Experimental Aspects of Autoionization

Above ionization limits, lines in the absorption spectrum are often broad or diffuse. This diffuse character results from an interaction of a very highly excited neutral molecule state, AB, with the continuum of an ionized molecule, AB+, plus an electron. This continuum reflects the fact that the electron can be ejected from the molecule over a continuous range of kinetic energy  

The state AB, often called the superexcited state, is an autoionized or resonance state. Autoionization is called preionization by Herzberg (1950). This can be justified by the analogy between preionization and predissociation. In predissociation, the interaction of a discrete state with the vibrational continuum of the nuclei allows this discrete state a finite probability of dissociation. In preionization, it is the mixing of a discrete state with the electronic continuum that provides a finite ionization probability. 

We consider in the following the case of a single isolated resonance, far removed from other resonances. If an absorption line has a Lorentzian profile, the linewidth/ T, is related to the total lifetime, r, of the state by the usual relation P oc r 1 [cf. Eq. (7.4.11)]. This width includes contributions from each of these three different types of decay. If interactions between continua of the electron, nuclei, and radiation field are neglected, one can write 

These partial widths, T r, and Tr, associated with autoionizing, dissociative, and radiative decay, respectively, and the corresponding partial lifetimes ra,Td, and Tr, cannot be directly observed. Only the total width T can be observed experimentally. This width is identical whether the line is observed by absorption, photoionization, or fluorescence spectroscopy, but the associated cross-sections can be different. Here we address only the problem of autoionization in absorption spectroscopy. 

These /-levels, in addition to being weakly predissociated, are also very weakly autoionized (see also Fig. 8.23). Another example appears in the spectrum of Li2 (Chu and Wu, 1988). In many other cases, decay by predissociation can be very fast, particularly for electrostatic predissociations (see Tables 7.3 and 7.4). For example, in the spectrum of the NO molecule, most 2Il Rydberg states are predissociated by the vibrational continuum of the B2n valence state so rapidly that autoionization cannot compete (Giusti-Suzor and Jungen, 1984). 

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