Removing Electrons from More Than One Orbital: Direct and Indirect Ionization to Excited States of Molecular Cations

We use velocity map imaging of photoelectrons in coincidence with molecular cations to determine which ionic states are populated via strong field ionisation, and whether the ionisation to excited ionic states proceeds indirectly via the ground ionic state or directly from the neutral. We carry out measurements on a series of molecules that have different energy gaps between the ground ionic state and dissociative excited states. We measure both direct and indirect ionisation to excited states of the molecular cations, and find that the energy gap between non-dissociative and dissociative states plays an important role in determining the amount of excited state ionisation. Direct ionisation to dissociative states is generally comparable to ionization to the ground state for gap energies less than the photon energy, but is suppressed for gap energies larger than the photon energy.