Ionization and Autoionization

In Fano s parameterization it is implied—differently from (14) and (IS)— that autoionizing transitions from different states do not interfere with each other. The same is true for a parameterization given by Shore. Except for this, however, the parameterizations are equivalent and the parameters are related by simple formulas. Autoionization spectra due to photoionization as well as collisional ionization with fast electrons, ions, and atoms, have successfully been analyzed by means of Fano s parameterization. In collisions with low-velocity projectiles, however, other effects become important that influence the relative phases of the ionization amplitudes and thus the spectral shapes. Also interferences between different autoionization amplitudes have to be taken into account. 

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This experiment may be regarded as the forerunner of mass spectro-metric appearance-potential determination in that both are threshold techniques, that is they depend on slow variation in the energy supplied by the impacting electron until a change in the electron-molecule interaction is observed. Thus, just as the Hertz experiment did not distinguish between excitation and ionization potentials, mass spectrometric appearance potential measurements are subject to similar ambiguities in interpretation as between ionization and autoionization. 

After this schematic discussion of possible processes around the 4d ionization shell in xenon, Fig. 5.2(h) can be compared with the experimental results shown in Fig. 5.1. The main structures can be related to 4d, photoexcitation/ionization, and autoionization/Auger decay to 5s25p4fa electron configurations can be identified in the energy regions of overlap. 

Using an ordinary El mass spectrometer with electron energies in the range 50—100 eV, the internal energy distribution, P(E), of the molecular ion is typically extremely broad (possibly tens of eV) and poorly defined. Both direct ionization and autoionization contribute to formation of molecular ions [517]. 

Understanding the spur structure as a function of the electron energy, especially below 1 keV. What is the relative significance of ionization and autoionization processes What is the origin of the observed spin effects ... 

Fig. 5.30 Direct ionization and autoionization resonances just above the ionization threshold observed through the ion yield Mon ( 2) in Li2 when the first laser excites the level k) = (v = 2, J = 1) in the B state (a) with Li off (b) with Li on...

Figure 5. Autoionization spectrum from He, excited by electron impact. The line shapes are strongly influenced by interferences between direct ionization and autoionization. (From Ref. 10.)...

Figure 24. Noncoincident angular distributions of He autoionization electrons. There is no longer a symmetry with respect to 90°. This indicates that interferences of direct ionization and autoionization processes become important. (From Ref. 56.)...

Fig. 10.27. Direct ionization and autoionization resonances close above ionization threshold observed through the ion yield Njo (A2) in Li when the first laser excites the level (v =2, J =7) in the B Ilu state and the second laser is tuned a) without and b) with pump laser LI [10.69]... 

Figure 28. Electron spectrum for collision system He -Kr at various collision energies. Broad distribution at low electron energies is a result of Penning ionization, and narrow peaks arise from atomic autoionization of krypton following excitation transfer from He to Kr.77...

Fig. 11.26 Rb-Rb potential curves showing the origin of the differing rates for Penning and associative ionization. In associative ionization the initial state Rb n( + Rb 5s only is above the lower 2g ionic state at small R where the g - 2U exchange splitting is large. Only at small R does autoionization to the ionic molecular state occur. In contrast, in Penning ionization the initial state Rb n + Rb 5p always lies above the ionic final state, and autoionization can occur at any R.

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... 

Hitherto the discussion of Fig. 5.2 has neglected the possibility of non-radiative decay following 4d shell excitation/ionization. These processes are explained with the help of Fig. 5.2(h) which also reproduces the photoelectron emission discussed above, because both photo- and autoionization/Auger electrons will finally yield the observed pattern of electron emission. (In this context it should be noted that in general such direct photoionization and non-radiative decay processes will interfere (see below).) As can be inferred from Fig. 5.2(h), two distinct features arise from non-radiative decay of 4d excitation/ionization. First, 4d -> n/ resonance excitation, indicated on the photon energy scale on the left-hand side, populates certain outer-shell satellites, the so-called resonance Auger transitions (see below), via autoionization decay. An example of special interest in the present context is given by... 

Ionization techniques can lead to the formation of not only the ground electronic state of a molecular ion, but also excited electronic states, both by direct ionization and by autoionization. Such excited states may react directly and they may emit radiation. They may decay to other electronic states (vibronic relaxation) and then undergo reaction. All these processes may be in competition with each other [711]. 

Here we are chiefly interested in the intrinsic causes of line broadening. We do not include among these dissociation, ionization, predissociation, autoionization, and pieisomerization, since few unambiguous examples of their occurrence have been reported for the low-lying excited electronic states. Attention is devoted to broadening through anharmonicity (vibrational relaxation) and in particular to electronic relaxation. 

The pre-dissociation results from a non-adiabatic coupling between electronic states (NA v) and (N A ec) at internuclear distances (usually smaller than the equilibrium distance) that correspond to the vibrational continuum of N A state, or when the N A state is a purely dissociative one. Autoionization and pre-dissociation probabilities for Npa(v) and Npir(v)(N > 4) states have been calculated in [68], while for the states with N < 3 they have been evaluated from experimental measurements [69,70]. Auto-ionization and pre-dissociation rates are very high ( 108s 1 for N = 3,4), and rapidly increase with increasing v. More experimental or theoretical information is needed for these processes. 

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