Standoff chemical detection using SUPER CARS

Increasingly stringent requirements in the manufacture of integrated circuits and microscale devices are demanding new approaches to the design and operation of glow-discharge plasma process reactors. Several approaches have been proposed recently, one of which is the operation of plasma reactors in the pulsed mode where the power input to the reactor is modulated using square-wave pulses (SWPs). In this article, we propose a completely general technique for pulsed operation of plasma reactors where the power input is modulated using pulse shapes that are determined systematically using a computational method. We call this technique optimal pulse shaping (OPS) and it relies on a physical model of the plasma reactor used in conjunction with an optimal control algorithm. The OPS technique enables simultaneous control of several plasma process parameters, thus expanding the accessible plasma parameter space over that achievable by SWPs. We apply the OPS technique to a pure argon high-density plasma reactor. Optimal power input pulse shapes and pulsing frequencies are determined in order to control time-averaged values of the ion number densities, the ratio of metastable-to-ion number densities, and the electron temperatures in the bulk plasma. Results indicate that all optimal power input pulse shapes can be characterized by “on” and “off” periods, with a typical on period consisting of power input spikes accompanied by a lower plateau input. The off period consisting of either a true zero input power or a small input power. The power input spikes during the on period control the averaged ion densities, while the plateau input controls the metastable densities. The average electron temperature is controlled by the off power input.

Investigation of matrix effects in laser-induced breakdown spectroscopy plasmas of high-alloy steel for matrix and minor elements

NMR spectroscopy is ubiquitous in many areas of science, including chemistry, material science, biophysics, and structural biology. Heteronuclear two-dimensional (2D) NMR experiments form the basis of several NMR-based investigations, especially those involving biomolecules like proteins. However, in solution, as the molecule of interest becomes larger, transverse relaxation times of the spins of interest become shorter. This makes it difficult to record 2D 1H-13C or 1H-15N correlation maps of large protein molecules using standard Fourier transform-based experiments as they contain transfer delays that are long compared to the short relaxation times. Herein, we explore the possibility of leveraging deep neural networks (DNNs) to obtain 2D correlation maps of proteins while eliminating transfer delays from experiments. In this study, we show that 2D methyl 1H-13C correlation maps can be obtained from experiments containing only a single 1H excitation pulse followed by off-resonance 13C continuous-wave decoupling. A DNN was successfully trained to reconstruct the 2D 1H-13C correlation map from datasets recorded with two 13C decoupling fields using protein samples enriched with isoleucine, leucine, and valine (ILV) 13CHD2 methyl-groups in a 2H background. The efficacy of this strategy is demonstrated by using the DNN to reconstruct methyl 1H-13C correlation maps of the ~8 kDa FF domain from human HYPA/FBP11, the ~18 kDa T4 phage lysozyme, the ~360 kDa α7α7 particle from the Thermoplasma acidophilum proteasome and a ~530 kDa ancestrally reconstructed L8S8 Rubisco complex. This study illustrates the potential for improving the sensitivity and resolution of NMR spectra by designing experiments tailored for use with DNNs.

Summary form only given. Pulsed power gas switches operate under extreme conditions holding off voltages at millions of volts and greater, conducting currents at 100's of kiloamps, and can reliably trigger with less than 10ns of jitter at optimal operating conditions. However as Martin and Braginskii had noted the impedance of these switches is dynamic and a function of many parameters such as gas pressure, current, and the number of channels. In systems such as PROTOGEN or URSA MINOR many gas switches are utilized to enable optimal performance and pulse shaping. A challenge with either system is that, if the switches are tightly coupled and one switch triggers early, then the fields on the second switch change dramatically and can move the second switch into a new operating regime that will result in a different dynamic resistance. This paper presents a least squares approach to extracting the Martin and Braginskii dynamic resistance from measured data. Results from this approach are compared to previous approaches utilizing piecewise nonlinear least squares.

The growing interest in backscatter communications as a low-powered solution in various fields necessitates pushing the envelope of current backscatter systems in multiple frontiers, among which is spectral efficiency. The increase in spectral efficiency can drive real-time applications such as augmented reality. In most of backscatter-system applications such as radio-frequency identification, backscatter modulation is implemented using binary schemes realized with square pulses, which are not spectral efficient. To address the spectral efficiency concern, this article reviews—with a scope limited to prototyped systems—some of the existing works in the literature that pertain to increasing the spectral efficiency of backscatter systems. The prototyped systems can be grouped—based on the implementation technique used—into three groups: High-order modulation, single-sideband modulation, and pulse shaping. Based on the current trends and studied literature, the article concludes with discussions on some future directions and open-ended research problems such as optimal pulse shaping, the use of artificial intelligence, multi-carrier modulation, and agile modulation; which are all aim for overcoming the spectral-efficiency limit of contemporary backscatter systems.

By using a double-pulse excitation scheme, we demonstrate the optical control of higher-order spin waves in a ferromagnetic permalloy thin film. Three spin precession modes are observed in the film under single pulse excitation. Based on a theoretical calculation, these spin precession modes are assigned to the fundamental Kittel mode and the first- and second-order perpendicular standing spin-wave (PSSW) modes. In order to excite the first-order PSSW mode selectively, we use double-pulse excitation at 400 nm. We show that, by choosing an appropriate time interval between the two pump pulses, it is possible to obtain a relatively large amplitude of the first-order PSSW mode while the amplitudes of the other two modes are almost completely suppressed. By analyzing the spin dynamics, it is found that the spin precession motion under the double-pulse excitation condition can be explained by the interference between the spin waves that have been induced by the first and second pulses. Our finding indicates that the selective excitation of a spin precession mode with a high precession frequency in a ferromagnetic metal may be realized by a multiple-pump pulse excitation scheme with more than two pulses.

