Ilija Stefanovic

Kinetics of ozone and nitric oxides in dielectric barrier discharges in O2/NOx and N2/O2/NOx mixtures

Concentrations of NO, NO2, NO3, N2O5, and O3 were measured by classical absorption spectroscopy in dielectric barrier discharges in flowing O2/NOx and N2/O2/NOx mixtures. The results of measurements in different parts of the discharge chamber and in its exhaust are compared to a numerical zero-dimensional kinetic model and good agreement is found. The experimentally found upper limit of the NOx concentration allowing ozone production is confirmed by the kinetic calculations for both gas mixtures. The rotational temperature of different nitrogen bands was measured by high-resolution emission spectroscopy. The results are explained on the basis of a simplified model and related to the gas temperature in the microdischarge channel and the surrounding gas.

Infrared fingerprints and periodic formation of nanoparticles in Ar/C2H2 plasmas

The formation of dust particles in argon diluted C2H2 plasmas was studied by means of Fourier transform infrared absorption spectroscopy and mass spectroscopy. The detection limit for infrared absorption was significantly improved by the use of a multipass technique. Measuring the intensity of the Rayleigh/Mie scattering of the infrared signal we found a periodicity of dust formation/vanishing (period of about 35 min in our experimental conditions). The fast disappearance of the dust from the plasma region at the end of every period is the evidence of a narrow particle size distribution, as confirmed by secondary electron micrographs of the collected powder. Characteristic infrared absorption features have their origin in absorption within the dust particles. Besides the strong presence of aliphatic hydrocarbons characteristic for amorphous hydrocarbon films, a significant amount of aromatic structures was detected. Heavy positive ions measured by ion-mass spectroscopy originate from polyacetilenic (C2nH2...

A Candidate Analog for Carbonaceous Interstellar Dust: Formation by Reactive Plasma Polymerization

Carbonaceous compounds are a significant component of interstellar dust, and the composition and structure of such materials is therefore of key importance. We present 1.5‐15 � m spectra of a plasma-polymerized carbonaceous material produced in radio-frequency discharge under low pressure, using C2H2 as a precursor component. The infrared spectra of the resulting spheroidal carbonaceous nanoparticles reveal a strong aliphatic band (3.4 � m feature), weak OH and carbonyl bands, and traces of aromatic compounds, all characteristics identified with dust in the diffuse interstellar medium of our Galaxy. The plasma polymerization process described here provides a convenient way to make carbonaceous interstellar dust analogs under controlled conditions and to compare their characteristics with astronomical observations. Here we focus on a comparison with the IR spectra of interstellar dust. The IR spectrum of carbonaceous dust in the diffuse interstellar medium is characterized by a strong 3.4 � m

Dust particle formation in low pressure Ar/CH4 and Ar/C2H2 discharges used for thin film deposition

Abstract This paper deals with the study of the temporal and spatial evolution of the dust formation in two types of capacitively coupled discharges in Ar/C 2 H 2 and Ar/CH 4 gas mixtures used for thin film deposition. To initiate the particle growth in the Ar/CH 4 discharge it is necessary either to apply transiently high power to the discharge or to inject transiently a pulse of C 2 H 2 . In the Ar/C 2 H 2 discharge, however, the particles are formed spontaneously at constant low power. Due to the different initiation process the further temporal evolution of the dust formation is significantly different for both kind of gas mixtures. In the case of argon/acetylene the formation of dust particles shows a periodical behavior, which is not observed in the argon/methane mixture. The dust particles are detected by means of laser light scattering and by measuring the extinction of the laser after passing the discharge. The chemical nature of the particles was studied in situ by means of a multi pass FTIR-spectrometer. The thin film deposition was measured with an in situ ellipsometer.

