Aromatic core-modified twisted heptaphyrins[1.1.1.1.1.1.0]: syntheses and structural characterization

[reaction: see text] First examples of aromatic core-modified twisted heptapyrrolic systems with six meso links have been obtained via a general [3 + 4] acid-catalyzed condensation method. Despite the twisted conformation, these heptaphyrins exhibit aromaticity in solution, unlike most of the all-aza analogues.     

Dynamical hysteresis in a three-level atomic system

We have experimentally investigated dynamical hysteresis behavior by changing the sweeping frequency of the cavity input field in the optical bistability of rubidium atoms (in a three-level atomic-type configuration) inside an optical cavity. The shape, width (or area), and direction of the hysteresis cycle are sensitive functions of the sweeping frequency for such an optical bistable system.     

Evidence for nontermination of rotational bands in 74Kr

Three rotational bands in 74Kr were studied up to (in one case one transition short of) the maximum spin I(max) of their respective single-particle configurations. Their lifetimes have been determined using the Doppler-shift attenuation method. The deduced transition quadrupole moments reveal a modest decrease, but far from a complete loss of collectivity at the maximum spin I(max). This feature, together with the results of mean field calculations, indicates that the observed bands do not terminate at I = I(max).     

Structural and functional analogue of the active site of polysulfide reductase from Wolinella succinogenes

Synthesis of [PPh4]2[Mo(SPh)2(S2C2(CN)2)2] (2) from [PPh4]2[MoO(S2C2(CN)2)2] (1) has been achieved to mimic the postulated [Mo(S)6] core of polysulfide reductase with two thiolates and two bis(ene-dithiolate) ligands. Compound 2 reacts with polysulfide to yield H2S, modeling the function of polysulfide reductase. The facile conversion of 2 back to 1 in moist solvent suggests that the interconversion of the [MoIV = O] and [MoIV - X] (X = O-Ser, S-Cys, Se-Cys) moieties might occur in the DMSO reductase class of enzymes under appropriate hydrophobic/hydrophilic conditions.     

High-resolution spectrum of xenon ions at 13.4 nm

The spectrum of xenon excited in a low-inductance vacuum spark was photographed at high resolution in the region of 9.5-15.5 nm. The observed transitions were identified as belonging to ions from Xe8+ to Xe13+. In the region of importance for extreme-ultraviolet lithography around 13.4 nm, the strongest lines were identified as 4d8-4d7 5p transitions in Xe10+. The identifications were made by use of energy parameters extrapolated along the isoelectronic sequence.     

Single-photon all-optical switching using coupled microring resonators

We study the nonlinear phase response of a microring resonator coupled to a bus waveguide and the use of this nonlinear phase shift to store information in the microring resonator and enhance the switching characteristics of a Mach-Zehnder interferometer (MZI). By introducing coupling between adjacent microring resonators, the switching characteristics of the MZI can be exponentially enhanced as a function of the number of microring resonators, when compared to the linear enhancement for uncoupled resonators. With only a few moderate-finesse microring resonators, the switching power can be reduced to attowatt level, allowing for photonic switching devices that operate at single-photon level in ordinary optical waveguides.      

On the genericity of spacetime singularities

We consider here the genericity aspects of spacetime singularities that occur in cosmology and in gravitational collapse. The singularity theorems (that predict the occurrence of singularities in general relativity) allow the singularities of gravitational collapse to be either visible to external observers or covered by an event horizon of gravity. It is shown that the visible singularities that develop as final states of spherical collapse are generic. Some consequences of this fact are discussed.