Intermediate and high-mass ion beams from a 10-cm Duopigatron

Experimental studies of a 10-cm Duopigatron as a source of argon, krypton, and xenon ion beams are reported. Source plasma instabilities are examined, and the mass dependence of oscillation frequencies and instability onset conditions are determined. Arc current and density oscillations are found to be associated with ion acoustic fluctuations with frequencies scaling as 1/M^1/2. Langmuir probe measurements within the source plasma double layer are used to indicate the physical mechanism responsible for the observed large-amplitude are current shifts. Ion beams have been extracted at energies up to 18 kV, and drain currents up to 540 mA for argon, 440 mA for krypton, and 520 mA for xenon have been achieved with source plasma densities in the range 10¹¹–10¹² cm^–3. Excellent agreement with existing theoretical models has been obtained in the mass and density dependence of the extraction current, as well as the voltage at which transition from space-charge limited to ion saturation emission occurs.     

Space- and direction-resolved Langmuir probe diagnostic in rf planar discharges

To consider the anisotropy of the plasma in the sheath regions, Langmuir probe characteristics are measured using a direction-resolving technique. The operation frequency, related to the neutral gas density (/p₀), is chosen in such a way that the electron velocity distribution function (VDF) may be regarded as frozen and the probe diagnostic may be performed without time resolution. In order to prevent convolution of the VDF from the effect of the time-dependent plasma potential, the rf component of the probe bias voltage is compensated to a minimum. The plasma potential, the mean energy of the electrons, and the electron density, averaged over the discharge bulk, are presented as functions of the discharge current and the neutral gas pressure for the O₂ gas. By means of a reactor model based on the theory of plane probes and using the plasma parameter measured, the sheath and bulk portions of the maintaining voltage are separated. In this procedure the thickness of the sheaths in front of the electrodes and the phase difference between discharge current and maintaining voltage are also obtained.     

Analytical theory of hysteresis in ion channels: two-state model

Channel-forming proteins in a lipid bilayer of a biological membrane usually respond to variation of external voltage by changing their conformations. Periodic voltages with frequency comparable with the inverse relaxation time of the protein produce hysteresis in the occupancies of the protein conformations. If the channel conductance changes when the protein jumps between these conformations, hysteresis in occupancies is observed as hysteresis in ion current through the channel. We develop an analytical theory of this phenomenon assuming that the channel conformational dynamics can be described in terms of a two-state model. The theory describes transient behavior of the channel after the periodic voltage is switched on as well as the shape and area of the hysteretic loop as functions of the frequency and amplitude of the applied voltage. The area vanishes as the voltage period T tends to zero and infinity. Asymptotic behaviors of the loop area A in the high- and low-frequency regimes, respectively, are A approximately T and A approximately T(-1).     

Study of reactive ion etching of Si and SiO2 for CFxCl4−x gases

A parametric study of the etching of Si and SiO₂ by reactive ion etching (RIE) was carried out to gain a better understanding of the etching mechanisms. The following fluorocarbons (FCs) were used in order to study the effect of the F-to-Cl atom ratio in the parent molecule to the plasma and the etching properties: CF₄, CF₃Cl, CF₂Cl₂, and CFCl₃ (FC-14, FC-13, FC-12, and FC-11 respectively). The Si etch rate uniformity across the wafer as a function of the temperature of the wafer and the Si load, the optical emission as a function of the temperature of the load, the etch rate of SiO₂ as a function of the sheath voltage, and the mass spectra for each of the FCs were measured. The temperature of the wafer and that of the surrounding Si load strongly influence the etch rate of Si, the uniformity of etching, and the optical emission of F, Cl, and CF₂. The activation energy for the etching reaction of Si during CF₄ RIE was measured. The etch rate of Si depends more strongly on the gas composition than on the sheath voltage; it seems to be dominated by ion-assisted chemical etching. The etching of photoresist shifted from chemical etching to ion-assisted chemical etching as a function of the F-to-Cl ratio and the sheath voltage. The etch rate of SiO₂ depended more strongly on the sheath voltage than on the F-to-Cl ratio.     

Dependence of the self-bias on the plasma parameters and on the electrode areas in RF plasma reactors

From the current-voltage characteristics for the collisionless sheath the dependence of the self-bias on the plasma parameters (electron temperature, ratio of electron temperatures and electron densities at the two electrodes), on the applied external voltage, and on the ratio of the electrode areas is investigated. Sinusoidal (is well as periodic rectangular and triangular time dependences of the voltage are considered. The integral equation for the self bias voltage is solved numerically. For large external RF voltages in comparison to the floating potential, simple analytical formulas result for the dependence of the bias voltage on the plasma parameters and the dimensions of the electrodes, which can be useful in practice.     

