Threshold photoelectron spectroscopy of unstable N-containing compounds: Resolution of ΔK subbands in HNCO⁺ and vibrational resolution in NCO⁺

The threshold photoelectron spectra (TPES) of two unstable nitrogen-containing species, HNCO and NCO, were recorded utilizing vacuum ultraviolet synchrotron radiation. Both are intermediates in combustion processes and play a role in the removal of nitrogen oxides from exhaust gases. The rovibronic structure of the first band in the TPES of HNCO⁺ was analyzed within the framework of an orbital ionization model, and the resolved structure of the origin band was assigned to ΔK subbands. An ionization energy of 11.602 ± 0.005 eV was determined and the vibrational structure of the cationic ground state was analyzed by a Franck-Condon fit. Low lying electronically excited states of HNCO⁺ were also observed. In a second series of experiments, the NCO radical was generated by flash pyrolysis from chlorine isocyanate. The ionization energy to the X^{+ 3}Σ⁻ ground state was determined to be 11.76 ± 0.02 eV, while for the a⁺¹Δ state, a value of 12.93 ± 0.02 eV was obtained. Vibrational structure was observed for both states, and bands were assigned by Franck-Condon simulations