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In studies of extragalactic radio sources with multiple compact components the determination of which components, if any, are stationary and which moving is of importance. In order to learn about the radio properties of the individual components it is also relevant to be able to register maps made at several wavelengths. Both tasks are usually not possible with VLBI because of the irrecoverable corruption of the fringe phase introduced by the propagation medium and the instrumentation. However, when two or more compact radio sources are separated by only a small angle from each other difference techniques can be used to help tackle both questions.
Mark III VLBI observations of the pulsars PSR 0329+54 and PSR 1133+16 were made at 2.3 GHz using antennas with diameters and locations as follows: 100m, Effelsberg, West Germany (but only for SPR 0329+54); 43m Green Bank, WV, USA; and 40m, Big Pine, CA, USA. The Mark III processor at the Haystack Observatory was “gated” to compute visibility amplitudes and phases as a function of pulsar longitude. This method allowed a) an improvement of the signal to noise ration, by as much as a factor of ten in the case of PSR 1133+16, and b) an interferometric investigation of the pulse structure.
On 1983 May 10–11 we undertook simultaneous λ3.6 and λ13 cm Mark III VLBI observations of the quasars 1038+528 A,B. Our experimental conditions (i.e., synthesized band, uv-coverage, etc.) were almost identical to those we used on 1981 March 17–18. Thus, we could make a direct comparison of the results from both epochs.
We have made 3.6 cm VLBI observations of the RS CVn binary star IM Pegasi (HR 8703) approximately four times per year since 1997 in support of the NASA/Stanford Relativity Gyroscope Experiment (Gravity Probe B). Phase-referenced maps reveal structural changes in the radio emission of the star on hour time scales during several of the sessions. Analyses of the VLBI phase delays with a Kalman-filter estimator reveal submilliarcsecond motions of the radio centroid of the star on hour and even subhour time scales. The observed structural changes and centroid motions often coincide with rapid changes in the star's flux density, as measured with the VLA. We report on our latest results and summarize our findings to date.
The NASA/Stanford Relativity Mission (Gravity Probe B) is to test the unverified “frame-dragging” prediction of general relativity through measurements of the precessions of orbiting gyroscopes. For mission accuracy goals to be met, the proper motion of a “guide star,” whose position will be used as an inertial reference, must be determined in an extragalactic reference frame with a standard error less than 0.5 mas/yr. We discuss our VLBI observations of the current guide-star candidates (radio stars HR 1099, HR 5110, and HR 8703) and our techniques for obtaining differential astrometric positions with the needed accuracy.
We determine the relative separations of the sources in the triangle 1803+784/1928+738/2007+777 with submilliarcsecond accuracy from global 8.4 GHz VLBI observations. We remove the ionospheric contribution to the phase-delay observable using ionospheric total electron content estimates obtained from Global Positioning System (GPS) data. The triangular geometry provides a consistency check through sky closure.
Third-order optical susceptibilities (γijkl) can be efficiently analyzed for a variety of molecular structures employing perturbation theory and a PPP-SCF-SECI-DECI π-electron model Hamiltonian. The key, frequency tripling second hyperpolarizability γijkl(−3ω;ω,ω,ω) is calculated with full single and double CI. It is found that double excitations play a major role in third-order processes, and that γijkl, like the polarizability αij, is sensitive largely to the overall size (volume) of the π system, although charge transfer excitations may also contribute. The frequency dependence of v and correlations between γijkl and conjugation length are found for a series of trans polyenes.
Factors contributing to the polarizability (αij) and frequencydoubling hyperpolarizability (βijk) of 2-(p-dimethylaminophenyl)-6-(pnitrophenyl) benzo(l,2-d:4,5-d′ )blsthiazole (DNBT) are analyzed via perturbation theory and the PPP-SCF-MECI π-electron model Hamiltonian. While the observable part of β (βvec) is clearly identifiable with a small number of charge transfer excitations along the molecular dipole direction, a (and by inference, the second-order hyperpolarizability γ) is more closely related to the overall size (volume) of the π-electron cloud. As a consequence, βvec is far more sensitive to molecular distortions which affect donor-acceptor charge transfer interactions than is π. The more sensitive frequency dependence of βvec can be understood in terms of the three-photon character of this nonlinearity.
The progress of dynamically disordered hopping (DDH) in modelling charge transport in polymer electrolytes is reviewed. The DDH model successfully describes many of the salient features of polymer electrolytes, most notably, the frequency and temperature dependence of the conductivity. Furthermore, analyses and simulations based on the DDH model provide rich mechanistic information. The general picture of charge transport that emerges from the DDH model is one in which two classes of charge carriers exist in thermal equilibrium:quasi-free and bound. The quasi-free carriers dominate the conductivity response and diffuse freely over short distances (≈1Å) with longer range diffusion requiringlocal segmental motions, renewal in the language of DDH, of the polymer solvent. The bound carriers, which are likely polymer solvated ion clusters, are immobile on the time-scale of renewal and contribute relatively little to the conductivity.
