Photodissociation of HNCO in the Vacuum Ultraviolet; Production of NCO A 2Σ and NH(A 3π, πc1)

Abstract

Strong emission bands originating from NCO A2Σ and NH (A 3π, c 1π) were found in the photodissociation of HNCO in the vacuum ultraviolet. In addition, weak bands coming from NCO B 2Π were observed. Threshold energies of incident photons to produce these bands were measured, from which the following bond dissociation energies and heats of formation were obtained: D(H–NCO) = 4.90 ± 0.01 eV, D(HN–CO) = 3.38 ± 0.1 eV, D(N–CO) = 2.14 ± 0.15 eV, ΔHf(HNCO) = − 1.0 ± 0.14 eV, ΔHf(NCO) = 1.6 ± 0.15 eV. The emission spectrum, NCO A 2Σ → X 2π, produced from the photolysis at the Xe and Kr lines shows that NCO A 2Σ is highly excited in bending vibration, indicating that the upper state of HNCO from which NCO A 2Σ predissociates has a bent NCO configuration. NH c1Π formed is highly rotationally excited. Experimental evidence is presented to show that NH A 3π is produced primarily by a secondary process, CO a 3π + NH X 3Σ → CO X 1Σ + NH A 3π. Primary processes in the near-ultraviolet photolysis of HNCO are discussed on the basis of bond dissociation energies obtained in the present work.