Ionization channels

This is a transient discrete electric discharge which takes place between two conductors which are at different potentials, bridging the gap in the form of a single ionization channel (Plate 4). Based on light emission measurements of sparks with symmetrical electrode geometry, the energy is dissipated approximately uniformly along the channel. This is in contrast with asym-... 

For collisions involving fast electrons, most of the relevant reactions given by Eqs. (9.1)-(9.13) occur with the primary and secondary electrons leaving the target molecule promptly, in about 10 sec. One the other hand, autoionization and dissociative ionization channels can result in a secondary electron being delayed relative to the primary, and in the case of resonant electron attachment, there may be a measurable delay in the exit of the primary electron. These processes are described in considerable detail by Mark et al. [19]. 

Notice that in our calculation we have both closed and open ionization channels. This means that the quantum defect/frame transformation approach appears to work very well both below and above the critical region for which n - 100,..., 1000.1 would find it surprising if the approach failed in that region. 

From photoelectron spectroscopic studies it is apparent that a second ionization channel starts at 11.4eV in the case of benzene, i.e., at about 2.15 eV above the first ionization potential.219 The tail of the calculated spectrum will, therefore, be buried beneath the second ionization threshold. The second ionization threshold is indicated on Figure 30 by an arrow. Finally, there have been recent suggestions that there are two a ionization potentials, at 10.35 and 10.85 eV.220 If correct, excitation of these a-electrons would also lead to absorption intensity obscuring the contribution of transitions of the elg orbitals. 

Regarding the formation of subexcitation electrons, many authors consider only two possibilities they assume that each subexcitation electron either has been ejected during ionization (and happened to have the energy below fcco01), or it is one of the fast electrons that has slowed down to energies below hw(n. However, there may be other possible ways for subexcitation electrons to be formed. One of them is via a decay of a superexcitation state according to the ionization channel. Such a process may occur both in gaseous and in condensed media, and the majority of electrons produced in this case are the subexcitation ones. 

A number of molecular targets have been studied in recent years and the effects of direct and dissociative ionization channels have been observed. Positron-methane collisions were also used in the first observation of positronium hydride, as will be described in section 7.5. 

However, to obtain results with good accuracy, it is also necessary to include a pseudostate (or localized) channel, which accounts for those closed channels that have not been included, as well as for the multiple ionization channels, since the set of single-ionization PWCs alone would be incomplete. The localized channel may comprise a large number of normalized many-electron functions /, built from localized orbitals... 

The mixing coefficients are still unknown, but as will be seen below, in the present context only the a0-coefficient, where a0 < 1, is needed. Hence one can add to the wavefunction in equ. (5.26) the photoelectron s contribution, in order to calculate the matrix element between the initial state (equ. (5.24)) and the selected 2p photoionization channel. Working out the relevant dipole matrix elements, one then has the advantage that the overlap factors are unity if there the electron configurations are the same on both sides, and zero for all other cases. Hence only two ionization channels remain, and they are given by... 

Kleiman, Gordon and co-workers [73] have also demonstrated control of ionization in H2S in a jet with a large distribution of j states. Although in this case both a dissociation and an ionization channel are possible, that is, H2S+ - H2S - H + HS, no discrimination between the possible outcomes of the photoexcitation has been observed The signals of all final channels oscillate in phase as the relative phase 03 — 3(j>l is varied. 

Ionization at a given photon energy may proceed in several channels. For example, the dipole selection rule, A l- 1, permits an initial electronic state of angular momentum / to decay into two degenerate ionization channels, the / +1 and / -I channels in which the photoelectrons have angular momenta (/ + 1) h and (/ - 1 )h. Since the parameters a and P contain the radial matrix elements for ionization into the two channels, and since these elements are proportional to the overlap of the electronic wavefunctions for the initial and final states of the ionization process, it follows that a and P are functions of these overlaps. Secondly, since the two photoelectron waves have different phase and nodal structures, they may interfere this interference is also determinative of o and p values. For atomic photoionization and LS coupling, one finds ... 

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