An attractive and challenging approach to the construction of robust, thin film materials with large second-order optical nonlinearities is the covalent self-assembly of aligned arrays of high-β molecular chromophores into multilayer superlattices. In this paper, we describe the dispersion of second harmonic generation (SHG) in a self-assembled (SA) monolayer containing a stilbazolium chromophore. The frequency-dependent measurements were performed on 25 Å thick monolayers on glass using a tunable (0.4–2 μm) light source based on optical parametric amplification (OPA). The SHG spectrum contains a clear two-photon resonance at hω = 1.3eV. The maximum in the second-order susceptibility coincides with a low energy chromophore-centered charge-transfer excitation at 480 nm. The experimental SHG dispersion values compare favorably with theoretical results computed using a sum-over-states (SOS) formalism. However, the measured values exhibit a somewhat broader band response than the theoretical curve, and the origin of this behavior is discussed.
Angular-resolved ESCA was used to study single cadmium arachidate monolayers transferred to Si (100) wafers by the Langmuir-Blodgett technique. We find the monolayers to be of high integrity with respect to those defects which enhance the escape probability of substrate photoelectrons through the overlayer. The inelastic mean-free pathlengths of Si (2p) and C (1s) electrons were calculated to be 49±6 Å and 45±6 Å for the kinetic energies of 1388 eV and 1202 eV, respectively. The overall ordering of the hydrocarbon chains is less than for alkane thiols assembled on noble metals. We find that the precision of the Tyler algorithm to deconvolute angular-resolved ESCA data into depth profiles is accurate within 10% for predicting the thickness of the hydrocarbon overlayer but less precise for intermediate layers.
A new process has been developed for the high rate vacuum deposition of solid films from high molecular weight/low vapor pressure liquid, or even liquid/solid suspension, monomer precursors. The gas resulting from the flash evaporation of a liquid monomer mixture, or from a suspension of insoluble solid particles in liquid monomer, is used as the support medium for a glow discharge in a Plasma Enhanced Chemical Vapor Deposition-like (PECVD) process. Due to the high molecular weight/low vapor pressure nature of the precursors, the plasma of the flash evaporated gas cryocondenses at extremely high rate on substrates at ambient, and higher, temperatures. Upon condensation the liquified plasma immediately begins to polymerize to form a solid film due to the high concentration of radicals and ions contained in the liquid film. The process has been successfully implemented in a vacuum roll coating system in a roll-to-roll deposition process. Polymer films, Molecularly Doped Polymer (MDP) composite films of polymer containing light emitting organic molecules, and MDP composite films of polymer containing organic dye molecules have been deposited at thicknesses ranging from 0.1 microns to 24 microns at webs speeds as high as 96 linear meters per minute. This new deposition process will be discussed along with some properties of the films fabricated with this new process.
Recent advances in atomic force microscopy (AFM) allow one to perform complex analysis of materials surfaces and near-surface regions with a nanometer scale resolution; this includes the imaging of surface topography and the measurements of stiffness, elasticity, hardness and nano-tribological characteristics. Understanding of the complex AFM response is still in its early stage, and it frequently requires the use of multiple complementary techniques. In the present work we have applied AFM for the study of the surface and nearsurface mechanical properties of plasma-treated polycarbonate. The results from AFM analysis, such as the stiffness and the relative Young modulus, were compared with the hardness values obtained by depth-sensing indentation and by the micro-scratch technique. The characteristics of the observed hardened surface layer are discussed with respect to the role of the “interphase” in adhesion en1hancement of plasma-deposited films on polymers.
Amine-functionalized thin films were prepared by plasma induced deposition of allylamine. Radio frequency (rf) plasma polymerization was carried out under both continuous wave (CW) and pulsed plasma conditions to control the film chemistry, all other process variables being held constant. Using plasma polymerized films for subsequent grafting reactions or for direct biomedical applications usually involves exposure of the films to a solvent environment. It was the major focus of this work to investigate both the chemical and physical effects of solvents on plasma polymerized allylamine films. Film properties were determined using Fourier Transform Infrared Spectroscopy (FTIR), X-ray Photoelectron Spectroscopy (XPS), Waveguide Mode Spectroscopy (WaMS) and Surface Plasmon Spectroscopy (SPS). WaMS provided an ideal opportunity to study the complex swelling and drying behaviour of these films by allowing a simultaneous study of the changes in film thickness and refractive index. Although the amine groups of the monomer were increasingly retained in the films as the RF duty cycle was lowered, a larger amount of oxygen was also found to be incorporated upon exposure to air. Extraction in ethanol led to a decrease in film thickness, especially for the films produced at low duty cycles, but appeared to have little effect on the film composition, as measured by XPS and FTIR.
