Gregor Bánó

EFFECT OF DIFFERENT ELEMENTARY PROCESSES ON THE BREAKDOWN IN LOW PRESSURE HELIUM GAS

We investigated the breakdown in low-pressure helium gas both experimentally and by computer simulations. At low breakdown voltages (VBR 1000 V) the experimental and simulation results show a good agreement (differences are within 20%), while at higher voltages the simulations and experiments agree qualitatively. Our simulations indicate that several processes contribute to the particular shape of the Paschen curve in helium at low pressures. These processes are: (1) the dependence of the (ion-induced) secondary electron emission yield on the ion energy, (2) the appearance of ion impact ionization of the gas at high electric fields and (3) the secondary electron emission from the cathode due to fast neutral atoms.

On the Diffusion of Hypericin in Dimethylsulfoxide/Water Mixtures— The Effect of Aggregation

Hypericin (Hyp) is a natural photosensitizing pigment with a possible application in the photodynamic therapy of cancer. Hyp is readily dissolved in dimethylsulfoxide (DMSO) but forms nonsoluble aggregates in an aqueous environment. Fluorescence spectroscopy and diffusion coefficient measurements are used to investigate the self-association of Hyp molecules in DMSO/water mixtures. Fluorescence measurements reveal that Hyp remains in its monomeric form in DMSO/water mixtures containing up to ∼20-30 wt % water. At higher water concentration, Hyp starts to form nonfluorescent aggregates. To determine the size of the aggregates, the diffusion coefficient of Hyp is determined for different DMSO/water mixtures both experimentally and theoretically. Our data indicate that the size of the aggregates increases as more water is added into DMSO. At 50 wt % water content, the effective diffusion coefficient is about 30% smaller than the calculated value for the stacked Hyp tetramer. The results indicate that in an aqueous environment, Hyp presumably produces large molecular weight stacked H-aggregates. We have also confirmed that in an aqueous environment at alkaline pH, molecules of Hyp remain in the monomeric state.

Large area mass analyzer instrument for the chemical analysis of interstellar dust particles

A new instrument to analyze the chemical composition of dust particles in situ in space has been developed. The large target area ( approximately 0.2 m(2)) makes this instrument well suited for detecting a statistically significant number of interstellar dust grains or other dust particles with a low flux. The device is a reflectron-type time-of-flight mass spectrometer that uses only flat electrodes for the generation of the parabolic potential. The instrument analyzes the ions from the impact generated plasma due to hypervelocity dust impacts onto a solid target surface. The SIMION ion optics software package is used to investigate different potential field configurations and optimize the mass resolution and focusing of the ions. The cylindrically symmetric instrument operates with six ring electrodes and six annular electrodes biased to different potentials to create the potential distribution of the reflectron. The laboratory model of the instrument has been fabricated and tested. Hypervelocity dust impacts are simulated by laser ablation using a frequency doubled Nd:YAG laser with approximately 8 ns pulse length. The experimental data show typical mass resolution m/Deltam approximately 200.

Comparison of calculated and measured optical emission intensities in a direct current argon-copper glow discharge

q . Based on a collisional)radiative model for argon atoms and copper atoms and Cu ions, which was developed as a part of a comprehensive simulation network, optical emission intensities have been calculated for argon and copper lines in a direct current argon glow discharge with copper cathode. Comparison with experimental data has been made, both with respect to the optical emission spectra and to some selected emission lines as a function of axial position. From this study, information can be obtained about the importance of various plasma processes, like electron, fast ion and fast atom impact excitation, and reabsorption of resonant radiation. Q 2000 Elsevier Science B.V. All rights reserved.

Study of the electron ion recombination in high pressure flowing afterglow: recombination of NH4·+ (NH3)2

Abstract Rate coefficients for the dissociative recombination of O 2 + and NH 4· + (NH 3 ) 2 with electrons have been measured using the high pressure flowing afterglow (HPFA) technique. The electron energy distribution function, effective temperature, and electron number density of the recombination dominated plasma have been measured by a Langmuir probe at pressures up to 11 Torr. The measured value of the recombination rate coefficient of O 2 + was used for validation of the Langmuir probe measurements at medium pressures. It is found that the recombination rate coefficient of the recombination of the cluster NH 4· + (NH 3 ) 2 ion is 1.4 × 10 −6 cm 3 s −1 at 600 K.

