Far-infrared pump-probe measurement of the lifetime of the 2p?1 shallow donor level in n-GaAs

The low temperature lifetime of electrons excited in the 2p^–1 donor level of n-GaAs has been studied in a far-infrared pump-probe experiment. The measurement has been carried out using a pulsed far-infrared molecular gas laser working at a wavelength of 292µm, with the sample in a magnetic field of 5.1 T, resonant with the 1s_o–2p^–1 transition. Two FIR pulses are sliced from one FIR-laser pulse by means of optical switching techniques using two Q-switched Nd:YAG lasers. The first pulse is used to saturate the transition, while the second pulse probes the return of the population in the excited state towards thermal equilibrium as a function of the time delay after the excitation pulse. The value of 350±50 ns found for the lifetime falls in line with CW saturation results on materials with other doping concentrations.     

Infrared and Raman spectra, ab initio calculations, conformational stability and vibrational assignment of 1-bromo-1-silacyclopentane

Infrared and Raman spectra (3500-60 cm(-1)) of gas and/or liquid and solid 1-bromo-1-silacyclopentane (c-C4H8SiBrH) have been recorded and the vibrational data indicate the presence of a single conformer with no symmetry which is consistent with the twisted form. Ab initio calculations with a variety of basis sets up to MP2(full)/6-311+G(2df,2pd) predict the envelope-axial and envelope-equatorial conformers to be saddle points with nearly the same energies but approximately 900 cm(-1) (5.98 kJ/mol) lower in energy than the planar conformer. Density functional theory calculations by the B3LYP method predict slightly lower energies for the two envelope forms and considerably lower energy for the planar form compared to the MP2 predictions. By utilizing the MP2(full)/6-31G(d) calculations the force constants, frequencies, infrared intensities, band contours, Raman activities, and depolarization values have been obtained to support the vibrational assignment. Estimated r0 structural parameters have been obtained from adjusted MP2(full)/6-311+G(d,p) calculations. These experimental and theoretical results are compared to the corresponding quantities of some other five-membered rings.     

Monodisperse hydrogel microspheres by forced droplet formation in aqueous two-phase systems

This paper presents a method to form micron-sized droplets in an aqueous two-phase system (ATPS) and to subsequently polymerize the droplets to produce hydrogel beads. Owing to the low interfacial tension in ATPS, droplets do not easily form spontaneously. We enforce the formation of drops by perturbing an otherwise stable jet that forms at the junction where the two aqueous streams meet. This is done by actuating a piezo-electric bending disc integrated in our device. The influence of forcing amplitude and frequency on jet breakup is described and related to the size of monodisperse droplets with a diameter in the range between 30 and 60 μm. Rapid on-chip polymerization of derivatized dextran inside the droplets created monodisperse hydrogel particles. This work shows how droplet-based microfluidics can be used in all-aqueous, surfactant-free, organic-solvent-free biocompatible two-phase environment.     

The characteristics of chromized 1020 steel with electrical discharge machining and Ni electroplating pretreatments

A uniform and continuous chromized coating on AISI 1020 steel is produced by low-temperature pack chromization (LTPC) with electrical discharge machining and Ni electroplating pretreatments. The anticorrosive performance of the chromized steels is investigated in a 0.5 M H₂SO₄ solution at room temperature. The testing results indicate that the chromized specimen with electrical discharge machining and Ni electroplating pretreatments exhibits the lowest corrosion current density, 2.16 × 10⁻⁸ A cm⁻², among the tested specimens. The corrosion resistance of all tested specimens are in the order of bare 1020 < 1020-Cr(700-2) < 1020-Ni-Cr(700-2) < 1020-EDM-Ni-Cr(700-2). Moreover, the 1020-Ni-Cr(700-2) specimen have the best conductivity as a result of the less amount of oxides in the superficial coating.     

Proton ENDOR study of copper(II) bis(dithiocarbamate)

The proton hyperfine coupling tensors of the methylene protons in methyl-deuterated copper(II) bis(N,N-diethyldithiocarbamate) in a diamagnetic host crystal of the corresponding nickel complex have been measured by ENDOR spectroscopy. Two intermolecular and all four intramolecular proton coupling tensors could be determined. With the aid of spin densities, obtained from extended Hückel molecular orbital calculations, the anisotropic part of the tensors can be reproduced quantitatively, taking into account all two- and three-centre contributions. Comparison of the transition frequencies which are computed from the theoretical tensors with the experimental transitions enables the tracing of another five tensors which cannot be completely determined experimentally.     

Validated quantitation method for a peptide in rat serum using liquid chromatography/high‐field asymmetric waveform ion mobility spectrometry

The analysis of peptides presents serious challenges for bioanalytical scientists including low total ion current and non‐selective fragmentation during tandem mass spectrometry (MS/MS). During method validation of a peptide in rat serum matrix some interferences could not be easily removed and thus prevented accurate and precise measurement. These problems associated with peptide quantitation were resolved by using FAIMS (high‐Field Asymmetric waveform Ion Mobility Spectrometry). This selectivity‐enhancing technique filters out matrix interferences, and the resulting pseudo‐selected reaction monitoring (pseudo‐SRM) chromatograms were nearly free from interferences. Control blank matrix samples contained an acceptable level of interference (only 7% signal as compared to the lower level of quantitation). Chromatographic peaks were easily, accurately and precisely integrated resulting in a validated liquid chromatography (LC)/FAIMS‐MS/MS method for the analysis of a peptide drug in rat serum according to United States Food and Drug Administration (US FDA) bioanalytical guidelines. These results confirm that new selectivity‐enhancing technologies aid the pharmaceutical industry in reliably producing acceptable pharmacokinetic data. Copyright © 2009 John Wiley & Sons, Ltd.     

Confocal macro X‐ray fluorescence spectrometer on commercial 3D printer

Novel confocal X‐ray fluorescence (XRF) spectrometer was designed and constructed for 3D analysis of elementary composition in the surface layer of spatially extended objects having unlimited chemical composition and geometrical shape. The main elements of the XRF device were mounted on a moving frame of a commercial 3D printer. The XRF unit consists of a silicon drift detector and a low‐power transmission‐type X‐ray tube. Both the excitation and secondary X‐ray beams were formed and regulated by simple collimator systems in order to create a macro confocal measuring setup. The spatial accuracy of the mechanical stages of the 3D printer achieved was less than 5 μm at 100‐μm step‐size. The diameter of the focal spot of the confocal measuring arrangement was between 1.5 and 2.0 mm. The alignment of the excitation and secondary X‐ray beams and the selection of the measuring spot on the sample surface were ensured by two laser beams and a digital microscope for visualization of the irradiated spot. The elements of the optical system together with the XRF spectrometer were mounted on the horizontal arm of the 3D printer, which mechanical design is capable of synchronized moving the full spectroscopic device within vertical directions. Analytical capability and the 3D spatial resolution of the confocal spectrometer were determined. Copyright © 2017 John Wiley & Sons, Ltd.     

Stimulated terahertz stokes emission of silicon crystals doped with antimony donors

Stimulated Stokes emission has been observed from silicon crystals doped by antimony donors when optically excited by radiation from a tunable infrared free electron laser. The photon energy of the emission is equal to the pump photon energy reduced by the energy of the intervalley transverse acoustic (TA) g phonon in silicon (approximately 2.92 THz). The emission frequency covers the range of 4.6-5.8 THz. The laser process occurs due to a resonant coupling of the 1s(E) and 1s(A1) donor states (separation approximately 2.97 THz) via the g-TA phonon, which conserves momentum and energy within a single impurity center.