Electronic state dependence in dissociation of core-excited water

Water (H2O) is the most studied molecule in the history of science. Yet many things remain unknown about its nuclear motion and decay. In recent years some studies have been made on the behavior of core-excited water molecules and its subsequent Auger decay and dissociation. Particular attention has been drawn to the ultra-fast dissociation channel H2O* -> H + O*H, which occurs on a femtosecond time-scale in the H2O*(O1s^(-1)4a1^1) core-excited state. This study concerns the outcome of an electron energy resolved electron--ion coincidence (PEPICO) measurement on core-excited water . The experiments were carried out at the I411 beamline at the MAX II synchrotron in Lund. A setup consisting of a time-of-flight ion spectrometer and an electron energy analyser was used to measure the energy of Auger electrons in coincidence with the resulting ion fragments from the decay. Although this method implies intriguing features, issues concerning false coincidences and detector resolution leave room for improvement. The report discusses possible improvements of the experimental method and data analysis. We verify that ultra-fast dissociation channels exist, and we are able to directly observe the associated Auger decay O*H -> OH^+ + e^- in coincidence with the OH+ ionic fragments. In the same Auger electron energy region we study the spectator Auger decay to anti-bonding H2O+ electronic states. We find a state-specific effect where the intermediate state H2O*(O1s^(-1)2b2^1) has a strong preference for decay to states dissociating to OH + H+ , whereas H2O*(O1s^(-1)4a1^1).