Structural investigations of perlite and expanded perlite using 1H, 27Al and 29Si solid-state NMR

Photoacoustic imaging (PAI) used for noninvasive imaging of biological tissue has been reported in many literature. However, there are still some disadvantages in the novel technique, such as the poor efficiency of imaging. In the current PAI, multiple excitation of laser and multiple acquisition of signal are necessary for image reconstruction. In this case, laser pulses may injure biological tissue due to energy accumulation. To popularize PAI in clinical applications, it is necessary to develop a new imaging approach to increase the efficiency of PAI. A spatial phase-controlled algorithm is presented for full-field three-dimensional (3D) image reconstruction. By using the algorithm, photoabsorption sources at different depths can be reconstructed using just one set of data acquired in single laser shot. Unfocused plane transducer array and parallel data-acquisition (PDA) equipment are used for real-time photoacoustic (PA) signal detection and acquisition. The spatial resolution of the 3D PAI system was analyzed. Two graphite rods at various positions in a simulation model and two bifurcate vessels in the ear of rabbit were imaged. In addition, the motion trace of one particle flowing at constant velocity was captured dynamically. Experimental results showed that spatial phase-controlled algorithm based on plane transducer array and PDA system was capable of static and dynamic 3D PAI. Spatial phase-controlled algorithm is introduced for 3D image reconstruction. The PA signals are collected by plane transducer array and PDA system in single pulse excitation. The acquired volumetric data are sufficient for 3D image reconstruction. Therefore, tissue can avoid the long-term exposure to light source and it is safer than the current PAI for in vivo imaging. With an increase in the repetition rate of laser pulse and speed of image display, the imaging method will realize real-time 3D imaging, which will be significant in clinical detection and medical diagnosis.

Photothermal microscopy has enabled highly sensitive label-free imaging of absorbers, from metallic nanoparticles to chemical bonds. Photothermal signals are conventionally detected via modulation of excitation beam and demodulation of probe beam using lock-in amplifier. While convenient, the wealth of thermal dynamics is not revealed. Here, we present a lock-in free, mid-infrared photothermal dynamic imaging (PDI) system by MHz digitization and match filtering at harmonics of modulation frequency. Thermal-dynamic information is acquired at nanosecond resolution within single pulse excitation. Our method not only increases the imaging speed by two orders of magnitude but also obtains four-fold enhancement of signal-to-noise ratio over lock-in counterpart, enabling high-throughput metabolism analysis at single-cell level. Moreover, by harnessing the thermal decay difference between water and biomolecules, water background is effectively separated in mid-infrared PDI of living cells. This ability to nondestructively probe chemically specific photothermal dynamics offers a valuable tool to characterize biological and material specimens.

Sensitivity and resolution enhancement in solid-state NMR spectroscopy of bicelles

1H and 13C-n.m.r. spectra and spin-lattice relaxation behaviour (laboratory frame, T1 (13C and 1H), and rotating frame, T1ρ (1H)) are reported for a range of solid ethylene copolymers with α-olefins having different types and concentrations of branches. The spin-diffusion model for relaxation in semicrystalline polymers is summarised and some new theoretical results given. The 13C high-resolution n.m.r. spectra obtained using both cross-polarisation (c.p.) and single pulse excitation (s.p.e.) methods associate side chain resonances mainly with the mobile, short T1ρ protons and, hence, the more disordered region of the solids, consistent with the short 13C T1 components. 1H laboratory-frame spin-lattice relaxation is single component whilst those for 13C and the 1H on-resonance rotating-frame relaxation are both multiexponential processes, requiring a minimum of three components to describe them. Annealing of samples, which is known to increase the lamellar thicknesses of the crystalline region, causes large increases in the longer relaxation time components. The 1H and 13C spin-lattice relaxation data for a set of annealed and quenched ethyl-branched materials having different branch contents are compared in detail with the predictions of the spin-diffusion relaxation model. The results are internally and semi-quantitatively consistent with this theory and it is concluded that for the 1H spin-lattice relaxation the model is clearly appropriate and for 13C it is consistent with observations. Questions concerning the relevance of spin-diffusion for the magnetically dilute 13C nucleus at natural abundance are mentioned and possible alternative explanations for the relationships observed are referred to briefly.

The pump-probe excitation of a two-level dipolar $(d\ensuremath{\ne}0)$ molecule, where the pump frequency is tuned to the energy level separation while the probe frequency is extremely small, is examined theoretically as an example of absolute phase control of excitation processes. The state populations depend on the probe field's absolute carrier phase but are independent of the pump field's absolute carrier phase. Interestingly, the absolute phase effects occur for pulse durations much longer and field intensities much weaker than those required to see such effects in single pulse excitation.

A technique is described for recording all carbons in coals almost quantitatively using 200 MHz NMR-spectrometers. A spinning rate of 8 kHz and single pulse excitation are employed to achieve this aim. The determined aromaticities obtained by means of single-pulse excitation and cross-polarisation experiments are compared. Furthermore, dipolar dephasing experiments are applied to illustrate that a great part of Cq-carbon cannot be observed using the cross-polarisation technique.

H. Jork, W. Funk, W. Fischer and H. Wimmer, Thin-layer chromatography: Reagents and detection methods, vol. Ib, Physical and chemical detection methods: Activation reactions, reagent sequences, reagents II: VCH, Weinheim, 1994 (ISBN 3-527-28205-X). xvi + 496 pp. Price DM198.00/ £81.00.

A 27Al MAS, MQMAS and off-resonance nutation NMR study of aluminium containing silica-based sol-gel materials