Ion transmission in an electrospray ionization-mass spectrometry interface using an S-lens

We present the design and performance of an in-house built electrospray ionization-mass spectrometry (ESI-MS) interface equipped with an S-lens ion guide. The ion source was designed specifically for our ion beam experiments to investigate the chemical reactivity and deposition of the clusters and nanoparticles. It includes standard ESI-MS interface components, such as nanoelectrospray, ion transfer capillary, and the S-lens. A custom design enables systematic optimization of all relevant factors influencing ion formation and transfer through the interface. By varying the ESI voltage and flow rate, we determined the optimal operating conditions for selected silica emitters. A comparison of the pulled silica emitters with different tip inner diameters reveals that the total ion current is highest for the largest tip, whereas a tip with the smallest diameter exhibited the highest transmission efficiency through the ESI-MS interface. Ion transmission through the transfer capillary is strongly limited by its length, but the loss of ions can be reduced by increasing the capillary voltage and temperature. The S-lens was characterized over a wide range of RF frequencies and amplitudes. Maximum ion current was detected at RF amplitudes greater than 50 V peak-to-peak (p/p) and frequencies above 750 kHz, with a stable ion transmission region of about 20%. A factor of 2.6 increase in total ion current is observed for 650 kHz as RF amplitudes reach 400 V p/p. Higher RF amplitudes also focus the ions into a narrow beam, which mitigates their losses when passing through the ion guide.     

Sheath liquid interface for the coupling of normal-phase liquid chromatography with electrospray mass spectrometry and its application to the analysis of neoflavonoids

A novel interface that allows normal-phase liquid chromatography to be coupled with electrospray ionization (ESI) is reported. A make-up solution of 60 mM ammonium acetate in methanol, infused at a 5 microl min(-1) flow-rate at the tip of the electrospray probe, provides a sheath liquid which is poorly miscible with the chromatographic effluent, but promotes efficient ionization of the targeted analytes. Protonated molecules generated in the ESI source were subjected to tandem mass spectrometric experiments in a triple-quadrupole mass spectrometer. The main fragmentation reactions were characterized for each analyte and specific mass spectral transitions were used to acquire chromatographic data in the multiple reaction monitoring detection mode. Results obtained during optimization of the sheath liquid composition and flow-rate suggest that the electrospray process was mainly under the control of the make-up solution, and that it forms an external charged layer around a neutral chromatographic mobile phase core. This sheath liquid interface was implemented for the analysis of some neoflavonoid compounds and its performance was evaluated. Limits of detection were established for calophillolide, inophyllum B, inophyllum P and inophyllum C at 100, 25, 15 and 100 ng ml(-1), respectively.     

Preparation of Core‐Sheath Composite Nanofibers by Emulsion Electrospinning

Summary: Uniform core‐sheath nanofibers are prepared by electrospinning a water‐in‐oil emulsion in which the aqueous phase consists of a poly(ethylene oxide) (PEO) solution in water and the oily phase is a chloroform solution of an amphiphilic poly(ethylene glycol)‐poly(L ‐lactic acid) (PEG‐PLA) diblock copolymer. The obtained fibers are composed of a PEO core and a PEG‐PLA sheath with a sharp boundary in between. By adjusting the emulsion composition and the emulsification parameters, the overall fiber size and the relative diameters of the core and the sheath can be changed. A mechanism is proposed to explain the process of transformation from the emulsion to the core‐sheath fibers, i.e., the stretching and evaporation induced de‐emulsification. In principle, this process can be applied to other systems to prepare core‐sheath fibers in place of concentric electrospinning and it is especially suitable for fabricating composite nanofibers that contain water‐soluble drugs.

Schematic mechanism for the formation of core‐sheath composite fibers during emulsion electrospinning.     

Impedance of electrochemical cell with hysteresis effects in voltammetric characteristics

The effect of nonlinear processes on the electrochemical cell impedance is studied theoretically. It is shown that the presence of hysteresis effects in the voltammetric characteristics results in phase shift φ of the first harmonic of current vs. the applied sine voltage. The performed calculations showed that the tangent of φ is proportional to the area encompassed by the hysteresis loop. The sign of φ is determined by the direction of moving around the inner region. The obtained results allowed explaining number of experimental data obtained using the method of impedance spectroscopy at low frequencies.     

Hypochromism of polynucleotides in the nearest neighbour approximation

A method of calculating the hypochromism of polynucleotides in the nearest neighbour approximation is given by perturbation theory. The calculated hypochromism values of the first UV-absorption band of polynucleotides poly(dA)·poly(dT) and poly(dG)·poly(dC) are in good agreement with experiment. In this approximation the origin of the hypochromic effect is studied for the double-stranded polynucleotides; the dependence of the hypochromism upon the length of the polynucleotide is given.     

