Autoionization

Autoionization, in principle, is similar to predissociation, in that mixing between a bound state and a continuum state results in the formation of two 

A similar extended progression is observed with O2, and a number of other molecules and intensity 

The pseudo-continuum and interloper are strongly coupled, with the transition to the pseudocontinuum being strongly favoured. This results 

the interpretation of ZEKE signal intensities can be more complex than expected purely on the basis of Franck-Condon factors. 

For a wider discussion of autoionization the reader is referred to the existing literature, e.g. see Powis (1995). 

One consequence of this behavior is that a properly measured absorption cross-section curve of an atom or molecule will show no discontinuities at the positions of thresholds for formation of states of the corresponding ion. This is not necessarily true of an ionization cross-section curve for a molecule, as can be seen from Fig. 3. The reason for this difference is very simple. If nearly all absorption above the first ionization limit leads to ionization (as it must for an atom since autoionization is much faster than radiation, the only other competing process), there will be no discontinuity at higher thresholds. However, if the higher Rydberg states can be depopulated by some mechanism that does not produce ions (such as predissociation to neutral fragments) and that is much faster than autoionization, then the thresholds can become apparent. 

The radiationless decay of a quasidiscrete excited state of an atom or molecule into an ion and electron of the same total energy is called autoionization. The quasidiscrete state must, of course, lie above the first ionization potential of the atom or molecule. The occurrence of autoionization may be inferred from the appearance of absorption spectra or ionization cross-section curves which exhibit line or band structure similar to that expected for transitions between discrete states. However, in the case of autoionization the lines or bands are broadened in inverse proportion to the lifetime of the autoionizing state, as required by the uncertainty principle. In the simple case of one quasidiscrete state embedded in one continuum, the line profile has a characteristic asymmetry which has been shown to be due to wave-mechanical interference between the two channels, i.e., between autoionization and direct ionization. In an extreme case the line profile may appear as a window resonance, i.e., as a minimum in the absorption cross section. 

Quasidiscrete states lying above the ionization limit can be formed if the excited electron is not the most easily removed one or if the excitation energy resides in more than one degree of freedom. A simple example of 

Some important properties of Rydberg states can be illustrated by the Ar curve of figure 6. For high values of n, the spacing between adjacent Rydberg levels varies as as can be seen by differentiating Eq. (8). 

In the case of most molecules, autoionization also occurs predominantly by electronic interaction. Thus for O2 and N2, which are among the more thoroughly studied molecules, most autoionization occurs via Rydberg states converging to electronically excited states of the ion. When energetically possible, the resulting ion will be formed in a distribution 

Generally, when studying autoionizing levels, we have to take into consideration both (radiative and radiationless) channels of their decay. The total natural width of the autoionizing level will be the sum of its autoionizing and radiative widths. 

Configurations with vacancies in inner shells possess a number of peculiarities their states are autoionizing they are short-lived relativistic effects are essential for them their energy spectrum has particular characteristics. Short lifetimes of excited states lead to large widths of relevant spectral lines. 

In X-ray and electron spectroscopy the sets of shells with given n = 1,2,3,4,5,6. are often denoted by capital letters K,L,M,N,0,P. whereas the subshells with given nlj are numbered as 

A iV-electron vacancy (hole) in a shell may be denoted as nl N = ni4l+2-N (see aiso Chapters 9, 13 and 16). As we have seen in the second-quantization representation, symmetry between electrons and vacancies has deep meaning. Indeed, the electron annihilation operator at the same time is the vacancy creation operator and vice versa instead of particle representation hole (quasiparticle) representation may be used, etc. It is interesting to notice that the shift of energy of an electron A due to creation of a vacancy B l is approximately (usually with the accuracy of a few per cent) equal to the shift of the energy of an electron B due to creation of a vacancy A l, i.e. 

Unfortunately, vacancies cannot be treated as quasiparticles for the cases of strong configuration mixing, including mixing with continua. 

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Conventional associative ionization (AI) occurring at ambient temperature proceeds in two steps excitation of isolated atoms followed by molecular autoionization as the two atoms approach on excited molecular potentials. In sodium for example [44]... 

A water molecule has amphoteric character. This means it can act as both an acid and a base. The autoionization equilibrium process in water. 

Click Coached Problems for a self-study module on autoionization. 

Kw is also widely called the autoionization constant and sometimes the ion product constant of water. 

Aufbau principle See building-up principle. autoionization See autoprotolysis. autoprotolysis A reaction in which a proton is transferred between two molecules of the same substance. The products are the conjugate acid and conjugate base of the substance. Example ... 

It is well known that the value of the p parameter, more than the cross-section a, often shows a strong response to resonant structure embedded in the continuum. Given the sensitivity exhibited by the parameter in the foregoing there must be an a priori expectation that it would also show a strong response to resonant behavior. Computational methods do not yet exist to deal with autoionization phenomena in the systems of interest here, but one electron shape resonances can, in principle, be examined. 

From the available evidence Stener and co-workers [53, 60] conclude that the chiral parameter is more sensitive to small asymmetries in the molecular potential than to continuum collapse effects at resonance. At present, such conclusions must be provisional as there is little direct evidence. There is also no evidence regarding likely behavior at autoionization resonances, and this too deserves attention. 

The sitnation for the states is similar, because of the autoionizing state... 

The formal basis employed in the K-matrix calculation includes the relevant partial wave channel (pwc) subspaces plus a "localized channel" (/c) of discrete functions. These last are usual Cl states and their inelusion in the basis allows to efficiently reproduce the autoionizing states and the eorrelation effects. 

As discussed in (4), the K-matrix has a pole at energies near a resonance and this yields a convenient method for the analysis of the narrow autoionizing states. The matrix representation of equation [2] upon a finite basis may be in fact recast in the form (4)... 

Table 3. Wave energies and widths of autoionizing states. a — b) means a 10 ...

If we suppose that these Rydberg states have non-negligible lifetimes against autoionization, as could occur via dispersion of the potential energy of the molecule into various vibrational modes, then these states would be very sensitive to electric fields in the measuring apparatus. It is suggested here that this is the reason for the large discrepancies between cross sections measured in the various beam experiments. 

It is a straightforward task to construct a rate model for the reaction scheme shown in Figure 9. Electron capture and its inverse (autoionization),... 

The rate coefficients for H" in the autoionizing states (s, p) may differ from those in higher /-states and are treated differently (subscripts 0 and 1 ). A steady-state analysis of the rate equations gives a deionization coefficient of ... 

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