Stationary and nonstationary fluid flow of a bose-einstein condensate through a penetrable barrier

We experimentally study the fluid flow induced by a broad, penetrable barrier moving through an elongated dilute gaseous Bose-Einstein condensate. The barrier is created by a laser beam swept through the condensate, and the resulting dipole potential can be either attractive or repulsive. We examine both cases and find regimes of stable and unstable fluid flow: At slow speeds of the barrier, the fluid flow is steady due to the superfluidity of the condensate. At intermediate speeds, we observe an unsteady regime in which the condensate gets filled with dark solitons. At faster speeds, soliton formation completely ceases, and a remarkable absence of excitation in the condensate is seen again.     

Proton hyperfine couplings in VO(H2O)2+5: The validity of the point-dipole approximation

A double-zeta wavefunction has been used to calculate the dipolar electron—proton hyperfine interactions in VO(H₂O)²⁺₅ using numerical integration. The results indicate that a point-dipole model is a good approximation for this ion.     

Organic reactivity in liquid ammonia

Liquid ammonia is a useful solvent for many organic reactions including aliphatic and aromatic nucleophilic substitution and metal-ion catalysed reactions. The acidity of acids is modified in liquid ammonia giving rise to differences from conventional solvents. The ionisation constants of phenols and carbon acids are the product of those for ion-pair formation and dissociation to the free ions. There is a linear relationship between the pK(a) of phenols and carbon acids in liquid ammonia and those in water of slope 1.68 and 0.7, respectively. Aminium ions exist in their unprotonated free base form in liquid ammonia. The rates of solvolysis and aminolysis by neutral amines of substituted benzyl chlorides in liquid ammonia show little or no dependence upon ring substituents, in stark contrast with the hydrolysis rates of substituted benzyl halides in water which vary 10(7) fold. However, the rates of the reaction of phenoxide ions and amine anions with 4-substituted benzyl chlorides gives a Hammett ρ = 1.1 and 0.93, respectively. The second order rate constants for the substitution of benzyl chlorides by neutral and anionic amines show a single Br?nsted β(nuc) = 0.21 whereas those for substituted phenoxide ions generate a Br?nsted β(nuc) = 0.40. The rates of aromatic nucleophilic substitution reactions in liquid ammonia are much faster than those in protic solvents indicating that liquid ammonia behaves like a typical dipolar aprotic solvent in its solvent effects on organic reactions. Nitrofluorobenzenes (NFB) readily undergo solvolysis in liquid ammonia but oxygen nucleophiles, such as alkoxide and phenoxide ions, displace the fluorine of 4-NFB in liquid ammonia to give the corresponding substitution product with little or no competing solvolysis product. The Br?nsted β(nuc) for the reaction of 4-NFB with para-substituted phenoxides is 0.91, indicative that the decomposition of the Meisenheimer σ-intermediate is rate limiting. The aminolysis of 4-NFB occurs without general base catalysis by the amine and the second order rate constants generate a Br?nsted β(nuc) of 0.36, which is also interpreted in terms of rate limiting breakdown of the Meisenheimer σ-intermediate.     

E.S.R. spectrum of 3,5-lutidine anion and comments on nitrogen hyperfine couplings

The E.S.R. spectrum of the 3,5-lutidine anion has been observed. The methyl groups prevent the dimerization which occurs for pyridine. From the observed splitting constants and those of the pyrazine negative ion an estimate is made of the relative contribution to the nitrogen hyperfine splitting of the π-orbital spin densities on the nitrogen atom and on the adjacent carbon atoms.     

Observations of focal conic domains in smectic liquid crystals aligned on patterned self‐assembled monolayers

Patterned Self‐Assembled Monolayers (SAMs) promoting both homeotropic and planar degenerate alignment of nCBs in their smectic‐A phase were created using microcontact printing of functionalized organothiols on gold films. By patterning the surface with homeotropic and planar aligning SAMs, the location and formation of the focal conic domains (FCDs) can be controlled. Polarizing microscopy was used to study the formation of FCDs in circle, stripe and checkerboard pattern geometries. Fluorescent confocal microscopy (FCM) was used for the first time to measure the eccentricity of FCDs that form along a stripe pattern.     

A hybrid OPCPA/Nd:phosphate glass multi-terawatt laser system for seeding of a petawatt laser

To enhance radiographic abilities on its Z-Accelerator, Sandia National Laboratories is incorporating a petawatt laser system into the existing Z-Backlighter laser facility. As part of this work, a short-pulse laser has been constructed to seed the larger Beamlet type Nd:phosphate glass slab amplifiers. This seed laser consists of an optical parametric chirped pulse amplification (OPCPA) system joined to a Nd:phosphate glass rod amplifier system in order to achieve multi-Joule sub-picosecond operation. The rod amplifier system has been modeled using the Miró code which shows good agreement with the experimental results. This system can achieve focal intensities up to 10¹⁸ W/cm² at a repetition rate of once every 20 min and has been applied to produce k-alpha X-rays in copper.     

Eine neue Brommethode zur Bestimmung von Thymol, Salizylaten und ?hnlichen Verbindungen

Ohne Zusammenfassung     

Proton distributions in the target fragmentation region in proton-nucleus and nucleus-nucleus collisions at high energies

We present measurements of the rapidity and transverse-momentum distributions of the protons emitted in S+W, O+W, andp+W reactions at 200 GeV/A around the target rapidity (y=1). The rapidity density rises linearly with the transverse energy for all three systems, but the slope forp+W is much steeper than for O+W and S+W. The rapidity density forp+W is much higher than predicted by summing single nucleonnucleon collisions without any nuclear effects, indicating substantial rescattering of the produced particles. The predictions of the VENUS 3 model, including rescattering, show reasonable agreement with the data for all three systems. We do not have evidence for a strong collective flow of the outgoing particles.