Electron Impact Dissociation Research Articles

Optical emission studies of the gas phase during the silicon suboxide deposition by reactive r.f. magnetron-sputtering

Silicon oxide (a-SiO) is one of the most used silicon-based materials in optoelectronic and microelectronic technology. It is well-known that the electronic properties are linked to the material structure, which depends on the deposition technique and on the details of the deposition. Silicon suboxide (a-SiO x 0 < x < 2) layers were prepared by r.f. magnetron-sputtering from a polycrystalline silicon target in a well defined oxidation environment. Optical Emission Spectroscopy (OES) was employed to study the plasma used in SiO x depositions. Theoretical calculations performed in the frame of electron impact excitation mechanism for argon and atomic oxygen corroborated with electron impact dissociation of the molecular oxygen have shown the strong influence of the electron temperature on the rate coefficients of photons' production. Correlation functions between the OES signals assigned to silicon and oxygen atoms from plasma (gas phase) and the SiO x layer composition (infrared and energy dispersive X-ray investigated) have been found. Based on these functions, the OES plasma monitoring is proposed as a tool to control in-situ and in direct time the SiO x layer composition.

Electron-induced reactions in condensed films of acetonitrile and ethane

Reactions in pure and mixed films of C(2)H(6) and CD(3)CN deposited on a Au surface at 35 K have been induced by low-energy electrons and investigated by Thermal Desorption Spectrometry (TDS). The incident electron energy (E(0)) was varied between 5 and 16 eV and a number of different products were identified. Beside the main products, CD(4), CD(3)H, and C(2)D(6), molecules resulting from atom scrambling during radical chain reactions (C(2)H(5)D) and recombination products (CD(3)CD(2)CN and C(2)H(5)CD(3)) were identified while others were characteristically absent. The quantity of the different products varied with E(0). The observed electron-driven processes are in accord with previous findings from gas phase experiments on dissociative electron attachment and electron impact ionization. On this basis, reaction mechanisms leading to the formation of the observed products are suggested for different ranges of E(0).

Decomposition of Ethane in Atmospheric-Pressure Dielectric-Barrier Discharges: Experiments

It is well known that electric fields can influence combustion processes. When the magnitude of an external applied electric field exceeds the breakdown field of the fuel gas or fuel/oxidizer mixture, plasma effects dominate. The earlier work in the field of plasma-assisted combustion has demonstrated that dielectric-barrier-discharge (DBD)-driven nonthermal plasmas (NTPs) can increase flame speed and extend the combustion of hydrocarbon fuel gases into very lean-burn regimes. In this paper, results on the decomposition of ethane (C2H6) by DBDs at atmospheric pressure will be presented. The authors have chosen ethane for this paper because its gaseous electronics properties (electron-impact dissociation cross sections, drift velocity) are available in the literature. A subsequent paper will present results on the calculated yield of DBD-driven plasma decomposition products of ethane, as predicted by plasma-chemistry modeling. In this paper, results on experiments carried out to determine the decomposition products of ethane, as measured by gas chromatography are presented. An atmospheric-pressure DBD reactor processed a flowing gas stream of chemically pure ethane in the regime of plasma specific energy ranging from 1200 to 2400 J/std lit. The major stable decomposition products were H2, CH4, C2H2, and C2H4. These results are important in assessing the possibility of using NTPs to enhance the combustion of hydrocarbons

Experimental observation of neutral radical formation fromCH4by electron impact in the threshold region

An apparatus combining the cross-beam and threshold-ionization techniques has been used to measure absolute cross sections for electron impact dissociation of ${\mathrm{CH}}_{4}$ molecules into the ${\mathrm{CH}}_{3}$ neutral radicals at energies from threshold up to $13\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The threshold energy for the lowest-lying $^{3}T_{2}$ state has been observed to be $7.5\phantom{\rule{0.3em}{0ex}}\ifmmode\pm\else\textpm\fi{}0.3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, and attributed to neutral ${\mathrm{CH}}_{3}$ formation. Peaks have been observed at $\ensuremath{\sim}9.6$ and $11.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, in agreement with electronic excitation and photon impact neutral dissociation literature data. The current results indicate that all excited states of ${\mathrm{CH}}_{4}$ predominantly result in dissociation via the ${\mathrm{CH}}_{3}$ neutral radical channel below $12.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$.

