Juraj Országh

Electron impact excitation of methane: determination of appearance energies for dissociation products

In this work, we present an experimental study of dissociative excitation of CH4?utilizing a crossed electron molecular beam experiment. Methane was excited by nearly monochromatic electrons generated by a trochoidal electron monochromator. The dissociative products were identified on the basis of the emission spectra in the ultraviolet?visible (UV/VIS) spectral range. The excitation functions were recorded as the function of the electron energy for different emission bands of the fragments (Balmer series for H: n = 3,4?9?2, and moreover, CH: A2??X2?, CH: B2??? X2?, CH: C2?+? X2?, CH+: B1?? A1?, and CI: 2p3s 1Po1?2p2 1S0). From the experimental data we have determined the threshold energies for excitation of particular fragments. Present experimental results indicate that the threshold energies for some dissociative excitation channels could be lower by ?1?2?eV in comparison to earlier studies and indicate that different dissociative processes may be operative at the threshold than assumed in the former studies.

Positive and negative corona discharges in flowing carbon dioxide

The effect of the gas flow rate (10–320 cm3 min−1) on the process of ozone formation in both positive and negative corona discharges has been studied using a coaxial cylindrical system of electrodes fed by dry CO2. The source of ozone is electron impact dissociation of carbon dioxide to liberate oxygen atoms and their subsequent formation of oxygen molecules, which may then form ozone by the well-known Chapman mechanism. Small increases in flow rate were found to cause a major increase in the discharge current measured in the negative corona discharge. This effect was found to correspond to observed changes in the ozone concentration within the discharge and is a consequence of dissociative electron attachment to ozone leading to negative ion formation in the discharge. In contrast no direct effect of ozone on the positive corona discharge current was observed. The observed increase in average positive ion mobility in the positive corona is ascribed to the conversion of ions into more mobile ions. Considerably higher ozone concentrations (up to 100 ppm) were found in the negative corona discharge. The effect of the geometry of the system was also explored by using three different stainless steel outer electrodes with diameters of 10, 15 and 27 mm. Ozone concentrations were found to decrease significantly with increasing radius of the outer electrode at the same average input energy density.

A mass spectrometric study of ions extracted from a point-to-plane dc corona discharge in N2O at atmospheric pressure

In this paper we report the first study of ions formed in both positive and negative point-to-plane corona discharges fed by N2O containing less than 0.1% of H2O at atmospheric pressure. Considerable differences were observed in the mass spectra of the ions produced from corona discharges of different polarities. In all the discharges the product ions were observed mainly in the form of clusters. However, in positive corona discharges only four major groups of cluster ions were monitored whereas in all corona discharges operated with negative polarity a much more varied and complex variety of negative ions was observed. The spectrum of positive ions is highly sensitive to the content of water vapour in nitrous oxide with H3 O+ · (H2O)n clusters (especially H3O+ · (H2O)3) and NO+ · (H2O)n clusters being the dominant products. The yield of NO+ · (H2O)n was found to increase strongly with increasing discharge current while the yield of H3O+ · (H2O)n clusters was reduced. Spectra of ions detected in negative corona discharges contain clusters of NO−, , ions and also clustered with N2O, NO, NO2, H2O and HNO3. This paper reports how the yield of such ions is influenced by the plasma characteristics and discusses how these results may be explained by physical and chemical processes in the plasma.

A Study of the Physical and Chemical Processes Active in Ozone Generation by Carbon Dioxide Fed Corona Discharges

Ozone generation in both positive and negative corona discharges DC corona, both operated in glow regime, feed by dry CO2 has been studied. Higher ozone concentrations were observed in negative corona discharges. Ozone formation was found to be strongly dependent upon both the flow rate of the gas and on the radius of the outer electrode. The physical characteristics of the discharge were monitored through measurement of the discharge current. Small increases in the gas flow rate were observed to cause a significant increase in the discharge current of a negative corona discharge but little/no effect was observed in positive corona.

A Study of the Physical and Chemical Processes Active in Corona Discharges Fed by Carbon Dioxide

Ozone generation in both positive and negative corona discharges DC corona, both operated in glow regime, feed by dry CO2 has been studied. Higher ozone concentrations were observed in negative corona discharges. Ozone formation was found to be strongly dependent upon both the flow rate of the gas and on the radius of the outer electrode. The physical characteristics of the discharge were monitored through measurement of the discharge current. Small increases in the gas flow rate were observed to cause a significant increase in the discharge current of a negative corona discharge but little/no effect was observed in positive corona.

