Photoelectron spectroscopy and dissociative photoionization of fulminic acid, HCNO

We report a joint experimental and computational study of the photoelectron spectroscopy and the dissociative photoionization of fulminic acid, HCNO. The molecule is of interest to astrochemistry and astrobiology as a potential precursor of prebiotic molecules. Synchrotron radiation was used as the photon source. Dispersive photoelectron spectra were recorded from 10~eV to 22~eV, covering four band systems in the HCNO cation and an ionization energy of 10.83~eV was determined. Transitions into the Renner-Teller distorted $X^+{}^2\Pi$ state of the cation were simulated using wavepacket dynamics based on a vibronic coupling Hamiltonian. Very good agreement between experiment and theory is obtained. While the first excited state of the cation shows only a broad and unstructured spectrum, the next two higher states exhibit a well-resolved vibrational progression. Transitions into the excited electronic states of \cation{HCNO}{+} were not simulated, due to the large number of electronic states that contribute to these transitions. Nevertheless, a qualitative assignment is given, based on the character of the orbitals involved in the transitions. The dissociative photoionization was investigated by photoelectron-photoion coincidence spectroscopy. The breakdown diagram shows evidence for isomerization from \cation{HCNO}{+} to \cation{HNCO}{+} on the cationic potential energy surface. Zero Kelvin appearance energies for the daughter ions \cation{HCO}{+} and \cation{NCO}{+} have been derived.