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We have used complete correlated momentum mapping of the photoelectron and heavy ion products from the dissociation of the di-cation of acetylene, induced by photoionizing the carbon K shell of one of the atoms, to map out the angular correlation between the electron and the axis of the target molecule. The (quasi-) symmetric decay is found to proceed through both acetylene and vinylidene configurations. By using the strongly peaked photoelectron emission to "start a clock," an upper limit of 60 fs is placed on the isomerization time from the acetylene to the vinylidene configuration.

We present kinematically complete measurements of the photo double ionization of ethylene (double CC bond) and acetylene (triple CC bond) hydrocarbons just above the double ionization threshold. We discuss the results in terms of the coincident kinetic energy of the photo electrons and the nuclear kinetic energy release of the recoiling ions. We have incorporated quantum chemistry calculations to interpret which of the electronic states of the dication have been populated and trace the various subsequent fragmentation channels. We suggest pathways that involve the electronic ground and excited states of the precursor ethylene dication and explore the strong influence of the conical intersections between the different electronic states. The nondissociative ionization yield is small in ethylene and high in acetylene when compared with the dissociative ionization channels.The reason for such a striking difference is explained in part on the basis of a propensity rule which influences the population of states in the photo double ionization of a centrosymmetric closed shell molecule by favoring singlet ungerade and triplet gerade final states. This propensity rule and the calculated potential energy surfaces clarify a picture of the dynamics leading to the observed dication dissociation products.

We have used cold target recoil ion momentum spectroscopy to study the continuum correlation between the photoelectron of core-photoionized neon and the subsequent Auger electron. We observe a strong angular correlation between the two electrons. Classical trajectory Monte Carlo calculations agree quite well with the photoelectron energy distribution that is shifted due to the potential change associated with Auger decay. However, a striking discrepancy results in the distribution of the relative angle between Auger and photoelectron. The classical model predicts a shift in photoelectron flux away from the Auger emission direction, and the data strikingly reveal that the flux is lost rather than diverted, indicating that the two-step interpretation of photoionization followed by Auger emission is insufficient to fully describe the core-photoionization process.

We present experimental measurements and theoretical calculations for the photoionization of CH 4 at the carbon K-edge. Measurements performed using cold target recoil ion momentum spectroscopy (COLTRIMS) combined with complex Kohn variational calculations of the photoelectron in the molecular frame demonstrate the surprising result that the low energy photoelectrons effectively image the molecule by emerging along the bond axes. Furthermore, we observe a dynamic breakdown of axial recoil behaviour in one of the dissociation pathways of the intermediate dication, which we interpret using electronic structure calculations.