The response of a capacitively coupled discharge to the formation of dust particles: Experiments and modeling

The influence of dust particles on the properties of a capacitively coupled Ar–C2H2 discharge is studied both experimentally and theoretically. The results of measurements of the intensity and spatial distribution of the emitted light, the line width of the fast component of Hα line and of the electron density during the particle growth are presented. To analyze the experimental results a one-dimensional discharge model is developed. Using the model the effects of dust grains on the power absorption (taking into account stochastic and Ohmic heating in the plasma sheaths), the optical emission intensity profile, the sheath size, the rf electric field and on the energy of positive ions bombarding the electrodes are investigated. In particular, it is shown that the decrease of the power absorption in the sheaths of complex plasmas is due to the dependence of the stochastic and Ohmic heating in the plasma sheaths on the electron temperature and the current flowing across the discharge plates. The results of t...

The influence of C2H2 and dust formation on the time dependence of metastable argon density in pulsed plasmas

Diode laser absorption at 772.38 nm is used to measure the time resolved density of Ar* (3P2) metastable atoms in a capacitively coupled radio-frequency (RF) discharge running in an argon/acetylene mixture at 0.1 mbar. The RF power is pulsed at 100 Hz and the density of Ar* (3P2) atoms in the 5 ms ON time and in the afterglow is recorded. Different plasma conditions, namely (1) pure argon, (2) argon +7% acetylene before powder formation, (3) argon +7% acetylene after dust particles have been formed and (4) argon with dust particles remaining in the plasma volume but without acetylene, are studied. The measured steady-state Ar* (3P2) density in the middle of the reactor is always about 10 times larger in the dusty argon plasma than in the pure argon discharge. This is mainly a consequence of the enhancement of electron temperature after dust formation. Both steady-state densities and decay times in the afterglow indicate that the degree of dissociation of C2H2 in the plasma volume can be as high as 99%. It is shown that under our plasma conditions, the loss of Ar* (3P2) atoms on the surface of dust particles is negligible compared to their loss by diffusion to the electrodes.

Controlled dust formation in pulsed rf plasmas

This paper deals with the formation of nanoparticles in a pulsed discharge. Experiments are performed in a capacitively coupled discharge operated in a mixture of argon and acetylene. The paper focuses especially on the influence of the pulse frequency on the dust formation. The experiments reveal the existence of a rather narrow frequency band that separates a frequency region with no dust formation from a frequency region where dust formation occurs. The decisive point in the observations is that a small change in the pulse frequency (from 700 to 725 Hz) is enough to induce or, respectively, suppress the formation of dust particles. The experimental results are discussed by means of a simple model that allows one to calculate the density of negative ions (C2H−, C4H−, etc.) as a function of the pulse frequency.

Dust formation in Ar/CH4 and Ar/C2H2 plasmas

This paper discusses the growth of nanoparticles in capacitively and inductively coupled radio-frequency plasmas operated in hydrocarbon gases and the back-reaction of particles on the plasma properties. The focus is on the growth mechanism in CH4- and C2H2-containing plasmas, on the role of atomic hydrogen and on the dynamic charging and decharging of particles in pulsed plasmas.

Hα emission in the presence of dust in an Ar-C2H2 radio-frequency discharge

We present the results of time resolved spectroscopy measurements of the Hα line of atomic hydrogen in an Ar-C2H2 radiofrequency plasma. The presence of the fast (high-energy) and slow (low-energy) components of the hydrogen atoms can be deduced from the Doppler broadening of the Hα line. With the appearance of the dust particles, the Hα profile became narrower, indicating reduction of the fast component. We discuss several mechanisms for the formation of the fast hydrogen atoms in our plasma. The main cause for the reduction of the hydrogen atom fast component is the plasma change from electropositive to electronegative, decreasing the sheaths width and voltage. The change of plasma conditions coincides with the end of the dust coagulation/agglomeration when the dust particles become multiply negatively charged. We propose here a new experimental technique for detection of the dust coagulation/agglomeration phase.

Volt Ampere Characteristics of Low Current DC Discharges in Ar, H 2, CH 4 and SF 6

In this paper we show new results for a wide range of characteristics of low current diffuse (Townsend-dark discharge regime) discharges in Ar, H 2 , CH 4 , SF 6 . The measured observables include: volt-ampere characteristics, ranges of free running oscillations, frequency and damping of induced oscillations and we also determine the negative differential resistance (the negative differential voltage to current ratio). The measurements were performed for pressures between 0.027 and 1 Torr and for distances between electrodes in the range from 1 to 3 cm.