The DAΦNE electron-positron collider at the Frascati National Laboratories has made available a unique “beam" of negative kaons providing
unprecedented conditions for the study of the low-energy kaon-nucleon interaction, a field still largely unexplored. The DEAR
(DAΦNE Exotic Atom Research) experiment at DAΦNE and its successor SIDDHARTA (SIlicon Drift Detector for Hadronic Atom Research by Timing Application) aim at a precision
measurement of the strong-interaction shift and width of the fundamental 1s level, via the measurement of the X-ray transitions to this level, for kaonic hydrogen and kaonic deuterium. The aim is to
extract the isospin-dependent antikaon-nucleon scattering lengths and to contribute to the understanding of aspects of chiral
symmetry breaking in the strangeness sector. 相似文献
The formation and properties of short-lived homo- and heteronuclear metal clusters in aqueous solutions were investigated
by pulse radiolysis. These species arise in early stages of the aggregation of metal atoms and ions in unusual oxidation states.
The conditions favoring the formation of positively charged “magic” clusters and intermediate heterometallic clusters were
elucidated.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 1–15, January, 1999. 相似文献
The evolution of Internet topology is not always smooth but sometimes with unusual sudden changes. Consequently, identifying patterns of unusual topology evolution is critical for Internet topology modeling and simulation. We analyze IPv6 Internet topology evolution in IP-level graph to demonstrate how it changes in uncommon ways to restructure the Internet. After evaluating the changes of average degree, average path length, and some other metrics over time, we find that in the case of a large-scale growing the Internet becomes more robust; whereas in a top-bottom connection enhancement the Internet maintains its efficiency with links largely decreased. 相似文献
An oxetane ring can be constructed from 5α-acyloxy-Δ4(20)-taxoids. Hie facile intramolecular acyl migration from 5- to 20-position under slightly basic conditions enabled the construction of the oxetane ring in a convenient short cut, whereas the acyl migration from 2- to 20-position left the 2-hydroxyl accessible to a later benzoylation. An unexpected five-mem-bered 4-O, 20- O sulfite ring was formed in the attempted construction of the oxetane ring with 5α-triflate as a leaving group. After the construction of the oxetane ring, treatment with strong base LiHMDS and acetyl chloride gave the expected 4-O-acetate while treatment with acetic anhydride and DMAP gave a 4-O-acetoacetate. 相似文献
The time-dependence of the population of muonic hydrogen states in hydrogen-helium mixtures is calculated for principal quantum
number n. The number of muons transferred to helium nuclei is also determined. The dependence of the population of the ground state
of muonic hydrogen on time and target density and the helium concentration is also considered. The results are in agreement with recent experimental
data. The comparison of the calculated yield of K lines of X-ray in pure hydrogen and deuterium with experimental data indicates the essential role of the Coulomb deexcitation
process. Possible Stark mixing is also analysed.
Received 17 February 1999 and Received in final form 9 June 1999 相似文献
For linear and cyclic coupling reactions of CO, among other products, the formation of the hexapotassium salt of hexahydroxybenzene is of particular interesting. The interaction of metallic potassium and CO offers, via the assumed K[OC≡CO]K as the result of several carbon monoxide coupling reactions, the formation of C6(OK)6 among other products. To date, only speculations exist about the reaction pathway for these products, which were first described by Liebig in 1834. A novel concept is suggested here, which consists of the single steps (i) reductive coupling of CO, (ii) formation of dihetero-metallacyclopentynes (cis-2,5-diheterobutatriene as formal ethylenedione O=C=C=O complexes), (iii) formation of its dinuclear 1-metalla-2,5-dioxo-cyclopentyne complexes by external coordination of the triple bond, (iv) insertion of CO into the M−C bond of the formed metallacyclopropene, and (v) the reductive elimination of C6(OK)6. The novel aspect of this concept is the formation of dihetero-metallacyclopentynes (in analogy to the well characterized all-C-metallacyclopentynes), which have not been considered in the mechanism of reductive CO coupling reactions. It is expected that the presence of transition-metal impurities would promote the reaction. 相似文献
UV‐irradiation of aqueous suspensions of amino acid‐derived amphiphilic diacetylene supramolecules promotes a process that involves initial formation of species that absorb at 640 nm followed by the generation of polymers that have longer wavelength (686 nm) absorbance. The initially formed intermediate polydiacetylenes display substantial colorimetric reversibility while the long wavelength absorbing polymers show irreversible thermochromism during heating and cooling cycles. The long wavelength absorbing polydiacetylenes, formed from amino acid‐derived amphiphilic diacetylene supramolecules, are suggested to have more planer backbone structures that allow more efficient overlap of the conjugated p‐orbitals.