On the accuracy and limitations of fluid models of the cathode region of dc glow discharges

This paper compares the performance and limitations of different models of the cathode region of cold-cathode low-pressure dc glow discharges: (i) we review known modelling approaches, (ii) develop our own simulation codes based on these approaches, (iii) perform calculations using these codes for reference sets of discharge conditions, which allows a critical comparison of the models and (iv) for a further check of the simulation results we carry out Langmuir probe measurements of electron densities in abnormal Ar glow discharges. The theoretical approaches include fluid models both neglecting and including the electron energy balance equation, as well as hybrid models, which combine the fluid treatment of slow plasma species with the kinetic simulation of fast electrons. We also test the effect of the choice of the ionization source term in fluid models. We find that the electron densities calculated from the fluid models are far (several orders of magnitude) below the experimental values even if the electron energy equation is considered in the calculations. This weakness of fluid models clearly points out the importance of an accurate calculation of the ionization source term, which can only be accomplished by a kinetic approach under the conditions of highly nonlocal electron transport in the cathode region of glow discharges. In hybrid models Monte Carlo simulation is used for this purpose, and indeed, this approach gives electron densities comparable to our experimental data.

Selected ion flow drift tube study of the formation and dissociation of CO+.N2 ions in nitrogen buffer gas: the CO+.N2 bond energy

Abstract Efficient vibrational quenching of CO+ (v = 1) by N2 led to the deduction of an unexpectedly strong attractive potential between CO+ and N2. This was subsequently confirmed by ab initio quantal calculations of the CO+.N2 bond strength. The bond energy of CO+.N2 has now been determined in a selected ion flow drift tube, by measuring collisional destruction and formation of CO+.N2. The bond energy is found to be 0.7 ± 0.2 eV. This is sufficiently large to rationalize the fast vibrational quenching but somewhat less than an ab initio value. The large value of D(CO+.N2) can unambiguously be explained by resonance interaction between CO+.N2 and the charge-transfer CO.N2+ states.

Investigations on the effect of constriction in the cathode region of argon glow discharges

We have investigated the effect of constriction on the characteristics of low-pressure glow discharges in argon. In a series of experiments four discharge tubes with plane-parallel electrodes of different diameters (D = 31.4, 20, 10 and 5 mm) and same electrode separation (L0 = 45 mm) were studied. The discharges were surrounded by floating-potential metal tubes. We measured voltage-current characteristics of the discharges and recorded the spatial intensity distribution of selected spectral lines (Ar-I 750.3 nm, 811.5 nm and Ar-II 476.5 nm) in the electrode gap at current densities 0.2 mA cm-2 j 1 mA cm-2. We also observed copper lines in the spectrum originating from the sputtering of the copper cathode, even at these relatively low discharge current densities. The `effective surface of the cathode - that actively participates in discharge operation - is always smaller than the cathode surface. This results from the radial electric field formed in the cathode dark space due to the accumulation of charges on the metal tube surrounding the discharge. Using a simple model we could calculate the effective cathode diameter (which determines the current density for given current). Taking into account the current density obtained this way, the voltage of the tubes was found to obey the V = f(j/p2) scaling relation. The increasing loss of charge on the metal wall with decreasing tube diameter also resulted in structural changes in the discharge. While at large diameter the cathode dark space and the negative glow filled the interelectrode gap, at low D a positive column-like part was also formed.

Optically Trapped Surface-Enhanced Raman Probes Prepared by Silver Photoreduction to 3D Microstructures

3D microstructures partially covered by silver nanoparticles have been developed and tested for surface-enhanced Raman spectroscopy (SERS) in combination with optical tweezers. The microstructures made by two-photon polymerization of SU-8 photoresist were manipulated in a dual beam optical trap. The active area of the structures was covered by a SERS-active silver layer using chemically assisted photoreduction from silver nitrate solutions. Silver layers of different grain size distributions were created by changing the photoreduction parameters and characterized by scanning electron microscopy. The structures were tested by measuring the SERS spectra of emodin and hypericin.

Combined Langmuir probe, electrical and hybrid modelling characterization of helium glow discharges

We present a systematic study of electrical characteristics and Langmuir probe measurements on plane-parallel electrode helium glow discharges operated in normal and abnormal glow regimes. The experimental studies are complemented with numerical modelling based on a hybrid approach combining fluid and particle descriptions of the discharge plasma. A comparison between the experimental data and the two-dimensional modelling results is presented. The temperature of slow Maxwellian electrons is found to be ≈0.055 eV in the negative-glow region nearly independent of the discharge conditions. This temperature value is used as an input parameter of the hybrid model. Good agreement is obtained between the calculated spatial density distribution of electron impact excitation events and the CCD image of the light emission from the discharge. The calculated electron densities are 1.3–2 times higher than the corresponding Langmuir probe data; the possible reason for these differences is discussed.