  Double and triple ionization spectra of isocyanic acid have been measured
using multi-electron and ion coincidence techniques combined with synchrotron
radiation and compared with high-level theoretical calculations. Vertical
double ionization at an energy of 32.8+/-0.3 eV forms the 3A'' ground state in
which the HNCO2+ ion is long-lived. The vertical triple ionization energy is
determined as 65+/-1 eV. The core-valence double ionization spectra resemble
the valence photoelectron spectrum in form, and their main features can be
understood on the basis of a simple and rather widely applicable Coulomb model
based on the characteristics of the molecular orbitals from which electrons are
removed. Characteristics of the most important dissociation channels are
examined and discussed.
