Scattering of low energy He2+ ions by Ne,Ar, and Kr atoms

Experimental studies of collisions of He²⁺ ions with Ne, Ar, and Kr atoms have been carried out at laboratory kinetic energies in the range 8 ? E₁ ? 10 eV. For each collision pair, relative differential cross sections for elastic scattering, and for the formation of He⁺ by single charge transfer [e.g., He²⁺ + R = He⁺ + (R⁺)^*] were measured. Information concerning the initial states of the charge transfer products was also obtained, from measurements of the kinetic energy distributions of the He⁺ + He = Ne⁺(2s 2p⁶²S) ± He⁺(²S), whereas for the other systems, transfer proceeds via a number of channels. The He⁺-ion kinetic energy measurements indicated that for He²⁺. Ar both Ar⁺ both Ar⁺ and Ar²⁺ are formed in transfer, and that for He²⁺, Kr only Kr²⁺ (and no Kr⁺) was formed.The differential elastic scattering patterns were analyzed by means of cross section calculations based on an approximate form of the optical model. These calculations indicated that the pronounced shoulders observed in the σ_el(θ) versus θ curves arose from scattering from an attractive potential well, in the presence of concurrent inelastic scattering. Using parametrized Morse potentials to represent the ground electronic states of (HeNe)²⁺, (HeAr)²⁺, and (HeKr)²⁺, the corresponding well-depth are estimated to be, respectively: 1.0 eV, 2.1 eV and 2.6 eV.     

Distribution of irreducible polynomials of small degrees over finite fields

D. Wan very recently proved an asymptotic version of a conjecture of Hansen and Mullen concerning the distribution of irreducible polynomials over finite fields. In this note we prove that the conjecture is true in general by using machine calculation to verify the open cases remaining after Wan's work.

     

The determination of nitrogen in nitrocellulose

A method for the determination of nitrogen in nitrocellulose has been developed. The sample is dissolved in concentrated sulphuric acid and a salicylic acid solution is added. The nitrosalicylic acid formed is reduced at room temperature with an excess of titanous chloride solution and the excess is estimated by titration with ferric alum. Statistically designed experiments were used to establish the conditions necessary for accurate results. The procedure is more rapid than the usual nitrometer method and gives results of high precision; e.g. on a mean result of about 13.8% nitrogen, the standard deviation is 0.014.     

Analytical representations of the Mössbauer recoilless fraction,Debye-Waller factors,lattice energies,and heat capacities

Simple analytical expansions are given for the recoilless fraction in Mössbauer spectroscopy, the Debye-Waller factor in X-ray scattering, and the lattice energy and heat capacity of solids. While this problem has been discussed in an earlier paper [1], computer technology has now advanced to the point that direct evaluations of the simple expansions of these quantities are useful for quick curve fitting to experimental data at any desired temperature, and these expansions are easier to evaluate than using graphs to estimate recoilless fractions and Debye temperatures. We compare this approach with a polynomial expansion in terms of Bernoulli numbers, which has only a limited domain of convergence. We explicitly evaluate the convergence of these Debye integral expansions as a function of the number of terms used and the time required.This work was prepared with the support of the U.S. Department of Energy, Grant No. DE-FG02-85 ER 45199.     

Determination of arsenic in high-purity copper by flameless atomic-absorption spectrophotometry

Arsenic is separated from an ammoniacal solution containing 1 g or less of copper(II) by co-precipitation with ferric hydroxide. The precipitate is dissolved in nitric acid, the solution almost neutralized with ammonia, made up to volume and the As determined by flameless atomic-absorption spectrophotometry. Less than 0.5 ppm can be determined.     

Toward multistation rotaxanes using metalloporphyrin coordination templating

This paper describes some approaches toward the templated synthesis of rotaxanes incorporating strapped metalloporphyrin moieties as the shuttle unit, with the thread component containing both a neutral diimide "station" and a functionalized pyridine moiety, the latter acting not only as a template but also as a second binding motif. In the first instance, the use of appropriately 3,5-difunctionalized pyridine esters and naphthoquinol-strapped rhodium(III) chloride porphyrins in a stoppering approach to rotaxanes produced only unlinked components: the flexibility of the strap allowed sufficient room for the potential thread unit to bind on the same face of the porphyrin as the strap, while not being interlocked through it. An alternative strategy involving a 1,3-dipolar cycloaddition reaction (a "click" reaction) between azides and alkynes, producing triazole linkers in the thread component of rotaxanes, was more successful. Both porphyrinic (zinc, free base, and rhodium(III) derivatives) and crown ether rotaxanes were successfully produced, with multifunctional (triazole and naphthodiimide) thread units. The potential for molecular motion through the use of stimuli such as acid, solvent, and competing ligands was investigated, with limited success. The same cycloaddition methodology was extended to pyridine-templated analogues of the thread components in the Rh(III)-strapped porphyrins, but again, only unlinked thread and porphyrin shuttle units were produced.     

A concise synthesis of a new xylyl-biaryl diphosphine ligand for asymmetric hydrogenation of ketones

A concise synthesis of a symmetrical biaryl diphosphine ligand bearing 3,5-dimethylphenyl substituents at phosphorus is described. The ruthenium catalysts [diphosphine RuCl₂ diamine] containing the new ligand Xyl-TetraPHEMP were found to be as active and as selective as the state-of-the-art catalysts for homogeneous asymmetric ketone hydrogenation.     

Three-dimensional orientational colocalization of individual donor--acceptor pairs

We report on the determination of the three-dimensional orientation of the donor and acceptor transition dipoles in individual fluorescence resonance energy transfer (FRET) pairs by means of scanning optical microscopy with annular illumination. Knowledge of the mutual orientation of the donor and acceptor dipole is mandatory for reliable distance determination based on FRET efficiency measurements. In our model system perylenediimide as the donor and terryelenediimide as the acceptor are coupled via a stiff p-terphenyl linker. The absorption dipoles of the donor and acceptor are selectively addressed by the 488 nm and 647 line of an Ar/Kr mixed gas laser, respectively. A clear deviation from collinearity is observed with a distribution of misalignment angles peaked around 22 degrees.