one-electron operators. Example calculations are performed on the butadiene molecule within the context of the -electron approximation.

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Zusammenfassung Für das Problem einer Störung von Einelektronen-Operatoren für angeregte Zustände werden drei Verfahren vorgeschlagen: Erstens die Verwendung einer Zustandsfunktion nullter Ordnung, die eine Linearkombination einfach angeregter SCF-Konfigurationen ist, zweitens die entsprechende Verwendung nur einer ausgewählten Konfiguration. Drittens läßt sich, wenn auch nur bei kleinen -Elektronensystemen, als nullte Näherung eine Linearkombination von allen möglichen angeregten Konfigurationen verwenden. Als Beispiel wird das -Elektronensystem des Butadiens gewählt.

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Résumé Discussion du problème de perturbation pour les états excités et développement de trois méthodes. La première utilise une fonction d'ordre zéro combinaison linéaire de configurations SCF monoexcitées, alors que la seconde utilise seulement une de ces configurations. La troisième méthode est restreinte à de petits systèmes , la fonction d'onde d'ordre zéro étant une combinaison linéaire de tous les déterminants construits dans la base utilisée. Les perturbations envisagées ici sont constituées par des opérateurs monoélectroniques. La molécule de butadiène sert d'exemple dans le cadre de l'approximation à électrons .

     

Absorption cross-sections of NO2 in the UV and visible region (200 – 700 nm) at 298 K

The absorption cross-sections of NO₂ have been measured in the wavelength range 200 – 700 nm at 298 K with a spectral resolution of 0.04 nm. The data were acquired digitally, allowing post-processing such as integration in different wavelength intervals. The cross-sections are averaged over 1 nm intervals and over the atmospheric wavelength intervals used in solar photolysis calculations.     

Crown moieties as cation host units in model polyamide compounds: Application in liquid-liquid cation extraction and in membrane cation transport

A new model polyamide compound that has a benzo-18-crown-6 moiety in the pendant structure is described. This model interacts with metal cations in the alkaline, earth alkaline, transition metal and heavy metal series. The interaction has been analyzed in terms of competitive cation extraction from aqueous solution by liquid model/dichloromethane phase. In each cation series, K(I), Ba(II), Cr(III), and Hg(II) have been selectively extracted by liquid model polyamide phases.The interaction of a dense composite model polyamide-cellulose acetate membrane with lead(II) has been studied through its adsorption isotherm, infrared spectra and scanning electron microscopy study of the membranes before and after Pb(II) adsorption. The transport of lead nitrate through the membrane together with that of sodium chloride (for comparison), have been evaluated.