Atomic and Molecular Processes in Reactive Plasmas

The primary activation of parent molecules in reactive plasmas is through the collision of molecules with electrons in a wide energy range. Molecules thus receive energies from electrons and form reactive species such as excited or ionized states of molecules, free radicals, and electrons of low energies. These species interact with each other or with stable molecules. The succession of events in atomic and molecular processes that follow the primary activation is sununarized in Table I (Hatano, 1991 Tanaka et al, 1996). In analyzing these processes, workers in a reactive-plasma research field should understand, at least 

Following are a brief description of and related comments on the atomic and molecular processes in reactive plasmas listed in Table I. 

Molecules AB in collisions with electrons distributed over a wide range of their energy, which is characterized with an electron temperature or a mean energy, are directly ionized and excited into superexcited states (Hatano, 1999) above their first ionization potentials and excited states below them. 

Superexcited states AB may be autoionized or dissociated to neutral fiagments, i.e., free radicals or stable product molecules. Electronically excited states AB may also be dissociated to neutral fiagments. Parent ions directly formed via direct ionization or indirectly formed via autoionization are dissociated to fiagment ions. It should be noted that firee radicals are formed simultaneously in the dissociation of the parent ion. Absolute cross sections as a function of the electron-molecule collision energy are needed, therefore, for both ionization and dissociation. 

In some cases, particularly in a gas system of polar molecules AB, fiee electrons or ions are solvated with 15-dipoles to form solvated electrons and ions, respectively (Hatano, 1986). Such species would have an important role in reactive plasmas. 

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PHYSICOCHEMICAL ASPECTS OF ATOMIC AND MOLECULAR PROCESSES IN REACTIVE PLASMAS... 

V Comments on Atomic and Molecular Processes in Reactive Plasmas from... 

Reactive plasmas are generally characterized as plasmas in which component polyatomic molecules have an important role. Information and ideas, as well as experimental techniques in physical chemistry, particularly in reaction dynamics and kinetics studies, are greatly needed to control the essential features of atomic and molecular processes in reactive plasmas and thus to obtain desired products of reactive-plasma processing such as chemical-vapor deposition (CVD) and etching (Hatano, 1991). 

Future perspectives and comments on atomic and molecular processes in reactive plasmas from physicochemical viewpoints are given as follows (Hatano, 1991). 

Hatano, Y. (1991). In T. Goto [Ed.], Proceedings of the International Seminar on Reactive Plasmas, Nagoya University, p. 341. This paper summarizes physicochemical aspects of atomic and molecular processes in reactive plasmas which have been discussed in detail in the joint research project entitled Control of Reactive Plasmas (R. Itatani, head) as supported by Grant-in-Aid for Scientific Research on Priority Areas, Ministry of Education, Science, and Culture, Japan. Hatano, Y. (1999) Phys. Reports., 313, 109. 

A brief survey is given of physicochemical aspects of atomic and molecular processes that are of great importance in reactive plasmas. The processes are composed of the interaction of molecules, in most cases polyatomic molecules, with reactive species such as electrons, ions (both positive and negative), free radicals, and excited atoms and molecules. Topics are chosen from recent studies of some elementary processes in reactive plasmas. Some comments are also given on future problems that call for more work in reactive-plasma research from the viewpoints of physicochemical studies of gas-phase reaction dynamics and kinetics, such as radiation chemistry and photochemistry. 

Free radicals, or radicals, are defined as atomic or molecular species with unpaired electrons on an otherwise open shell configuration. Because a radical has a half-filled orbital, it easily sucks up an electron from another bond, hence exhibiting highly reactive properties and therefore likely to take part in chemical reactions. The first organic free radical is the triphenylmethyl radical, which was identified by Moses Gomberg in 1900 [34], Radicals play an important role in chemical processes such as combustion, atmospheric chemistry, polymerisation, plasma chemistry, biochemistry, etc. [35],... 

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