Electron-molecule collision processes

Mengoni, A., and Shirai, T. (1991), Algebraic-Eikonal Approach to the Electron-Molecule Collision Process Vibrational Excitation and Quadrupole Interaction, Phys. Rev. A 44, 7258. 

In what follows we shall consider an excitation process which is short lived on the time scale of the resulting excited state. This excitation process may involve an electron-molecule collision or absorption of a light pulse. We start with the system in the ground state (f>0. The excitation process introduces a time-dependent perturbation Tx(t) for a duration of time r. An excited nonstationary state is thereby produced which can be represented as a superposition of molecular eigenstates (see eq. 6-2)... 

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... 

Processes 4a and 4b are the principal ways by which new electrons are generated by electron-molecule collisions (and by which existing ones are slowed down) through process 4b a multiplicity of positive ions can be produced. 

Lucchese, R.R. and McKoy, B.V. 1983, in Electron-Molecule Collisions and Photoionization Processes, eds. V. McKoy et al. (Verlag Chemie), p. 13. 

With these ideas in mind we can make a partial catalog of the processes in electron-molecule collisions. 

The same reactant ions are formed in positive-ion mode in a point-to-plane corona discharge. Approximately 3-9 kV is applied at the corona discharge, creating a corona current of 1-5 pA, depending on the point-to-plane distance and the composition of the vapours in the ion source. Corona discharges are very complicated processes that involve avalanches of ionization reactions by ion-molecule and electron-molecule collisions together with quenching... 

A partial wave decomposition provides the frill close-coupling quantal method for treating A-B collisions, electron-atom, electron-ion or atom-molecule collisions. The method [15] is siumnarized here for the inelastic 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.

Molecular Interaction. The examples of gas lasers described above involve the formation of chemical compounds in their excited states, produced by reaction between positive and negative ions. However, molecules can also interact in a formally nonbonding sense to give complexes of very short lifetimes, as when atoms or molecules collide with each other. If these sticky collisions take place with one of the molecules in an electronically excited state and the other in its ground state, then an excited-state complex (an exciplex) is formed, in which energy can be transferred from the excited-state molecule to the ground-state molecule. The process is illustrated in Figure 18.12. 

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. 

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