Electron-impact Dissociation Processes Research Articles

The formation of O and H radicals in a pulsed discharge in atmospheric pressure helium with water vapour admixtures

The spatio-temporal distribution of O and H radicals in a 90 ns pulsed discharge, generated in a pin–pin geometry with a 2.2 mm gap, in He + H2O (0.1% and 0.25%), is studied both experimentally and by 1D fluid modelling. The density of O and H radicals as well as the effective lifetimes of their excited states are measured using picosecond resolution two-photon absorption laser induced fluorescence. Good agreement between experiments and modelling is obtained for the species densities. The density of O and H is found to be homogenous along the discharge axis. Even though the high voltage pulse is 90 ns long, the density of O peaks only about 1 μs after the end of the current pulse, reaching 2 × 1016 cm−3 at 0.1% H2O. It then remains nearly constant over 10 μs before decaying. Modelling indicates that the electron temperature (Te) in the centre of the vessel geometry ranges from 6 to 4 eV during the peak of discharge current, and after 90 ns, drops below 0.5 eV in about 50 ns. Consequently, during the discharge (<100 ns), O is predominantly produced by direct dissociation of O2 by electron impact, and in the early afterglow (from 100 ns to 1 μs) O is produced by dissociative recombination of O2 +. The main loss mechanism of O is initially electron impact ionisation and once T e has dropped, it becomes mainly Penning ionisation with He2* and He* as well as three-body recombination with O+ and He. On time scales of 100–200 μs, O is mainly lost by radial diffusion. The production of H shows a similar behaviour, reaching 0.45 × 1016 cm−3 at 1 μs, due to direct dissociation of H2O by electron impact (<100 ns) followed by electron–ion recombination processes (from 200 ns to 1.5 us). H is dominantly lost through Penning ionisation with He* and He2* and by electron impact ionisation, and by charge exchange with O+. Increasing concentrations of water vapour, from 0.1% to 0.25%, have little effect on the nature of the processes of H formation but trigger a stronger initial production of O, which is not currently reproduced satisfactorily by the modelling. What emerges from this study is that the built up of O and H densities in pulsed discharges continues after electron-impact dissociation processes with additional afterglow processes, not least through the dissociative recombination of O2 + and H2 +.

Temporal dynamics of femtosecond-TALIF of atomic hydrogen and oxygen in a nanosecond repetitively pulsed discharge-assisted methane–air flame

The temporal dynamics of the spatial distribution of atomic hydrogen and oxygen in a lean methane–air flame, forced by a nanosecond repetitively pulsed discharge-induced plasma, are investigated via femtosecond two-photon absorption laser-induced fluorescence technique. Plasma luminescence that interferes with the fluorescence from H and O atoms was observed to decay completely within 15 ns, which is the minimum delay required for imaging measurements with respect to the discharge occurrence. During discharge, H atoms in the excited state rather than the ground state, produced by electron-impact dissociation processes, are detected at the flame front. It was found that the temporal evolution of H and O fluorescence intensity during a cycle of 100 µs between two discharge pulses remains constant. Finally, the decay time of O-atoms produced by the discharge in the fresh methane–air mixture was about 2 µs, which suggests a faster reaction between O-atoms and methane than in air.

Non-equilibrium plasma kinetics of reacting CO: an improved state to state approach

Non-equilibrium plasma kinetics of reacting CO for conditions typically met in microwave discharges have been developed based on the coupling of excited state kinetics and the Boltzmann equation for the electron energy distribution function (EEDF). Particular attention is given to the insertion in the vibrational kinetics of a complete set of electron molecule resonant processes linking the whole vibrational ladder of the CO molecule, as well as to the role of Boudouard reaction, i.e. the process of forming CO2 by two vibrationally excited CO molecules, in shaping the vibrational distribution of CO and promoting reaction channels assisted by vibrational excitation (pure vibrational mechanisms, PVM). PVM mechanisms can become competitive with electron impact dissociation processes (DEM) in the activation of CO. A case study reproducing the conditions of a microwave discharge has been considered following the coupled kinetics also in the post discharge conditions. Results include the evolution of EEDF in discharge and post discharge conditions highlighting the role of superelastic vibrational and electronic collisions in shaping the EEDF. Moreover, PVM rate coefficients and DEM ones are studied as a function of gas temperature, showing a non-Arrhenius behavior, i.e. the rate coefficients increase with decreasing gas temperature as a result of a vibrational–vibrational (V–V) pumping up mechanism able to form plateaux in the vibrational distribution function. The accuracy of the results is discussed in particular in connection to the present knowledge of the activation energy of the Boudouard process.

Experimental scaling of plane-Born cross sections and ab initio assignments for electron-impact excitation and dissociation of XF4 (X = C, Si, and Ge) molecules.

Electron energy loss spectra of carbon tetrafluoride, silicon tetrafluoride, and germanium tetrafluoride molecules (CF4, SiF4, and GeF4) have been measured for incident electron energies of 50-360 eV at 1.5°-15.5° and for 30 eV and 30° scattering angle, while sweeping the energy loss over the range 9.0-20.0 eV. Low-lying valence excited triplet and singlet states are investigated by quantum chemical ab initio calculations. The Rydberg series converging to the (lowest) ionisation energy limits of XF4 (X = C, Si, Ge) are also identified and classified using the systematic behaviour according to the magnitude of the quantum defects. A generalized oscillator strength analysis is employed to derive oscillator strength f0 value and the apparent Born integral cross sections from the corresponding differential cross sections by using the Vriens formula for the optically allowed transitions. The f0 value is compared with the optical oscillator strength of the photoabsorption, pseudo-photon measurements, and theoretical values. The binary-encounter and f-scaled Born cross sections of the most intense optically allowed transitions have been also derived from the excitation threshold to the high energy region where the Born approximation is valid. Potential energy curves were obtained along the XF3 + F coordinate with two different basis sets to lend support on electron impact dissociation processes yielding radical formation. We found that in CF4, the lowest-lying dissociative character is due to intramolecular conversion from Rydberg 3s to valence character (σ*(C-F)), whereas in SiF4 and GeF4, an antibonding behaviour prevails.