Electron induced fluorescence of the H2 molecule—Balmer lines and Fulcher α system

We report results of an experimental study focused on the fluorescence of H2 initiated by the impact of quasi-monochromatic electrons. The emission spectra of H2 molecules at electron energies of 25 eV, 50 eV and 100 eV have been measured in the spectral range of 320–670 nm. The emission spectrum is dominated by Balmer emission lines, Fulcher α, β, γ and δ bands and different singlet transitions of H2. In addition to the atomic lines and molecular bands, the presence of continuum radiation is present in the whole studied range. We have determined the emission cross sections (ECS) for particular processes leading to the emission of the Balmer (n = 3, 4, 5, 6, 7, 8 → 2) lines and for selected bands of the Fulcher α system of H2(d 3Πu → a 3 ) as a function of electron energy in the range 5–100 eV. Emission cross sections for the Balmer lines (n = 6, 7, 8 → 2) and Fulcher α (1, 0), (1, 1), (2, 2), (3, 3) bands were measured for the first time.

Current transport mechanisms of amorphous n-doped silicon carbide/crystalline silicon heterostructure: Impact of nitrogen dopation

Amorphous silicon carbide (a-SiC) thin films were prepared by PECVD deposition technique. A gas mixture of SiH4, CH4 and NH3 was directly introduced into the reaction chamber through a shower head. Properties of deposited films were studied by electrical measurement. Temperature dependences of forward (FW) current-voltage (I-V) characteristics of Al/a-SiC/c-Si(p)/Al structures are shown and analyzed in this paper. Parameters as saturation current and activation energies were measured and calculated from forward biased I-V curves. Identification of bonds at surface of amorphous layers by FTIR was presented.

The Mass Spectrometric Analysis of Negative Ions Extracted from Point-to-Plane Negative Corona Discharge in Ambient Air.

In this paper we report the detection and mass analysis of negative ions formed in a negative corona discharge using ambient air at atmospheric pressure. All of the detected anions are observed in the form of clusters, predominately containing water demonstrating the importance of humidity in atmospheric discharges. The formation mechanisms for such clustering phenomena are discussed.

Electron impact study of H2 and D2 continuum radiation

We report our recent experimental study regarding H2 and D2 continuum radiation (a3Σg + → b3Σu +) in the spectral range of 200 – 650 nm, excited by electron impact at near-threshold impact energies. In contrast to earlier studies, present results indicate that H2 and D2 continuum radiation is detectable in the spectral range above 500 nm. To supplement this, we present excitation-emission photon efficiency curves (PEC) of D2 measured at 650 nm, with precautions taken to rule out other possible signal sources.

Absolute excitation-emission cross section of electron induced argon excitation

The argon deexcitation 4P[1/2] → 4S[1/2]° produces one of the strongest argon emission lines (750.39 nm) and the absolute value of the cross section for electron-atom reaction is often utilized in optical emission spectroscopy of electrical discharges. The cross section value was re-evaluated experiementaly in this work. The experiment was conducted on Electron Induced Fluorescence (EIF) apparatus (described in detail in [3]) utilizing crossed beams of monochromatic electrons and atoms/molecules. The electron beam is formed by thermal emission from hairpin tungsten filament and subsequently it is energy filtered by trochoidal electron monochromator. For this experiment the electron energy resolution was set to ~200 meV (FWHM) and the electron current was in the range of 600 – 800 nA. Argon beam was created by effusion through a capillary. The background pressure in vacuum chamber was set to ~10 4 mbar to ensure single-collision conditions. Photons emitted from excited atoms were collected by an optical system, consisting of collimating optics, Jobin Yvon THR1500 Czerny-Turner monochromator and thermoelectrically cooled Hamamatsu R3869 photomultiplier working in photon counting mode with background noise averaging at 3.6 cps, primarily due to photomultiplier thermal noise. The narrow spectral range around the expected argon line was measured at several different electron energies (25 eV, 50 eV and 100 eV) well above the excitation limit for the process in question at high optical resolution to ensure there are no other spectral lines present in the vicinity that might influence the measured cross section. Identical procedure was used to measure photon efficiency curve of the He 728.13 nm line. The pressure in the effusive capillary was kept constant for both measurements ensuring equal sample densities in the reaction region. Electron energy step was 0.5 eV before the excitation threshold, 0.1 eV around the threshold and cross section maximum and 1 eV for the rest of the curve. Process of calculating the absolute cross section has multiple steps, as there are some properties of the experiment for which we have to make corrections, specifically pressure, electron current and spectral sensitivity corrections. For the argon 4P2[1/2] → 4S2[1/2]° transition the main peak and broader secondary peak was observed. It is typical for cross sections with contribution of cascade processes from the higher excited states, in addition to direct excitation. This phenomenon can be observed also at the rising slope of the cross section, which exhibits multiple threshold structure. This will be a subject of further study using higher electron beam resolutions and also more complex numerical fits of the rising edge. The values of the argon cross section were determined at electron energies 25 eV, 50 eV and 100 eV along with the value of excitation threshold. Experimental uncertainties were estimated by taking into account the statistical uncertainties of the measurements, pressure and current uncertainties and finally the uncertainty of helium reference cross section.