Coatings of biologically active molecules on synthetic ”bulk“materials are of much interest for biomedical applications since they can in principle elicit specific, predictable. controlled responses of the host environment to an implanted device. However, issues such as shelf life. storage conditions, biological safety, and enzymatic attack in the biological environment must be considered; synthetic proteins may offer advantages. In this study we investigated the covalent immobilization onto polymeric materials of synthetic proteins which possess some properties that mimic those of the natural protein collagen, particularly the ability to form triple helical structures, and thus may provide similar bio-responses while avoiding enzymatic degradation. In order to perform immobilization of these collagen-like molecules (CLMs) under mild reaction conditions, the bulk materials are first equipped with suitable surface groups using rf plasma methods. Plasma polymer interlayers offer advantages as versatile reactive platforms for the immobilization of proteins and other biologically active molecules. Application of a thin plasma polymer coating from an aldehyde monomer is particularly suitable as it enables direct immobilization of CLMs by reaction with their terminal amine groups, using reductive amination chemistry. An alternative route is via plasma polymer layers that contain carboxylic acid groups and using carbodiimnide chemistry. A third route makes use of alkylamme plasma polymer interlayers, which are less process sensitive than aldehyde and acid plasma coatings. A layer of poly-carboxylic acid compounds such as carboxylic acid terminated PAMAM-starburst dendrimers or carboxymethylated dextran is then attached by carbodiimide chemistry onto the amine plasma layer. Amine-terminated CLMs can then be immobilized onto the poly-carboxylic acid layer. Surface analytical methods have been used to characterize the immobilization steps and to assess the surface coverage. Initial cell attachment and growth assays indicate that the biological performance of the CLMs depends on their amino acid sequence.
A cylindrically-configured plasma treatment system in Radio Frequency Glow Discharges fed with ammonia was used to modify the internal surface of ePTFE arterial prostheses. The effects of RF-power on the surface chemical composition were characterized by XPS. Results show that the treatment at 20 W, for 250 seconds and under an ammonia pressure of 300 mtorr yielded a good compromise between ablation and substitution phenomena on the surface. With this treatment, fluorine content was decreased, while up to 20 % of the surface atoms were substituted by nitrogen.
The interest in incoherent sources for wavelength selective photochemistry has increased lately, but little is still known about the behavior of polymers when exposed to far UV and vacuum UV (VUV) radiation, for example that emitted from low-pressure plasmas. In order to study VUV-UV effects on several polymers (polyethylene - PE, polystyrene - PS, hexatriacontane - HTC, polymethylmethacrylate - PMMA and polycarbonate - PC), we used the wellcharacterized emissions from hydrogen and hydrogen/argon mixture plasmas as light sources. Mass changes were measured in-situ using a quartz crystal microbalance (QCM); irradiated samples were analyzed ex-situ by XPS, spectroscopic ellipsometry, and AFM.
Nanocomposite films consisting of gold nanoclusters embedded in a fluorocarbon matrix were prepared by simultaneous plasma polymerization of a fluorocarbon gas and magnetron sputtering of a gold target. The optical constants of the films were determined using variable angle spectroscopic ellipsometry (VASE). The Maxwell-Garnett (M-G) effective medium theory (EMT) was used to extract the microstructural characteristics (gold volume fraction, cluster size). The predictions of the M-G EMT were compared with the results from x-ray photoelectron spectroscopy. Angle-resolved measurements suggest the presence of a carbonaceous layer at the cluster surface. The presence of such a layer is discussed in the context of its influence on the validity of the optical model
We describe a new approach to deposition of Parylene N thin films. It utilizes a small scale, sonic speed, Jet Vapor DepositionTM (JVDTM) process technology in place of the conventional larger scale, slow flow, Gorham apparatus. It employs a simple but powerful strategy to promote radical polymerization: exposure of the growing film, during deposition, to a high flux of atomic hydrogen. We believe that H atoms have two effects: they clean oxygen from the substrate, and they promote crosslinking in the Parylene film by abstraction of H atoms from the Parylene ring or side groups. With “H atom assisted JVD” Parylene N deposits and adheres even on warm substrates; it has reduced index of refraction and dielectric constant.