Photoionization of highly excited atomic sodium involving core-excited resonances

Resonant photoionization in thenl outer-shell of 2p ⁶ nl excited sodium is studied between 32.50 and 33.50 eV. Resonances are of the type 2p ⁶ nl → 2p ⁵ 3snl → 2p ⁶ ελ, i.e. corresponding to the excitation of a 2p core-electron to a 3s orbital. Experimental technique combines laser and synchrotron radiation for exciting sequentially sodium atoms. The main features of the resonances — energies and total oscillator strengths — are measured or estimated. From the results, one concludes that the valence electron is almost decoupled from the core, with respect to the core excitation.     

A Titration Microcalorimetric Study of the Effects of Halide Counterions on Vesicle-Forming Aggregation in Aqueous Solution of Branched-Chain Alkylpyridinium Surfactants

Titration microcalorimetry is used to study the influences of iodide, bromide, and chloride counterions on the aggregation of vesicle-forming 1-methyl-4-(2-pentylheptyl)pyridinium halide surfactants. Formation of vesicles by these surfactants was characterised using transmission electron microscopy. When the counterion is changed at 303 K through the series iodide, bromide, to chloride, the critical vesicular concentration (cvc) increases and the enthalpy of vesicle formation changes from exo- to endothermic. With increase in temperature to 333 K, vesicle formation becomes strongly exothermic. Increasing the temperature leads to a decrease in enthalpy and entropy of vesicle formation for all three surfactants. However the standard Gibbs energy for vesicle formation is, perhaps surprisingly, largely unaffected by an increase in temperature, as a consequence of a compensating change in both standard entropy and standard enthalpy of vesicle formation. Interestingly, standard isobaric heat capacities of vesicle formation are negative, large in magnitude but not strikingly dependent on the counterion. We conclude that the driving force for vesicle formation can be understood in terms of overlap of the thermally labile hydrophobic hydration shells of the alkyl chains. Copyright 2000 Academic Press.