This project will explore the physics of the double photoionization of molecules by one photon and by the non-sequential absorption of two photons. These processes add molecular many-electron correlation to the already challenging three-body Coulomb breakup problem, by placing two electrons in the continuum of a molecular ion. In recent years the atomic versions of these problems have begun to yield to new theoretical methods. The even richer physics of molecular double photoionization is now becoming accessible experimentally. A new class of momentum imaging experiments has matured over roughly the past decade. Those experiments detect the momenta of all the charged particles emerging from the double photoionization event in coincidence and reconstruct the alignment, and even the internuclear distance, of the molecule at the time of photoabsorption. Therefore the powerful new experimental ingredients for a novel kind of spectroscopy are in place, but there is insufficient theory available to understand it. The research develops the methods that will open the way to an accurate description of double photoionization of diatomic molecules, treating the many-electron aspects of the problem as well as the dynamics of the outgoing electrons in ab initio numerical calculations. The basic theoretical approach makes use of the method of exterior complex scaling (ECS), which has been successful in treating two electron systems. A hybrid basis method is proposed that combines the grid methods that have been used for ECS calculations in the past with the Gaussian basis methods of quantum chemistry used to describe the target. This technology simultaneously addresses the problems of treating two continuum electrons and the correlation of the initially and finally bound electrons of the molecular target. Non-sequential two-photon double ionization of atoms and molecules in intense fields will be treated with the same methods. A novel approach to that problem is proposed that can completely describe two photon double ionization of helium and H2. Together with these theoretical predictions, new experiments that can be performed with existing instrumentation will be able to answer many open questions about this fundamental intense field process.
