Plasma electron-molecule collisions

However in most processes of practical relevance such as electron-molecule collisions in industrial plasmas and upper atmosphere, orientations of the molecules seem to be not fixed. On another hand typical interaction times for 1-30 eV of collision energy is 10 14-10 15 s. Timescale for the rotations of the polyatomic molecules at room temperature is 10 12 s and longer. This comparison allows us to assume that scattered electron responds adiabatically to the rotations of the molecules and validates the fixed-nuclei approximation19,20 implicitly assumed in equations (14) and (15). Nevertheless orientation of the molecule with respect to the incoming electron is random and therefore cross sections must be averaged over all the orientations of the molecule. This is carried out by the following technique. Inelastic differential cross sections are obtained from (11) as... 

This chapter discusses the contribution that theoretical methods can make to a knowledge of electron-molecule collision behavior, and thereby to an imder-standing of low-temperature plasmas. Its aim is to survey both the relevant problems and the methods that have been developed to treat those problems. 

If one hopes to develop detailed, predictive models of plasmas, microscopic information such as electron-molecule collision probabilities clearly is needed. But why obtain that information from theory The short answer is that experimental data are often absent and—given the difficulty of the measurements and the paucity of research groups conducting them—in many cases are likely to remain so indefinitely. A longer answer would add that, as both theoretical methods and computer hardware improve, theory is, at least in some areas, becoming competitive with experiment in terms of accuracy and time to solution. 

In the following section, we will describe some of the cross sections that are most important in low-temperamre plasmas. We will then turn to methods for computing such cross sections, describing the methods that are in most common use for low-energy electron-molecule collisions and giving a few examples of their application. A concluding section discusses areas where further progress is needed. 

In this section, we look at several methods in current use for calculating electron-molecule collision cross sections relevant to low-temperature plasmas. For the most part, we will avoid technical details, which in any case can readily be found elsewhere (Huo and Gianturco, 1995 Winstead and McKoy, 1996), although we will attempt to describe enough of the implementation to bring out the advantages, disadvantages, and limitations of each method. We will conclude with illustrative examples in which different methods are applied to the same elastic and inelastic electron-molecule collision problems. 

Electron-Molecule Collisions in Low-Temperature Plasmas The Role of Theory, Carl Winstead and Vincent McKoy... 

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... 

If the electric current and thus the density of electrons and excited atoms and molecules grows in the plasma, electron collisions with excited atoms and molecules and the Coulomb interaction between the electrons become increasingly important and have to be included in the kinetic study of the electron behavior. 

Relaxation of electronically excited atoms and molecules is due to different mechanisms. Superelastic collisions (energy transfer back to plasma electrons) and radiation are essential mostly in thermal plasma. Relaxation in collision with other heavy particles dominates in non-thermal discharges. Relaxation of electronic excitation into translational degrees... 

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