Anionic photofragmentation of CO: A selective probe of core-level resonances

Anion-yield spectroscopy using x rays is shown to be a selective probe of m olecular core-level processes, providing unique experimental verification o f shape resonances. For CO, partial anion and cation yields are presented f or photon energies near the C K edge. The O- yield exhibits features above threshold related only to doubly excited states, in contrast to cation yiel ds which also exhibit pronounced structure due to the well-known sigma* sha pe resonance. Because the shape resonance is completely suppressed for O-, anion spectroscopy thus constitutes a highly selective probe, yielding info rmation unobtainable with absorption or electron spectroscopy.