Photoionization direct double ionization

There are other electron emission processes which lie between such well-defined limiting cases, e.g., resonance affected two-electron emission which lies between direct double photoionization and photon-induced two-step double ionization (photoelectron and Auger electron emission). 

All the previous reviews [3-9] have been concerned mainly with what is called direct double photoionization in atoms. This is a process in which a single photon ejects two electrons simultaneously, without the intervention of any real intermediate state of either the neutral molecule or the singly ionized cation. It can be written... 

Its chief characteristic is that the excess energy of the photon above the double ionization threshold is shared as a continuous monotonic distribution between the two outgoing electrons. This direct double photoionization is contrasted with several indirect processes, of which by far the most important can be written... 

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. 

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