Jaime de Urquijo

Scattering cross sections for electrons in C2 H2F4 and its mixtures with Ar from measured transport coefficients

Previous measurements of the drift velocities, W, and the density-normalized effective ionization (multiplication) coefficients (ionization minus attachment), (?????)/N, measured in C2H2F4 (1,1,1,2 tetrafluoroethane) and in C2H2F4?Ar mixtures have been analysed with a standard swarm procedure. As a result of this analysis a set of electron collision cross sections for the C2H2F4 molecule has been obtained. This set has been further used to calculate other transport parameters such as the characteristic energies and rate coefficients for individual processes.

Ion Motion in Dielectric Gases

Dielectric gases are used in a wide variety of important applications such as gas insulation and semiconductor fabrication (Christophorou 2000). As important as their use is their abatement (Kiehlbauch 2001), since gases like SF6 or some perfluorocarbons are potent greenhouse gases. The above processes require that at some stage the gas be subjected to discharge conditions, which are nowadays optimized by previous modeling and process simulation, which in turn demand the knowledge of cross sections and/or swarm and transport data for the numerous physico-chemical processes occuring in the discharge. It is well known that the extent of the present quantitative knowledge on electron-molecule (atom) interactions with dielectric gases, like electron scattering, electron impact ionization and dissociation, and electron transport, by far overwhelms that on ion-molecule (atom) interactions, in the form of elastic and inelastic momentum transfer and reaction cross sections, coefficients of mobility, diffusion and reaction rates, to name only the most relevant quantities. There exist many cases in the literature where a researcher faces the problem of having only meagre, or even lacking, ion swarm or cross section data to provide a quantitative explanation to the phenomenon under study or simulation.

Using a Parallel Genetic Algorithm to Fit a Pulsed Townsend Discharge Simulation to Experiments

A genetic algorithm has been used to obtain the transport/reaction coefficients which describe the experimental ionic avalanches as obtained in a pulsed discharge operated in the Townsend regime. The calculation of the avalanche currents involves solving the continuity equations for the charged species present in the discharge by means of an explicit finite difference method. This is done by taking into account the drift and reactions of ions and electrons. In this work we apply this procedure to study the discharge of two pure gases, namely, \(SF_6\) and \(N_2\). In both cases the algorithm deals with a large number of transport/reaction coefficients (fitting parameters) and produces a solution in a small number of steps. For this purpose the algorithm generates a population of 480 individuals each one defined by a large number of genes (transport/reaction coefficients). Each individual determines a solution of the ionic avalanches which is compared to the experiment and a fitness function is defined to sort the individuals. This sorted population is then employed to obtained a new generation of individuals by using five evolution rules. The algorithm can be easily parallelized and produces an excellent agreement with the experimental data.

Cross sections for electron collisions with tetrafluoroethane (C 2 H 2 F 4 )

Summary form only given. Recent, growing interest in electron collision and transport data for tetrafluoroethane (C2H2F4) stems from the ever growing application of this gas and its mixtures in cooling systems and also in radiation particle detectors, resistive plate chambers in particular. Having that in mind and some new data we have decided to revise and extend our previous studies of electron interactions with that molecule. Our aim was to develop a more detailed set of cross sections and to improve its uniqueness as compared to the previously constructed set. Additional attention was focused on low electron energy region. A method that we used in our study was a standard swarm procedure. Our starting set consisted of elastic momentum transfer and 14 inelastic processes. Energy dependencies of eleven vibration excitations were assumed both in their magnitude and shape, while the threshold energies were taken from the literature. Electron attachment, dissociative excitation and ionization cross sections were calculated by using the Quantemol code. That set was normalized by using a Boltzmann equation solver and a Monte Carlo code, so as to obtain the electron swarm parameters - drift velocity, W, and effective ionization coefficient, (α-n)/N consistent with experimental data. Calculations were made for pure gas and mixtures with Ar of various compositions. Calculated data cover a wide interval of electron mean energies. After a number of iterative trials, during which the cross sections were modified in order to overcome some obvious inconsistencies, it was shown that two more cross sections had to be introduced to the set: the effective excitation with a broad peak around 15 eV, and the low energy attachment.

Negative Ion Motion in Pure SF6 and its Mixtures with Atmospheric, Halocarbon, and Rare Gases

The study of ion transport in gases has become increasingly important in view of the new and growing areas of applied research such as ion mobility mass spectrometry, in which the identification of the ionic species in the gas (pure or mixture) is carried out through the measurement of the ion mobility. In particular, SF6 is studied extensively because of its widespread use in the power and semiconductor plasma processing industries, despite its high global warming potential that has raised concerns on its release into the atmosphere. Thus research is currently aimed at finding replacements or mixtures with other gases that would retain many of its outstanding characteristics while reducing its atmospheric impact1.

Single electron capture of Ne+ in He in the energy range 0.5 to 5 keV

Absolute differential and total cross sections have been measured for the single electron capture of Ne+ in He at impact energies between 0.5 and 5 keV. Oscillations in the differential cross sections are interpreted as the envelope of the fast oscillations due to the interference of the scattering amplitudes. Total cross sections are in agreement with previous experiments.