Results 1 - 10 of 1682
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[en] We investigated the Autler–Townes splitting in photoelectron spectra of molecules steered by ultrashort laser pulses using the time-dependent wave-packet method. Structure of the Autler–Townes splitting was presented to analyze the information of a selective population of the dressed states. It was found that population transfer process, structure of photoelectron spectrum and pattern of Autler–Townes splitting can be controlled by adjusting the intensity, wavelength and delay time of laser pulses. (paper)
[en] We present 29 likely members of the young ρ Oph or Upper Sco regions of recent star formation that exhibit “accretion burst” type light curves in K2 time series photometry. The bursters were identified by visual examination of their ∼80-day light curves, though all satisfy the flux asymmetry criterion for burst behavior defined by Cody et al. The burst sources represent ≈9% of cluster members with strong infrared excess indicative of circumstellar material. Higher amplitude burster behavior is correlated with larger inner disk infrared excesses, as inferred from WISE color. The burst sources are also outliers in their large Hα emission equivalent widths. No distinction between bursters and non-bursters is seen in stellar properties such as multiplicity or spectral type. The frequency of bursters is similar between the younger, more compact ρ Oph region, and the older, more dispersed Upper Sco region. The bursts exhibit a range of shapes, amplitudes (∼10%–700%), durations (∼1–10 days), repeat timescales (∼3–80 days), and duty cycles (∼10%–100%). Our results provide important input to models of magnetospheric accretion, in particular, by elucidating the properties of accretion-related variability in the low state between major longer duration events such as EX Lup and FU Ori type accretion outbursts. We demonstrate the broad continuum of accretion burst behavior in young stars—extending the phenomenon to lower amplitudes and shorter timescales than traditionally considered in the theory of pre-main sequence accretion history.
[en] Alfvén Eigenmodes (AE) can be destabilized by energetic particles in neutral beam injection (NBI) heated plasmas through inverse Landau damping and couplings with gap modes in the shear Alfvén continua. We describe the linear evolution of the poloidal flux and the toroidal component of the vorticity in a full 3D system using the reduced MHD equations, density and parallel velocity moments for the energetic particles as well as the geodesic acoustic wave dynamics. A closure relation adds the Landau damping and resonant destabilization effects in the model. We apply the model to study the Alfvén modes stability in TJ-II, performing a parametric analysis in a range of realistic values of energetic particle β (), ratios of thermal/Alfvén velocities (), energetic particle density profiles and toroidal modes (n) including toroidal and helical couplings. The study predicts a large helical coupling between different toroidal modes and the destabilization of helical Alfvén eigenmodes (HAE) with frequencies similar to the AE activity measured in TJ-II, between 50–400 kHz. The analysis has also revealed the destabilization of GAE (global Alfvén eigenmodes), TAE (toroidal Alfvén eigenmodes) and EPM (energetic particle modes). For the modes considered here, optimized TJ-II operations require a profile in the range of to stabilize AEs in the inner and middle plasma. AEs in the plasma periphery cannot be fully stabilized, although for a configuration with , only AE are unstable with a growth rate 4 times smaller compared to the standard case and a frequency of 100 kHz. We reproduce the frequency sweeping evolution of the AE frequency observed in TJ-II as the profile is varied. The AE frequency sweeping is caused by consecutive changes of the instability dominant modes between different helical families. (paper)
[en] Highlights: • The need by industry for small area analysis spurred the advent of the XPS Microprobe. • The Microprobe design allows for rapid x-ray induced secondary electron imaging. • Small areas of interest can quickly be located and analyzed. • Industrial applications include varied types of failure and contamination analyses. • Multipoint depth profiles can be obtained in one acquisition with one sputter crater. - Abstract: We will review the evolution of x-ray photoelectron spectroscopy (XPS / ESCA) instrumentation and applications that led to the development of the scanning XPS microprobe, describe its unique capabilities, and how they have impacted the use of XPS for industrial applications.