Electron impact dissociation cross sections for C2F6

Absolute total dissociation cross sections for electron impact, σt,diss, from 8 to 700 eV are reported for C2F6. A dense set of data was obtained in the technologically important 8–30 eV energy range relevant to modelling the type of plasmas used in both fundamental and applied scientific investigations. The threshold for dissociation was found to be 12.0 ± 0.2 eV and appears to be associated with a Rydberg state. Estimated values for the total neutral dissociation cross section, σneut,diss, were obtained by subtracting the ionization cross sections (all ionizing events cause dissociation) from the total dissociation cross section. It is shown that a calibration error in a paper by one of us (HFW) caused a distortion of several previous investigations. When the data from the present work are used to recalibrate data from swarm experiments, agreement becomes quite reasonable. There is now a consistent set of data obtained from several investigators which describe the dissociation of C2F6. Neutral dissociation cross sections are obtained from electron-impact excitation calculations and found to be in reasonable agreement with measurements over most of the energy range.

Electron-impact dissociation and transient properties of a stored LiH2 - beam

The fragmentation of LiH2 - anions after electron impact was investigated at the heavy-ion storage ring TSR. The main reaction channel was found to be electron detachment followed by a breakup into LiH + H. In the first ms after production of the molecular ions in a cesium sputtering ion source, additional contributions were observed in the Li + H2 and Li- + H2 channels, hinting at an initial population of a short-lived state of the anion. To gain a better understanding of the mechanisms underlying the observed behavior of the system, ab initio calculations of relevant potential energy surfaces were performed at selected geometries. The experimental findings are discussed in the light of these calculations.

A crossed-beam experiment for electron impact ionization and dissociation of molecular ions: its application to CO+

A crossed electron–ion beam experimental set-up has been upgraded for the study of electron impact ionization and dissociation of molecular ions by means of ionic product detection. Both the experimental set-up and the data analysis procedures are described in detail for the estimation of (i) absolute cross sections, (ii) kinetic energy release distributions (KERD) and (iii) anisotropies of angular distributions. Absolute cross sections are obtained separately for dissociative excitation (DE) and for dissociative ionization (DI). A double focusing magnetic field analyser is used for the observation of product velocity distributions, in the laboratory frame, at selected electron energies. The KERD in the centre of mass frame is calculated from the measured velocity distribution as well as the anisotropy of the angular distribution with respect to the initial orientation of the molecular ions. Results are reported for dissociative ionization and dissociative excitation of CO+ to C+ and O+ fragments in the energy range from about 5 eV to 2.5 keV. Absolute cross sections for DE at maximum, i.e. for an electron energy around 35 eV, are found to be (9.69 ± 2.08) × 10−17 cm2 and (6.24 ± 1.33) × 10−17 cm2, for C+ and O+, respectively, and the corresponding threshold energies are found to be (8.5 ± 0.5) eV and (14.8 ± 0.5) eV. The DE process leading to C+ production is seen to dominate at low electron energies. For DI, the absolute cross section is found to be (12.56 ± 2.38) × 10−17 cm2 around 125 eV and the corresponding threshold energy is (27.7 ± 0.5) eV. KERDs, which extend from 0 to 24 eV both for C+ and O+, exhibit very different shapes at low electron energy but similar ones above 100 eV, confirming the role observed respectively for DE and DI. The groups of states contributing to the different processes are identified by comparing present energies thresholds values and the KERDs with theoretical values. Anisotropies are estimated to be in the range 3–6% for both C+ and O+.

Cross-section and rate coefficient calculation for electron impact excitation, ionisation and dissociation of H2 and OH molecules

The weighted total cross-section (WTCS) theory is used to calculate electron impact excitation, ionisation and dissociation cross-sections and rate coefficients of OH, H2, OH+, H2 +, OH- and H2 - diatomic molecules in the temperature range 1500–15000 K. Calculations are performed for H2(X, B, C), OH(X, A, B), H2 +(X), OH+(X, a, A, b, c), H2 -(X) and OH-(X) electronic states for which Dunham coefficients are available. Rate coefficients are calculated from WTCS assuming Maxwellian energy distribution functions for electrons and heavy particles. One and two temperature (θe and θg respectively for electron and heavy particles kinetic temperatures) results are presented and fitting parameters (a, b and c) are given for each reaction rate coefficient: k(θ) = a (θb)exp (-c/θ).

