Mode locking in a free-electron laser amplifier

A technique is proposed to generate attosecond pulse trains of radiation from a free-electron laser amplifier. The optics-free technique synthesizes a comb of longitudinal modes by applying a series of spatiotemporal shifts between the copropagating radiation and electron bunch in the free-electron laser. The modes may be phase locked by modulating the electron beam energy at the mode spacing frequency. Three-dimensional simulations demonstrate the generation of a train of 400 as pulses at gigawatt power levels evenly spaced by 2.5 fs at a wavelength of 124 angstroms. In the x-ray at wavelength 1.5 angstroms, trains of 23 as pulses evenly spaced by 150 as and of peak power up to 6 GW are predicted.     

EPR spectra and structure of the chlorotrifluoroethylene and bromotrifluoroethylene radical anions

EPR spectra attributable to the halogenotrifluoroethylene radical anions have been generated by electron attachment in solid solutions at low temperatures. The spin density distribution in these radicals strongly suggests that the unpaired electron occupies a σ^* orbital, a conclusion which is supported by CNDO/2 calculations.     

Optimisation of a gas chromatographic method for trace gaseous impurities in nitrogen trifluoride by column sequence reversal

Highly reactive fluorinated gaseous matrices require special equipment and techniques for the gas chromatographic analysis of trace impurities in these gases. The impurities that were analysed at the low mg/L levels included dioxygen (O2), dinitrogen (N2), carbon dioxide (CO2), carbon monoxide (CO), sulfur hexafluoride (SF6), methane (CH4) and nitrous oxide (N2O). Carbon tetrafluoride (CF4) is also present in the product at levels of 20-400mg/L and had to be analysed as well. This paper compares the use of a custom-built dual-channel gas chromatograph utilising single column back flush switching on one channel for the determination of O2, N2, CH4 and CO with column sequence reversal on a second channel for the determination of CO2, N2O, SF6 and CF4 to a similar system using a combination of dual-column back flush and heart-cut configurations. Pulsed discharge helium ionisation detectors were used on both channels in both configurations.     

Localized hydrogels based on cellulose nanofibers and wood pulp for rapid removal of methylene blue

Access to clean water has become increasingly difficult, motivating the need for materials that can efficiently remove pollutants. Hydrogels have been explored for remediation, but they often require long times to reach high levels of adsorption. To overcome this limitation, we developed a rapid, locally formed hydrogel that adsorbs dye during gelation. These hydrogels are derived from cellulose—a renewable, nontoxic, and biodegradable resource. More specifically, we found that sulfated cellulose nanofibers or sulfated wood pulps, when mixed with a water-soluble, cationic cellulose derivative, efficiently remove methylene blue (a cationic dye) within seconds. The maximum adsorption capacity was found to be 340 ± 40 mg methylene blue/g cellulose. As such, these localized hydrogels (and structural analogues) may be useful for remediating other pollutants.     

Carbon coating of fused silica ampoules

A reliable method for depositing a layer of carbon on the inner walls of a fused silica ampoule is described and characterized. Carbon deposition rates were found to be 0.33 μm/h at mid-length and range from 0.26 to 0.55 μm/h at the ends of a 150 mm long ampoule. Deposition rate was found to vary along the length of the ampoule, but not along the radial perimeter.