Electron-atom collision processes

Investigations on the doubly excited states of two electron systems under weakly coupled plasma have been performed by several authors. Such states usually occur as resonance states in electron atom collisions and are usually autoionizing [225]. Many of these states appear in solar flare and corona [226,227] and contribute significantly to the excitation cross-sections required to determine the rate coefficients for transitions between ionic states in a high temperature plasma. These are particularly important for dielectronic recombination processes which occur in low density high temperature plasma, occurring e.g. in solar corona. Coronal equilibrium is usually guided by the balance between the rates of different ionization and... 

The orders of magnitude of quantities involved in electron—atom collisions are seen in table 1.1. The units that are used for the description of experiments and of the individual atomic processes are chosen so that the order of magnitude of numbers to be discussed is not very far from 1. In particular, distances characteristic of atoms are far too small to observe directly, but energies and momenta are of roughly the same order of magnitude as the corresponding laboratory units. It is natural therefore to consider collisions in momentum space. 

We now introduce the general concepts and definitions behind studies of alignment and orientation in electron—atom collisions. Let us consider the scattering process (8.1) in some detail. We must first understand the concept of coherence. We illustrate it by considering the initial state of the atom... 

Farago, P. S. Free-electron physics. Baltimore Penguin Books, Inc. 1970, pp. 269 Feldman, D., Novick, R. Observations on the (Is2s2p) Ps/2 metastable state in lithium, in Atomic collision processes, McDowell, M. R. C. (ed.) Amsterdam North Holland Publishing Company 1964, pp. 201-210 Fox, R. E., Curran, R, K. J. Chem. Phys. 34, 1595 (1961)... 

T. Aberg, A. Blomberg and O. Goscinski Stochastic Analysis of Ion-Atom Collision Processes Electronic and Atomic Collisions (J. Eichler et al. Eds. Elsevier Science Publ. B. V., p. 215, North Holland, Amsterdam 1984). 

Bang and Hansteen (1959) and later Hansteen and Mosebekk (1973) treated the ion-atom collision process in a semiclassical approximation, considering the projectile motion classically and the transition of the inner-shell electron to the continuum quantum mechanically. 

As mentioned in Section 3.2 only a few such measurements have so far been performed for autoionization processes. In the case of electron-atom collisions the analysis of the data is complicated by the fact that direct ionization contributes considerably to the ejected electron intensity. With ions as projectiles the direct process can be neglected as long as the projectile is slow enough, i.e., <0.5 a.u. Only in such cases are the formulas given in Section 6.2 valid, and therefore we will discuss only such experiments in the following. 

At present, there is a considerable interest in investigations of atomic collision processes involving two-electron transitions. The reason is that such phenomena are often strongly influenced by electron-electron correlation and hence can be understood only on the basis of theories that go beyond the independent-particle approximation by taking into account the many-body aspects of the transitions. Recent developments within this field have been reviewed by Stolterfoht [6.1] and McGuire [6.2, 6.3]. 

Divergent couplings ai e a nuisance for the computational treatment of the nuclear dynamics. In cases of exact or near degeneracy of electronic potential-energy surfaces it is therefore preferable to introduce an alternative electronic representation, the so-called diabatic (or quasi-diabatic) representation, which avoids singular coupling elements. The basic concept of diabatic states has been introduced in early descriptions of atomic collision processes and vibronic-coupling phenomena in molecular spectroscopy. ... 

Plasma is an energetic environment in which a number of chemical processes may occur. Many of these chemical processes occur because of electron-atom collisions. 

There are two basic physical phenomena which govern atomic collisions in the keV range. First, repulsive interatomic interactions, described by the laws of classical mechanics, control the scattering and recoiling trajectories. Second, electronic transition probabilities, described by the laws of quantum mechanics, control the ion-surface charge exchange process. 

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.

The inelastic collision process is characterized by an inelastic mean free path, which is the distance traveled after which only 1/e of the Auger electrons maintain their initial energy. This is very important because only the electrons that escape the sample with their characteristic Auger energy are usefrd in identifying the atoms in... 

Figure 1 Schematic of DC glow-discharge atomization and ionization processes. The sample is the cathode for a DC discharge in 1 Torr Ar. Ions accelerated across the cathode dark space onto the sample sputter surface atoms into the plasma (a). Atoms are ionized in collisions with metastable plasma atoms and with energetic plasma electrons. Atoms sputtered from the sample (cathode) diffuse through the plasma (b). Atoms ionized in the region of the cell exit aperture and passing through are taken into the mass spectrometer for analysis. The largest fraction condenses on the discharge cell (anode) wall.

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