Determination of cross sections and rate coefficients for the electron impact dissociation of NO 2

Energy dependent and angle dependent differential cross sections for the production of the NO 2 + , NO +, O 2 + , N + and O + ions resulting from dissociative ionization of NO 2 by electron collision have been evaluated at a fixed incident electron energy. The semi-empirical formulation based on the well-known Jain–Khare formulation which requires the oscillator strength data as input has been employed. As no previous data seem to exist for differential cross sections, we have derived the partial and total ionization cross sections from these differential cross sections from ionization threshold up to 1000 eV and compared with available experimental and theoretical data. We also evaluated the ionization rate coefficients on the basis of calculated partial ionization cross sections and Maxwell–Boltzmann energy distributions.

Plasma parameters and chemical kinetics of an HCl DC glow discharge

The investigations of plasma parameters and active particles kinetics in an HCl DC glow discharge system were carried out. The investigation combines plasma diagnostics by electric probes and plasma modeling based on the self-consistent solution of Boltzmann kinetic equation and the balance equation of chemical kinetic for neutral and charged particles. It was shown that the direct electron impact dissociation of the HCl molecules represents the main mechanism for the formation of the Cl and H atoms while the total balances for all kinds of neutral particles are noticeably influenced by the volume atom–molecular reactions. The analysis of the vibrational kinetics shows low population of the vibrational energy levels for HCl molecules, but the role of the HCl V >0 in the negative ions formation process cannot be neglected. The assumption of the Eley-Rideal recombination kinetics for both chlorine and hydrogen atoms provides a good agreement between the calculated and experimental values of reduced electric field ( E/ N) and the total flux of positive ions.

Electron Interaction Cross Sections for CF3I, C2F4, and CFx (x=1–3) Radicals

The supply of absolute electron-impact cross sections for molecular targets and radicals is extremely important for developing plasma reactors and testing different types of etching gases. Current demand for such models is high as the industry aims to replace traditional plasma processing gases with less polluting species. New theoretical electron impact cross sections at typical etching plasma energies (sub 10 eV) are presented for the CFx (x=1–3) active radical species in a form suitable for plasma modeling. The available experimental and theoretical data are summarized for two potential feed gases, CF3I and C2F4. This data cover recommended cross sections for electron scattering (total, excitation, momentum transfer, and elastic integral), electron impact dissociation, and dissociative electron attachment, wherever possible. Numerical values are given as tables in the paper and are also placed in the electronic archive.

On determination of the degree of dissociation of hydrogen in non-equilibrium plasmas by means of emission spectroscopy: I. The collision-radiative model and numerical experiments

A new spectroscopic method of determination of the degree of dissociation of hydrogen is proposed. The method is based on measurements of the relative intensities of two atomic lines of the Balmer series, Hα and Hβ, and of one molecular line, the (2-2)Q1 line of the Fulcher-α system. These intensities are related to two plasma parameters: (i) the ratio of atomic and molecular hydrogen densities [H]/[H2], connected with the degree of dissociation of hydrogen and (ii) the artificial parameter , characterizing a tail of an electron energy distribution function (EEDF) in the threshold region of electron impact excitation cross sections. The link between measured relative line intensities and plasma parameters was found in the framework of a most simple collision-radiative model taking into account only the direct and dissociative electron impact excitation and spontaneous decay of excited states. Numerical simulations made it possible to analyse main peculiarities and to estimate the range of the applicability of the method concerning numerical values of [H]/[H2] and . For the first time, the fine structure of Balmer lines was taken into account in the spectroscopic determination of the degree of dissociation of hydrogen in non-equilibrium plasmas. The rate coefficients for electron impact excitation of the , v = 2, N = 1 rovibronic level of the hydrogen molecule were calculated with a Maxwellian EEDF for electron temperatures Te = 0.5–100 eV. Earlier calculated values of the emission rate coefficients for direct and dissociative electron impact excitation of Hα and Hβ lines were extended for electron temperatures up to 100 eV.

