Inelastic neutron scattering study of magnetic excitations in the helimagnetic and antiferromagnetic phases of NiBr2

The spin wave dispersion in NiBr₂ has been studied by medium and long wavelength inelastic neutron scattering in the [1 1 0], [1 0 0] and [0 0 1] directions at 4.2 and 30 K, i.e. in the incommensurate helical and collinear antiferromagnetic phases. The values of the intralayer Heisenberg exchange constant J_ij and XY anisotropy constant D at 4.2(30) K are J₀₁ 0.379(1)(0.379(1)), J₀₂ 0.0036(50)(0.0036(50)), J₀₃ - 0.105(5) (?0.105(5)), J′ - 0.0423(50)(?0.389(50))D 0.0364(50)(0.0290(50)), where J′; is the interlayer exchange constant. In fitting the 4.2 K data account is taken of the co-existence of three equivalent domains and of intensity arising from $\text{ω(}\text{q}\text{)}$ and $\text{ω(}\text{q ± k}{}_0\text{)}$ where $\text{k}{}_0$ is the wavevector of the helix. In the low frequency region of the dispersion curve such peaks are resolved. The results reinforce the hypothesis that in zero-field the commensurate-incommensurate phase transition is driven by fluctuations.     

The effect of ion source temperature on the fragmentation of 2-pentanone

A dramatic source temperature effect is observed in the low collision energy collision-induced dissociation spectra of molecular ions derived via 70 eV electron bombardment of 2-pentanone and via charge exchange. The observation cannot simply be attibuted to the increased thermal energy of the 2-pentanone molecular ions. A number of factors appear to contribute to the observed results, (i) the increase, with temperature, in the ratio of the concentration of enol to keto neutral molecules, (ii) temperature effects on the Franck-Condon factors which control the yields and internal energies of the keto and enol molecular ions, (iii) differences in the relative stabilities of the keto and enol molecular ions, (iv) temperature effects on the equilibrium between the isomeric molecular ions. This last item is probably most important, with the concentration of being most significant. These data highlight the role that ion source temperature can play as a variable in elucidating ion structure problems.     

Quantitative analysis of technical polymer mixtures by matrix assisted laser desorption/ionization time of flight mass spectrometry

We report quantitative MALDI-TOF measurements for polydimethylsiloxane (PDMS) of two different molecular weights using the relative ratio of the signal intensities of integrated oligomer distributions for these two molecular weight distributions. By reporting the ratio of intensities of the integrals of two oligomer distributions, we assume that the ionization and desorption efficiencies, crystallization conditions and other factors affecting intensity are similar. Poly(methyl methacrylate) (PMMA-33,000) was mixed with PDMS samples to show whether the presence of another material might affect the desorption efficiency. Quantitative values for the number-average molecular weight (Mn), weight-average molecular weight (Mw) and polydispersities (D) were calculated using the oligomer distributions. The results show a linear relationship between the analyte concentrations and the signal intensities in the range from 1,000 Da to 10,000 Da, and the desorption efficiency of these two PDMS materials was the same even in the presence of PMMA.     

Total synthesis of luminacin D

[reaction: see text] A highly convergent synthesis of the angiogenesis inhibitor luminacin D has been achieved in 13 linear steps (19 steps total, 5.3% overall yield) utilizing a samarium(II) iodide-mediated mixed tandem aldol/Evans-Tishchenko reaction to construct the carbohydrate precursor. The modular synthetic design will allow derivatization at key positions necessary for biochemical mode of action studies.     

Hyphenated techniques in anticancer drug monitoring. I. Capillary gas chromatography-mass spectrometry

Muramic acid (Mur) is present in the cell wall of Eubacteria and serves as a chemical marker for the trace detection of bacteria and bacterial cell wall debris in complex matrices. There have been numerous studies using a variety of derivatives of Mur, particularly in combination with gas chromatography-tandem mass spectrometry (GC-MS-MS) where the detection limit has been steadily lowered. A stable, halogenated derivative, the pentafluorobenzyl oxime (PFBO) acetate of Mur, has been developed by others and successfully used for GC with electron-capture detection. The current report is the first use of this derivative for GC-MS-MS analysis of Mur, or indeed any other carbohydrate, using negative ion chemical ionization (NICI) with GC-MS-MS. Mur was readily detected in settled surface dust (166 ng/mg), as well as dust collected from indoor air (1.4-5.9 ng/mg). Analyses of Mur as a PFBO acetate by GC-NICI-MS-MS or as alditol acetates by electron impact GC-electron impact ionization MS-MS serve as complementary approaches for trace detection in complex matrices.     

Degree constrained book embeddings

A book embedding of a graph consists of a linear ordering of the vertices along a line in 3-space (the spine), and an assignment of edges to half-planes with the spine as boundary (the pages), so that edges assigned to the same page can be drawn on that page without crossings. Given a graph G=(V,E), let be a function such that 1f(v)deg(v). We present a Las Vegas algorithm which produces a book embedding of G with pages, such that at most f(v) edges incident to a vertex v are on a single page. This result generalises that of Malitz [J. Algorithms 17 (1) (1994) 71–84].     

New mode of operating a magnetized coaxial plasma gun for injecting magnetic helicity into a spheromak

By operating a magnetized coaxial plasma gun continuously with just sufficient current to enable plasma ejection, large gun-voltage spikes (approximately 1 kV) are produced, giving the highest sustained voltage approximately 500 V and highest sustained helicity injection rate observed in the Sustained Spheromak Physics Experiment. The spheromak magnetic field increases monotonically with time, exhibiting the lowest fluctuation levels observed during formation of any spheromak (B/B>/=2%). The results suggest an important mechanism for field generation by helicity injection, namely, the merging of helicity-carrying filaments.