Anharmonic analysis of the vibrational spectra of some cyanides and related molecules of astrophysical importance

A detailed analysis of the vibrational spectra of carbonyl cyanide, diethynyl ketone and acetyl cyanide has been conducted in harmonic and anharmonic approximations. RHF, MP2 and density functional theory (DFT) methods with 6-311++G(2df,2p) basis sets and B3LYP functionals have been employed. Spectroscopic constants such as anharmonicity constants, rotational and centrifugal distortion constants, rotation-vibration coupling constants and Coriolis coupling coefficients have been calculated for each molecule and compared with the experimental data, where available. A close agreement between the calculated and experimental values of the spectroscopic constants has been obtained. Complete assignments have been provided to the fundamental bands, overtones and combination tones of the molecules. Density functional theory based anharmonic frequencies compare well with the experimental frequencies within +/-18 cm(-1) on an average. RHF and MP2 methods, however, give much higher values for the frequencies that need scaling even in the anharmonic approximation.     

Motion of drops on a surface induced by thermal gradient and vibration

It is well known that a liquid drop with a low contact angle (approximately 45 degrees ) and low wetting hysteresis moves toward the colder region of a temperature gradient substrate as a result of the thermal Marangoni force. A moderately sized water drop, however, usually does not move on such a surface because of the overwhelming effect of hysteresis. The water drop can, however, be forced to move when it is vibrated on a temperature gradient surface with its velocity exhibiting maxima at the respective Rayleigh frequencies. A simple model is presented that captures the dependence of drop velocity on hysteresis, vibration amplitude, and the forcing and resonance frequencies of vibration.     

Surface treatment of magnesium oxysulfate whiskers through plasma polymerization

The surface of magnesium oxysulfate (MOS) whiskers was treated through plasma polymerization to increase the compatibility between the MOS whiskers and a polymer matrix. Different plasma parameters were chosen to determine the most hydrophobic coating. The surface structure of the plasma-treated MOS whiskers was examined. The MOS whiskers retained their crystal structure after plasma treatment, as shown by X-ray powder diffraction (XRD) analyses. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy analyses revealed that a polymer sheath was formed on the surface of the MOS whiskers, and interfacial chemical bonds were generated between the polymer sheath and the MOS whiskers. The thin-layer polymer sheath was uniform around the entire surface of the MOS whiskers and exhibited a typical amorphous structure, as determined by transmission electron microscopy (TEM) combined with selected area electron diffraction (SAED) analyses. The possible reaction mechanism on the surface of the MOS whiskers under plasma treatment was then proposed. Finally, the effect of surface treatment was evaluated by scanning electron microscopy (SEM), measurement of the contact angle and contact angle hysteresis, and torque rheometer. Results showed that plasma treatment could markedly increase the hydrophobicity of the MOS whiskers' surface, effectively reducing the agglomeration and improving the dispersibility of the MOS whiskers in the matrix, which results in the improved compatibility between the MOS whiskers and the polyvinyl matrix, as well as the processability of the composites.     

Electrochemical diffusion potential in a flame plasma: theory and experiment

A flame doped with an appropriate additive to produce positive ions and free electrons is a quasineutral, weak, continuum plasma. When bounded by a metallic burner upstream and a metal plate downstream, the two electrodes and flame plasma can be viewed as a gas-phase electrochemical cell. When the ion (and electron) density varies continuously along the flame axis, an expression for the diffusion potential can be derived in terms of the concentration gradient. The familiar logarithmic dependence on the ion concentration is obtained. A plasma sheath develops at the metal plate electrode; it sustains a potential difference which can be modeled by a Boltzmann distribution of the electrons in the sheath. Since the plate has to be cooled in practice, the average sheath temperature is less than the flame temperature because the sheath occurs inside the thermal boundary layer which covers the plate electrode. Inevitably, the reduced sheath temperature affects the sheath voltage. Experimental measurements of the “cell” voltage are made for the two cases of a positive concentration gradient using a sodium plasma, and a negative gradient by doping the flame with methane. As predicted theoretically, the cell voltages have opposite signs. However, the magnitude of the cell voltage seems to depend significantly on the sheath temperature which appears to decrease steadily with increasing distance downstream from the burner. It is also possible that the measured cell voltages involve unknown surface contact potentials. When compared with solution concentration cells, gas-phase flame systems exhibit both similarities and differences.     