[en] Here, we study the behavior of the pseudogap in overdoped Bi2Sr2CaCu2O8+δ by electronic Raman scattering (ERS) and angle-resolved photoemission spectroscopy (ARPES) on the same single crystals. Using both techniques we find that, unlike the superconducting gap, the pseudogap related to the antibonding band vanishes above the critical doping pc = 0.22 . Concomitantly, we show from ARPES measurements that the Fermi surface of the antibonding band is holelike below pc and becomes electronlike above pc. This reveals that the existence of the pseudogap depends on the Fermi surface topology in Bi2Sr2CaCu2O8+δ, and more generally, puts strong constraint on theories of the pseudogap phase.
[en] Highlights: • Pyridine- and terpyridine terminated monolayers show different deposition qualities. • Distinction of different nitrogens by spectroscopic methods. • Monolayer qualities have a significant effect on layer-by-layer deposition. • When present, oxidization only occurs prior to monolayer deposition. • Preferential orientation in multilayers on one specific monolayer. - Abstract: The chemical composition of surfaces functionalized with self-assembled monolayers (SAMs) is an important parameter that determines their performance in a broad range of applications, from immobilizing molecular machines to initiation and growth control of MOFs (Metal-Organic Frameworks). In this article, a critical survey of XPS (X-ray photoelectron spectroscopy) and NEXAFS (near edge X-ray absorption fine structure) spectroscopy data for pyridine-functionalized monolayers on gold surfaces is presented to compile correlations that have been indiscernible before. Monolayers with aromatic backbones are compared to monolayers with aliphatic backbones. Monolayers with pyridine end-groups are compared to mixed monolayers formed terpyridine-functionalized by molecules end-groups and non-functionalized molecules. Thiol-oxidation during ageing of the SAMs in air is addressed. And finally, the addressability of the SAMs for the deposition of metal-ions and organic molecules is investigated and compared. This work consequently delivers a comprehensive set of spectroscopic data of (ter-)pyridine-terminated SAMs and their performance as template for the preparation of functional multilayers of macrocycles exhibiting a preferential orientation.
[en] Chitosan with its surface-properties and biodegradability is a promising biomaterial for green packaging applications. Till now, this application is still limited due to chitosan high sensitivity to water. Some existing studies deal with the incorporation of hydrophobic additives to enhance water-proof performances of chitosan films. As these additives may impair the film properties, our study focuses on chitosan efficient hydrophobization by means of simple and successful surface grafting reactions. Chitosan films prepared by solvent casting were modified by means of surface-initiated activators regenerated by electron transfer atom radical polymerization (SI-ARGET-ATRP) of 2-hydroxyethyl methacrylate (HEMA) followed by esterification reaction with fluorinated acyl compound. X-ray photoelectron spectroscopy (XPS), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) highlighted the surface chemical changes after each step. Surface properties were investigated by contact angle measurements and surface energy calculations. Hydrophobic surfaces with low surface energy and good water-repellent properties were obtained using a simple handling polymerization procedure. This is the first study in applying ARGET ATRP to prepare hydrophobic biopolymer films offering potential applications in packaging. (author)
[en] Highlights: • Several applications of using XPS in an industrial laboratory that supports manufacturing and R&D activities are discussed. • Common approaches by XPS to surface chemical analytical problems are discussed. • Problems such as contamination, characterization of thin films, and determination of surface chemistry are presented. - Abstract: X-ray photoelectron Spectroscopy (XPS) is a widely-used technique for surface chemical analysis. In an industrial laboratory XPS can be used for a wide variety of applications ranging from identification of contamination on a surface to characterization of materials as process control, or as a method for characterizing new materials in a research environment. Some analyses are performed to provide a ‘yes’ or ‘no’ answer to questions such as ‘is silicone present?’ or ‘was the surface plasma treated?’. In other cases, qualitative analysis or a simple approach for quantification does not adequately describe a material. A more detailed analysis may require imaging or depth profiling, and the use of numerical methods such as curve fitting, overlayer modeling, or linear least squares fitting to more accurately describe a material. We relate selected vignettes where XPS spectral analysis, imaging, and sputter profiling are applied to some of the many types of samples that have come into our industrial laboratory.