Plasma collisions

Encinar, J. R., Schaumloffel, D., Ogra, Y., and Lobinski, R., Determination of selenomethionine and selenocysteine in human serum using speciated isotope dilution-capillary HPLC-inductively coupled plasma collision cell mass spectrometry, Analytical Chemistry 76(22), 6635-6642, 2004. 

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... 

With these specifications, and with the appropriate neutral particle-plasma collision terms put into the combined set of neutral and plasma equations, internal consistency within the system of equations is achieved. Overall particle, momentum and energy conservation properties in the combined model result from the symmetry properties of the transition probabilities W indices of pre-collision states may be permuted, as well as indices of postcollision states. For elastic collisions even pre- and post collision states may be exchanged in W. 

Collisional ionization can play an important role in plasmas, flames and atmospheric and interstellar physics and chemistry. Models of these phenomena depend critically on the accurate detennination of absolute cross sections and rate coefficients. The rate coefficient is the quantity closest to what an experiment actually measures and can be regarded as the cross section averaged over the collision velocity distribution. 

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 ... 

Microwave discharges at pressures below 1 Pa witli low collision frequencies can be generated in tlie presence of a magnetic field B where tlie electrons rotate witli tlie electron cyclotron frequency. In a magnetic field of 875 G tlie rotational motion of tlie electrons is in resonance witli tlie microwaves of 2.45 GHz. In such low-pressure electron cyclotron resonance plasma sources collisions between tlie atoms, molecules and ions are reduced and the fonnation of unwanted particles in tlie plasma volume ( dusty plasma ) is largely avoided. 

As an example, we look at tire etching of silicon in a CF plasma in more detail. Flat Si wafers are typically etched using quasi-one-dimensional homogeneous capacitively or inductively coupled RF-plasmas. The important process in tire bulk plasma is tire fonnation of fluorine atoms in collisions of CF molecules witli tire plasma electrons... 

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. 

The probability for tliree-body collisions increases with increasing pressure making the use of an atmospheric pressure plasma desirable. The above process is used worldwide for ozone production for water purification. 

In a cascade process, one incident electron (e ) collides with a neutral atom ((S)) to produce a second electron and an ion ( ). Now there are two electrons and one ion. These two electrons collide with another neutral atom to produce four electrons and three ions. This process continues rapidly and — after about 20 successive sets of collisions — there are millions of electrons and ions. (The mean free path between collisions is very small at atmospheric pressures.) A typical atmospheric-pressure plasma will contain 10 each of electrons and ions per milliliter. Some ions and electrons are lost by recombination to reform neutral atoms, with emission of light. 

The cold plasmas tend to be unstable, are sometimes difficult to maintain, and provide ion yields that are less than those of the hot plasmas. To obviate the difficulties of the interfering isobaric molecular ions from hot plasmas, it has been found highly beneficial to include a collision cell (hexapole see Chapter 22) before the mass analyzer itself. This collision cell contains a low pressure of hydrogen gas. lon/molecule collisions between the hydrogen and, for example, ArO+... 

All three types of discharge involve the formation of ions as part of the process. For various reasons, most of the ions are positive. The ions can be examined by mass spectrometry. If small amounts of a sample substance are introduced into a corona or plasma or arc, ions are formed by the electrons present in the discharge or by collision with ions of the discharge gas. 

The multiple energetic collisions cause molecules to break apart, eventually to form only atoms, both charged and neutral. Insertion of sample molecules into a plasma discharge, which has an applied high-frequency electric field, causes the molecules to be rapidly broken down into electronically excited ions for all of the original component atoms. 

The two electrons emerging from the collision are again speeded until each produces another electron by collisional ionization of another atom of argon. The process continues so the first incident electron becomes two, the two become four, and so on. This cascade increases the number of electrons and ions in the gas to form a plasma within a few milliseconds. 

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