Electron-impact dissociation of the methane molecule into neutral fragments

The three most important electron-impact processes dissociating ${\mathrm{CH}}_{4}$ molecule into neutral fragments are $e+{\mathrm{CH}}_{4}\ensuremath{\rightarrow}{\mathrm{CH}}_{3}+\ensuremath{\cdots}$, $e+{\mathrm{CH}}_{4}\ensuremath{\rightarrow}{\mathrm{CH}}_{2}+\ensuremath{\cdots}$, and $e+{\mathrm{CH}}_{4}\ensuremath{\rightarrow}\mathrm{CH}+\ensuremath{\cdots}$. Neither experimental nor theoretical cross sections for the processes in a broad range of energy are available, except for the first process for which reliable measured cross sections for energies up to $500\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ are available. Only two measured values (at a single impact energy) of the cross section for the second process are available, and only two measured values (also at a single energy) of the cross section for the third process are available in the literature. Therefore, we derive in this work analytical dissociation cross sections for the second and the third processes mentioned above in the energy range up to $500\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. At higher (but still nonrelativistic) energies, we calculate the parameters of the Bethe-Born cross section for the second and the third processes.

An apparatus for studying momentum-resolved electron-impact dissociative and non-dissociative ionisation

A spectrometer for recoil ion momentum measurements has been built for studying electron impact ionisation and dissociation of molecules. The apparatus is described in detail, highlighting its capabilities, as well as differences in design from the ones already in use elsewhere. Momentum spectra of ions resulting from 1300 eV electron impact on CO2 are presented. We observe a broad momentum distribution for the dissociative ionisation reaction leading to the formation of C+, and two momentum groups in the CO+ and O+ channel. By recording multiple ions arising from the same dissociative ionisation event, we also demonstrate the formation of fragment pairs O+:CO+, C+:O+, and O+:O+.

Modeling the influence of anode–cathode spacing in a pulsed discharge nozzle

The pulsed discharge nozzle (PDN) is a spectrochemical source that is designed to produce and cool molecular ions in an astrophysically relevant environment in the laboratory with limited fragmentation. In order to gain a better understanding of the PDN and to optimize the yield of molecular ions and radicals in the PDN source, a parameter study of the influence of the interelectrode distance on the plasma properties is carried out by means of a discharge model, providing a qualitative as well as a quantitative picture of the plasma. We model the electron density and energy, as well as the argon ion and metastable atom number density for various interelectrode distances. The results reveal that increasing the interelectrode distance does not significantly influence the plasma at the cathode and at the anode. However, a positive column forms between the electrodes, which increases in length as the interelectrode distance increases. This is an additional evidence that the PDN is a glow discharge. This positive column does not contribute significantly to the formation of metastable argon atoms. Because metastable argon is thought to be the primary agent in the formation of molecular ions through Penning ionization of the neutral molecular precursor there is no benefit to be expected from an increase of the interelectrode distance. In fact, electron impact dissociation of the molecules in the column might even make the source less efficient for longer column lengths. The simulations presented here provide physical insight into the characteristics of interstellar species analogs in laboratory experiments.

Molecular size effect in NCO and NCS dianion resonances

Cross sections for electron-impact detachment and electron-impact dissociation of NCO- and NCS- were measured from about 3 to about 40 eV. The former are found to follow a classical prediction with a threshold energy of 9.1 +/- 0.1 eV for NCO- and 8.9 +/- 0.2 eV for NCS-. When the incoming electron binds to the monoanion, a short-lived dianion complex is formed, which is revealed as a resonance in the cross section. For NCO- a resonance is evident at 9.3 +/- 0.2 eV, which implies that the dianion lies above the monoanion by this amount of energy. In the case of NCS- two resonances are evident at 8.4 +/- 0.2 and 19.0 +/- 0.5 eV, respectively. The low-energy NCS dianion is less unstable than the dianion of NCO, which in turn is less unstable than the CN dianion (10-eV resonance). Thus the resonance shifts down in energy with the increasing size of the anion, a fact which is attributed to a decrease in Coulomb energy between the spatially separated electrons.

