Fast beam studies of NCO free radical photodissociation

The spectroscopy and dissociation dynamics of the NCO radical have been investigated by applying fast radical beam photodissociation spectroscopy to the B̃ 2Π←X̃ 2 Π electronic transition. Measurements of the photodissociation cross section as a function of dissociation wavelength show that even the lowest vibrational levels of the B̃ 2Π state predissociate. Analysis of fragment kinetic energy release reveals that the spin-forbidden N(4S)+CO(1Σ+) products are produced exclusively until 20.3 kcal/mol above the origin, at which point, the spin-allowed N(2D)+CO product channel becomes energetically accessible. The spin-allowed channel dominates above this threshold. By determining the location of this threshold, we obtain a new ΔHf0 for NCO of 30.5±1 kcal/mol, several kcal/mol lower than the previously accepted value.