Direct double photoionization molecules

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... 

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. 

If pairwise electron correlation is really an important progenitor of direct two-electron photoionization, as the usual introductory statements imply [3-9], it should be interesting to discover experimentally which particular pairs of electrons are removed most effectively in direct double photoionization transitions. Is removal of electrons from the same shell or the same orbital favored relative to removal of electrons from different shell or orbitals How about electrons of the same spin relative to pairs of different spin The TOF-PEPECO method, with its lack of bias on the basis of energy or emission angle, is ideally suited to make such comparisons and thus in principle to contribute direct measurements of electron correlation. Other known factors thought to affect transition strengths such as photon energy or electron pair symmetry should be allowed for, and direct photoionization must be isolated. So far, there are very few examples where this can be done, so the jury is still out. One possibly indicative observation is that in double photoionization of small molecules with a full 71-orbital, the lA term is always the most intense (e.g., HBr, Fig. 18, OCS, Fig. 26). To form 1 A, two electrons must be removed from exactly the same spatial orbital. The same intensity pattern is found in every full n 2 ionization, and it may be remembered that for small molecules, the process is mainly direct. 

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