Study of effect of H 2 addition on the production of fluorocarbon radicals in H 2 /C 4 F 8 inductively coupled plasma via optical emission spectroscopy actinometry

C4F8 plasma with the addition of H2 is generated by the inductively coupled plasma (ICP) method. The relative densities of CF, CF2, H and F radicals are determined by actinometric optical emission spectroscopy (AOES) as a function of the gas flow rate ratioR=H2/(H2+C4F8) at a pressure of 0.8 Pa and an input r.f. power of 400W, while that of HF is measured by quadrupole mass spectrometry (QMS). The results show that plasma activity increases firstly and then decreases with increasing R. As the gas flow rate ratio R changes from 0 to 0.625, relative densities of both CF and CF2 decrease and the relative [CF] has a similar tendency as the calculated [CF], indicating that CF radicals are generated mainly by the electron impact dissociation of CF2 radicals. Production of HF is also discussed.

Plasma parameters and volume kinetics in Cl2/O2 mixtures

We investigated plasma characteristics, plasma mass content and kinetic dependencies of both neutral and charged particle formation and decay in Cl2/O2 gas mixture. For these purposes we used a combination of experimental methods (optical emission spectroscopy, Langmuir probe) and a plasma modeling on the base of self-consistent solution of Boltzmann kinetic equation together with balance kinetic equations for neutral and charged particles in a quasi-stationary approximation. It was found that the change of O2/(Cl2+O2) mixing ratio from 0 to 0.2 leads to an increase of electron average energy and electron energy distribution function (EEDF) deformation. The main mechanisms of Cl and O atom formation are the direct electron impact dissociation of corresponding molecules while the contribution of all possible “secondary” processes is not significant in the case of a relatively low O2 addition.

Electron-impact dissociation of D13CO+ molecular ions to 13CO+ ions

Absolute cross sections for electron-impact dissociation of D13CO+ ions have been measured over a collision energy range from 4 to 100 eV with a crossed electron–ion beams method. Total experimental uncertainties are about 16% near the cross section peak. The role of resonant and direct dissociative excitation for energies below 21 eV is discussed in light of the energy levels and photo-dissociation cross sections of the HCO+ formyl cation predicted by ab initio multi-reference configuration interaction calculations.

Plasma–wall interactions during silicon etching processes in high-density HBr/Cl2/O2 plasmas

We have investigated the production and loss kinetics of SiClX radicals during silicon gate etching processes in HBr/Cl2/O2 plasmas. The absolute concentrations of SiClX (X = 0–2) radicals have been measured by broad band UV absorption spectroscopy at different O2 gas flow rates in the process gas mixture, and at different RF powers injected in the plasma. At the same time, the chemical composition of the layer deposited on the reactor walls has been investigated by x-ray photoelectron spectroscopy analysis. Without O2 in the plasma, the reactor walls stay clean because the silicon containing compounds redeposited on them (from Si, SiCl, Si+ and SiCl+ precursors) are subsequently etched by Cl atoms and recycled back into the plasma as SiCl2 and SiCl4 volatile species. Hence, the reactor walls are a region of production for these species, leading to their high concentrations in the gas phase. The introduction of O2 gas into the plasma results in the oxidation of the silicon chloride radicals resident on the surfaces and in the appearance of a silicon oxychloride layer on the reactor walls, whose deposition rate increases rapidly with the O2 flow. As a consequence, the production rate of SiCl2 by the reactor walls decreases because a part of the silicon containing species redeposited on the reactor walls is oxidized and incorporated in the silicon oxychloride film instead of being recycled back into the plasma as SiCl2 and SiCl4. Finally, a simple zero-dimensional model is built to predict the densities of SiClX radicals from the measured densities of SiClX+1 radicals and ions. The comparison between the calculated and measured densities at different RF powers allowed us to conclude that SiCl and Si radicals are produced both in the gas phase by electron impact dissociation of SiCl2 (SiCl) radicals, and at the reactor walls by the neutralization and reflection of approximately 50% of the SiCl+ (Si+) ions impinging on these surfaces. At the same time, SiCl and Si are estimated to be lost (adsorption and abstraction reactions) on the reactor walls with a probability ranging between 0.2 and 1.