Positive-Ion/Negative-Ion Coincidence (PINICO) Experiments on Water

Abstract: A new setup for versatile ion time-of-flight coincidence spectroscopy was built using a pre-existing negative ion time-of-flight spectrometer aligned with a short extension that may be used to detect photons, Auger electrons or positive ions. The extension was designed to be short to improve detection of photons, as they do not have a preferred direction for their emission, and high-momentum particles, which would miss the detector if it were placed farther from the interaction region. This new setup was tested for Positive-Ion/Negative-Ion Coincidence experiments near the O1s absorption edge of water. Positive ion detection was used as the start signal, due to the shorter time of flight in the short extension than in the negative ion time-of-flight spectrometer. The improvement of the setup in terms of high-momentum positive ions detection comes at the expense of resolution, and the use of positive ions as the start signal results in the absence of positive ion/electron coincidences in the spectrum. The O+/H-, O++/H- and the H+/H- coincidence peaks were observed, and identified as real peaks via the change in their position when the applied voltages in the experimental setup were varied. Their attribution was made so that they are consistent with one another for their intensity and position. The ion pair yields were then studied and used to discuss the dissociation pathways of water after O1s core ionization. The three dominant photoexcitation peaks of water (4a1 2b2 and 2b1) and the above threshold regime past 539.9eV were visible for each of the ion pairs. The high acceptance of the short extension enabled the detection of a larger number of H+ ions, resulting in clearly visible resonances above threshold, which can be attributed to simultaneous excitation of a O1s core electron and a valence electron. The H+/O- yield showed a lower energy resonance with a double peak structure that was not visible in the resonances in the yield of the other ion pairs at higher energy.