Boltzmann equation and Monte Carlo analysis of the electrons in spatially inhomogeneous, bounded plasmas

Kinetic studies of the electrons in spatially inhomogeneous, bounded plasmas have been performed by means of two different numerical techniques: the solution of the space-dependent electron Boltzmann equation (BE) using a multiterm approximation of the Legendre polynomial expansion of the electron velocity distribution function and Monte Carlo (MC) simulations. Appropriate conditions at the boundaries of gas discharge plasmas have been deduced, which are adequate for the direct comparison of electron BE and MC simulation results. In particular, extended boundary conditions at the electron emitting cathode are represented and discussed. The investigations are performed for dc discharges in oxygen at conditions typical of abnormal glow discharges. The analysis of the results shows extreme alterations of the properties of the electrons in the discharge region, where pronounced groups of electrons are found, which are generated by electron impact dissociation processes and ionization processes when assuming equal energy sharing in ionizing collisions and isotropic scattering in inelastic electron collisions. The influence of these assumptions on the results obtained is discussed. The very good agreement between the results of the BE and MC calculations verifies the consistence of the derived extended boundary conditions at the cathode and of both the kinetic approaches.

Analyses of Radicals in Etching Plasma Using Laser Spectroscopy in Association of Super-Advanced Electronics Technologies

Laser spectroscopy and optical emission spectroscopy have been applied to analyses of neutral radicals influoro-carbon etching plasma for ULSI mass production, as a fundamental approach to development ofnext-generation etching technologies. Electron-impact dissociation processes of parent molecules in the plasmaand ion-assisted surface reactions of radicals on the top electrode, were analyzed to extract the ways ofcontrolling reaction selectivity of different materials. Radial spatial distributions of radicals were measured todiscuss the technical issues of development of larger-diameter wafer processes. These show that the absolutedensities and spatial distributions are indispensable to quantitatively understand phenomena directly related toetching reaction.

Reactive ion etching of a-SiC:H films using CCl4 and O2 gas mixture

Reactive ion etching (RIE) of a-SiC:H thin films in a CCl4/O2 plasma has been investigated. The effects of process parameters, in particular radio-frequency power, oxygen addition, gas flow rate, interelectrode spacing, and pumping speed, on a-SiC:H etch rate and a-SiC:H/oxide etch selectivity are reported. For the RIE of a-SiC:H in CCl4/O2 plasmas, the gas phase plasma chemistry and surface reactions appear to be more important than ion bombardment. Reactive species generated from neutral chemical reactions of oxygen in the discharge play a more dominant role than those generated from the electron impact dissociation processes.

Isotopic dependences of the dielectric strength of gases-new observations, classification, and possible origins

It has been found previously that for both uniform and non-uniform fields, the direct current breakdown voltages, Vs, of H2 and NH3 exceed those of D2 and ND3, respectively, for positive polarity. In contrast, it was found that while for uniform fields CH4 and CD4 behave in a similar manner, the direct current breakdown voltage, Vs, of CD4 exceeds that of CH4 in non-uniform fields for positive polarity. The authors have now found that the CH4/CD4 non-uniform field behaviour is polarity dependent (i.e. the Vs of CD4 is lower than the Vs of CH4 for negative polarity). The origin of these observed isotope effects is related to changes in the effective ionisation coefficient, alpha /N, and to changes in the value of the secondary coefficient, gamma . Since the alpha /N changes are due to changes in either or both the ionisation, alpha /N, and the attachment, eta /N, coefficients, they have classified the isotopic pairs of molecules in three groups: non-electronegative (where only changes in alpha /N are important), weakly and moderately electronegative (where changes in alpha /N and/or eta /N are important), and strongly electronegative gases (where no isotopic dependence of Vs is expected). alpha /N depends on mass, especially when electron impact dissociation processes are significant and its isotopic dependence arises from the competition between dissociation into neutrals and auto-ionisation. eta /N depends on mass, especially when dissociative attachment proceeds via a negative-ion state which is attractive, and dissociative attachment is then strongly influenced by the time it takes for the dissociation fragments to separate as opposed to the time of auto-detachment. The latter led them to predict that the Vs of H2S should exceed the Vs of D2S since the dissociative attachment cross section is much larger for H2S than for D2S. This prediction has been confirmed by experiment. Basic data and Vs measurements in quasi-uniform and highly non-uniform fields for both positive and negative polarities suggest that the isotopic dependences of the Vs of gases are apparently of more general occurrence than hitherto recognised.

Electron-impact dissociation of HCl.

A recently developed impact-parameter method is applied to electron-impact dissociation processes involving the A /sup 1/Pi state of HCl. Calculations were performed for initial vibrational levels of the X /sup 1/..sigma../sup +/ state up to v/sub i/ = 2 and for rotational temperatures to 1000 K. The cross sections increase by approximately 50% from v/sub i/ = 0 to v/sub i/ = 2. The dependence on rotational temperature is small except for an anomalous enhancement at 0 K that the theory predicts for Pi states. Since the predicted cross sections are an order of magnitude larger than those for photodissociation, electron-impact dissociation processes will play an important role in lasers involving HCl.