Double photoionization Auger effects

The prototypical indirect double photoionization process is the classical Auger effect, where the single-hole state created by ejection of an inner-shell electron exists for a significant time before ejection of a second electron. Alternatively, an intermediate singly charged state may exist as a multiply excited valence state or a Rydberg state, but whatever its nature, a lifetime of at least a few femtoseconds will normally allow the whole process to be considered as a sequence of two distinct steps. A sharp peak in the photoelectron spectrum and also, therefore, in... 

In the following sections, we first consider molecules for which direct double photoionization seems to dominate, then we present clear examples of molecular Auger effects, and finally we discuss the dissociative double ionization mechanisms that have no counterpart in atoms. 

Figure 21. The inner valence Auger effect in formaldehyde (methanal). The broad peak in the regular photoelectron spectrum near 34 eV is an inner valence (IV) band based on C2s ] ionization. It recurs as a peak in the electron distribution for double photoionization and in the quantum yield. The uppermost curve shows the spectrum of HCH02+, where the ground state is strongly populated by this effect. 

Since correlations with 2p were omitted in the calculations, the theoretical curve fails to show this rise. It is probably due to interference effects between direct double photoionization and the Auger process described in lowest order by diagram (d) of Fig. 19 with r = 2p. It would be very interesting to study this region further, both theoretically and experimentally. 

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