Double and Triple Ionisation of Isocyanic Acid

Because of the importance of isocyanic acid, HNCO, in terrestrial environments1-3 and the interstellar medium4 , spectra of this molecule and its singly charged ion HNCO+ have been widely studied both experimentally and theoretically. A rather complete listing of the earlier spectroscopic work is given in a recent paper by Holzmeier et al. 5 on its normal and resonant Auger spectra. Te electronic and geometric structure of neutral HNCO and of its three most stable isomers have been calculated6,7 , as has the structure of the singly positive ion8,9 . Te dynamics of fragmentation of its positive ions, both singly and doubly charged, were studied ~30 years ago by mass-spectrometric methods10,11 and more recently by coincidence methods8,12. Te states of the singly charged ions, seen in the photoelectron spectrum, could be correlated to the dissociation pathways8 , but this was not possible for the doubly charged ions12 as no spectroscopic information on them existed at that time. We now report spectra of the double and triple ionisation of HNCO by single photon impact, obtained using a multi-electron coincidence technique combined with synchrotron radiation in the sof X-ray region. Te coincidence techniques used in the present work involve the detection and energy analysis of all the electrons or electrons and ions emitted in encounters between single high energy photons and single gas-phase molecules. Tey rely mainly on the use of a magnetic-bottle time-of-fight analyser13 for the electrons, and monochromatic photons from an electron storage ring or laboratory light source for ionisation.