Plasma collision processes

Table C2.13.1 Collision processes of electrons and heavy particles in non-thennal plasmas. The asterisk denotes short-lived excited particles, the superscript m denotes long-lived metastable excited atoms or molecules.

The probability for a particular electron collision process to occur is expressed in tenns of the corresponding electron-impact cross section n which is a function of the energy of the colliding electron. All inelastic electron collision processes have a minimum energy (tlireshold) below which the process cannot occur for reasons of energy conservation. In plasmas, the electrons are not mono-energetic, but have an energy or velocity distribution,/(v). In those cases, it is often convenient to define a rate coefficient /cfor each two-body collision process ... 

The reaction mechanisms of plasma polymerization processes are not understood in detail. Poll et al [34] (figure C2.13.6) proposed a possible generic reaction sequence. Plasma-initiated polymerization can lead to the polymerization of a suitable monomer directly at the surface. The reaction is probably triggered by collisions of energetic ions or electrons, energetic photons or interactions of metastables or free radicals produced in the plasma with the surface. Activation processes in the plasma and the film fonnation at the surface may also result in the fonnation of non-reactive products. 

An important point is that these advances have been complemented by the concomitant development of innovative pulse-characterisation procedures such that all the features of femtosecond optical pulses - their energy, shape, duration and phase - can be subject to quantitative in situ scrutiny during the course of experiments. Taken together, these resources enable femtosecond lasers to be applied to a whole range of ultrafast processes, from the various stages of plasma formation and nuclear fusion, through molecular fragmentation and collision processes to the crucial, individual events of photosynthesis. 

In general, the substrate temperature will remain unchanged, while pressure, power, and gas flow rates have to be adjusted so that the plasma chemistry is not affected significantly. Grill [117] conceptualizes plasma processing as two consecutive processes the formation of reactive species, and the mass transport of these species to surfaces to be processed. If the dissociation of precursor molecules can be described by a single electron collision process, the electron impact reaction rates depend only on the ratio of electric field to pressure, E/p, because the electron temperature is determined mainly by this ratio. 

In order to ionize the Ar atoms the electrons transfer their energy via collision processes to the plasma gas atoms. On the other hand, excited argon atoms are formed and photons emitted by radiative recombination of argon ions with electrons (see Equation 2.2.). The latter lead to a significant increase in the instrumental background in the ICP-MS ... 

In fact, plasma methods may belong to the most difficult methods to model completely as many as 24 different collision processes are incorporated in Bogaert s model and non-equilibrium situations, and the presence of imperfect solid surfaces must be accounted for. Similar arguments could, if the processes are studied in detail, be formulated for atomic emission or atomic absorption analysis. Diagnostics of the inductively coupled plasma has resulted in a quite well-characterised sample environment. 

Atmospheric pressure plasmas, just like most other plasmas, are generated by a high electric field in a gas volume. The few free electrons which are always present in the gas, due to, for example, cosmic radiation or radioactive decay of certain isotopes, will, after a critical electric field strength has been exceeded, develop an avalanche with ionization and excitation of species. Energy gained by the hot electrons is efficiently transferred and used in the excitation and dissociation of gas molecules. In a nonequilibrium atmospheric pressure plasma, collisions and radiative processes are dominated by energy transfer by stepwise processes and three-body collisions. The dominance of these processes has allowed many... 

R.K. Janev, D. Reiter, U. Samm Collision Processes in Low-Temperature Hydrogen Plasmas, Report JUEL-4105 (2003)... 

In the present review we shall discuss the current status of cross-section database for collision process of H and H2 in fusion plasmas, with particular attention to processes involving electronically and vibrationally excited states... 

Electron-impact electronic excitation, ionization and dissociation processes are among the most important collision processes that H2 molecules undergo in a low-temperature plasma. These include... 

Collision processes of 11 ions with molecular hydrogen and its ions have not received much attention until recently, although their cross-sections may be large. The most important among these processes for divertor plasma conditions are... 

E.L. Freeman, E.M. Jones, Atomic Collision Processes in Plasma Physics Experiments I, UKAEA Rept. No. CLM-R137 (Culham Laboratory, Abingdon, England 1974)... 

The charged particle collective behaviour (and thus the plasma) exists only if random collision processes do not smear out the coherence of the motion of the charged particles. This means in practice that the frequency of the electron-neutral collisions has to remain smaller than the electron plasma frequency. 

Collision frequencies, mean free paths, and cross sections. Collision processes are central to the description of chemical reactions in the plasma. The mean free path /Ia/b of particle A is the mean distance this particle travels before encountering particle B. AA/b is related to the mean velocity of particle A and to the mean collision frequency between A and B. 

With respect to plasma chemistry, the most important collision processes in a weakly ionised plasma occur between charged particles and neutral particles. Elastic collisions concern principally coulombic and polarisation scattering processes. Coulombic scattering applies when the characteristic interaction time... 

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