Multi-electron coincidence spectroscopy: Double photoionization from molecular inner-shell orbitals

The interest of molecular double core holes was predicted in 1986 by Cederbaum et al who showed that their spectroscopy can be more informative than that of single core holes, especially when the holes are located at different sites in the molecule (Cederbaum et al 1986 J. Chem. Phys. 85 6513). Their experimental study of single photon formation had to wait until 2009-2010 with progress in synchrotron sources and the development of efficient multi-electron coincidence experiments based on a magnetic bottle time-of-flight spectrometer. At the same time the advent of x-ray free electron lasers opened the possibilty of creating them in a two-photon process, and motivated new theoretical studies of their properties. We will illustrate here the progress made recently in the field with a few examples, including the formation of double core holes by double core photoionization, their spectroscopy and decay paths, and the related process of simultaneous core ionization and core excitation.