Frequency dependence of hysteresis associated with the electromechanical performance of PVDF film

The thickness strains, the electric displacements, and the hysteresis exhibited by poly(vinylidene fluoride) films under sinusoidal applied electric fields were measured over a range of frequencies from 0.05 to 100 Hz. The loops of strain‐ and electric displacement versus electric field exhibit a shape and hysteresis that undergoes a continuous and reversible change with frequency. At lower frequencies (<5 Hz), the shape and large hysteresis are characteristic of a ferroelectric with degrees of remanent polarization. At higher frequencies (>5 Hz), the shape and slight hysteresis of the loops are representative of a nonremanently polarized ferroelectric. This dependence of shape and hysteresis on frequency is attributed to the difference in rates between the rapid rate by which the polarization of domains of aligned dipoles is reversed in a periodic field and the slower rate by which the polarization is made remanent. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3207–3214, 2007     

Theory of radiationless multiphonon transitions in the electron shells of molecules

The probability of a multiphonon transition is calculated in the second approximation of the nonadiabatic perturbation theory. Allowance is made for resonance of the adiabatic potentials and for the change in the frequencies of the normal modes resulting from the transition.Read at the Symposium on Quantum Chemistry, Palanga, June 1965.     

辉光放电低温等离子体分解乙醇水溶液制氢

在辉光放电分解乙醇制氢过程中, 高能电子在反应中起到了最为关键的作用, 非法拉第效应使得电流效率获得大幅度提升, 产物产量远远高于理论产量. 本文研究了乙醇水溶液辉光放电等离子体电解制氢的过程. 实验研究发现, 辉光放电分解乙醇水溶液的产物主要以H₂和CO为主, 还有少量的C₂H₄、CH₄、O₂和C₂H₆. H₂体积分数能达到59%以上, CO为20%左右. 通过对影响辉光放电的因素进行实验后发现: 乙醇体积分数的大小不会影响辉光放电的伏安特性参数; 电导率的提高会使‘Kellogg 区’收窄, 同时使放电尽快进入辉光放电. 此外, 乙醇体积分数越高H₂体积分数越低, 产气速率在乙醇体积分数为30%和80%附近时达到极大值; 提高放电电压和电导率对辉光放电的影响规律是相类似的, 其实质都是增大了辉光放电加载在等离子鞘层两端的电压,H₂体积分数基本不随二者的变化而变化, 但提高溶液的电导率更有利于减少辉光放电引起的焦耳热.     

Electrospun core–sheath fibers for integrating the biocompatibility of silk fibroin and the mechanical properties of PLCL

In the process of preparing core–sheath fibers via coaxial electrospinning, the relative evaporation rates of core and sheath solvents play a key role in the formation of the core–sheath structure of the fiber. Both silk fibroin (SF) and poly(lactide‐co‐ε‐caprolactone) (PLCL) have good biocompatibility and biodegradability. SF has better cell affinity than PLCL, whereas PLCL has higher breaking strength and elongation than SF. In this work, hexafluoroisopropanol (HFIP)‐formic acid (volume ratio 8:2), HFIP and HFIP–dichloromethane (volume ratio 8:2) were used to dissolve PLCL as the core solutions, and HFIP was used to dissolve SF as the sheath solution. Then, core–sheath structured SF/PLCL (C‐SF/PLCL) fibers were prepared by coaxial electrospinning with the core and sheath solutions. Transmission electron microscopy images indicated the existence of the core–shell structure of the fibers, and energy dispersive X‐ray analysis results revealed that the fiber mat with the greatest content of core–shell structure fibers was obtained when the core solvent was HFIP–dichloromethane (volume ratio 8:2). Tensile tests showed that the C‐SF/PLCL fiber mat displayed improved tensile properties, with strength and elongation that were significantly higher than those of the pure SF mat. The C‐SF/PLCL fiber mat was further investigated as a scaffold for culturing EA.hy926 cells, and the results showed that the fiber mat permitted cellular adhesion, proliferation and spreading in a manner similar to that of the pure SF fiber mat. These results indicated that the coaxial electrospun SF/PLCL fiber mat could be considered a promising candidate for tissue engineering scaffolds for blood vessels. Copyright © 2014 John Wiley & Sons, Ltd.     

Molecular orbital theory of the electronic structures of one-dimensional molecular crystals

A non-empirical tight-binding LCAO SCF MO treatment of one-dimensional molecular crystals based on the SCF perturbation theory is presented. The simpler version of this method at the level of the CNDO/2 approximation is also given.     

Ion motion in the rectangular wave quadrupole field and digital operation mode of a quadrupole ion trap mass spectrometer

A quadrupolar electric field driven by a rectangular wave voltage can be used for mass-selective storage and analysis. The ion motion in such an electric field is derived, and the stability of ions is presented in the a-q diagram that is commonly used for sinusoidal wave quadrupole mass spectrometry in association with the solution of the Mathieu equation. The pseudo-potential well is discussed in an approximation that leads to the relation of secular frequency to operating parameters. A scheme for a digital ion trap mass spectrometer is described, based on this theory. An ion optics simulation was performed to check the theory of resonant ejection, and to prove the feasibility of the mass scan method for a practical ion trap of such geometry.