Electrons in plasmas

The reactions underlying CVD typically occur both in the gas phase and on the surface of the substrate. The energy required to drive the reactions is usually supplied thermally by heating the substrate or, in a few instances, by heating the gas. Alternatively, photons from an ultraviolet (UV) light source or from a laser, as well as energetic electrons in plasmas, are used to drive low-temperature deposition processes. 

The ionization temperature is relevant for all phenomena involving equilibria between analyte atoms, ions and free electrons in plasmas. In the case of thermal equilibrium, it occurs in the Saha equation [Eqs. (66, 68)] and can be determined from the intensity ratio of an ion and an atom line of the same element. In all other cases ionization temperatures can be determined from the ne value obtained from Stark broadening [see Eqs. (74, 77)]. 

TABLE 4. Bond energies of core electrons in plasma polymeric films measured by XPS... 

For the s mthesis of polymers orrly the low temperature plasma, in which the energy of molecules does not exceed a few electronvolts is used The temperature of electrons in plasma is always sigrrificantly larger than the temperature of ionized gas. Therefore, it is possible to run the reaction at a temperature close to ambient temperature where the free electrons have enough energy to break the covalent bonds. For this reason this type of plasma is also called the non-equilibrium plasma. 

The plasma state is often referred to as tire fourtli state of matter [1]. It is characterized by tire presence of free positive (and sometimes also negative) ions and negatively charged electrons in a neutral background gas. The... 

The velocity distribution of the electrons in a plasma is generally a complicated function whose exact shape is detennined by many factors. It is often assumed for reasons of convenience in calculations tliat such velocity distributions are Maxwellian and tliat tlie electrons are in tliennodynamical equilibrium. The Maxwell distribution is given by... 

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 ... 

Modelling plasma chemical systems is a complex task, because these system are far from thennodynamical equilibrium. A complete model includes the external electric circuit, the various physical volume and surface reactions, the space charges and the internal electric fields, the electron kinetics, the homogeneous chemical reactions in the plasma volume as well as the heterogeneous reactions at the walls or electrodes. These reactions are initiated primarily by the electrons. In most cases, plasma chemical reactors work with a flowing gas so that the flow conditions, laminar or turbulent, must be taken into account. As discussed before, the electron gas is not in thennodynamic equilibrium... 

Coburn J W and Winters H F 1979 Ion- and electron-assisted gas-surface chemistry—an important effect in plasma etching J. Appl. Phys. 50 3189-96... 

Plasmas at fusion temperatures cannot be kept in ordinary containers because the energetic ions and electrons would rapidly coUide with the walls and dissipate theit energy. A significant loss mechanism results from enhanced radiation by the electrons in the presence of impurity ions sputtered off the container walls by the plasma. Therefore, some method must be found to contain the plasma at elevated temperature without using material containers. 

This frequency is a measure of the vibration rate of the electrons relative to the ions which are considered stationary. Eor tme plasma behavior, plasma frequency, COp, must exceed the particle-coUision rate, This plays a central role in the interactions of electromagnetic waves with plasmas. The frequencies of electron plasma waves depend on the plasma frequency and the thermal electron velocity. They propagate in plasmas because the presence of the plasma oscillation at any one point is communicated to nearby regions by the thermal motion. The frequencies of ion plasma waves, also called ion acoustic or plasma sound waves, depend on the electron and ion temperatures as well as on the ion mass. Both electron and ion waves, ie, electrostatic waves, are longitudinal in nature that is, they consist of compressions and rarefactions (areas of lower density, eg, the area between two compression waves) along the direction of motion. 

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