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

Abstract

Strong emission bands originating from $\mathrm{NCO}{A}^2\Sigma$ and $\mathrm{NH}{\mathrm{(A}}^3{\mathrm{\pi ,\; c}}^1\mathrm{\pi )}$ were found in the photodissociation of HNCO in the vacuum ultraviolet. In addition, weak bands coming from $\mathrm{NCO}{B}^2\Pi$ 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: $\mathrm{D(}\mathrm{H}–\mathrm{NCO}\text{)\hspace{0.17em}=\hspace{0.17em}4.90\hspace{0.17em}±\hspace{0.17em}0.01}\mathrm{eV}\mathrm{,\; D(}\mathrm{HN}–\mathrm{CO}\text{)\hspace{0.17em}=\hspace{0.17em}3.38\hspace{0.17em}±\hspace{0.17em}0.1}\mathrm{eV}\mathrm{,\; D(}\mathrm{N}–\mathrm{CO}\text{)\hspace{0.17em}=\hspace{0.17em}2.14\hspace{0.17em}±\hspace{0.17em}0.15}\mathrm{eV}{\mathrm{,\; \Delta H}}_f(\mathrm{HNCO}\text{)\hspace{0.17em}=\hspace{0.17em}−\hspace{0.17em}1.0\hspace{0.17em}±\hspace{0.17em}0.14}\mathrm{eV}{\mathrm{,\; \Delta H}}_f(\mathrm{NCO}\text{)\hspace{0.17em}=\hspace{0.17em}1.6\hspace{0.17em}±\hspace{0.17em}0.15}\mathrm{eV}$ . The emission spectrum, $\mathrm{NCO}{A}^2{\mathrm{\Sigma \hspace{0.17em}\to \hspace{0.17em}X}}^2\pi$ , produced from the photolysis at the Xe and Kr lines shows that $\mathrm{NCO}{A}^2\Sigma$ is highly excited in bending vibration, indicating that the upper state of HNCO from which $\mathrm{NCO}{A}^2\Sigma$ predissociates has a bent NCO configuration. $\mathrm{NH}{c}^1\Pi$ formed is highly rotationally excited. Experimental evidence is presented to show that $\mathrm{NH}{A}^3\pi$ is produced primarily by a secondary process, $\mathrm{CO}{a}^3\mathrm{\pi \hspace{0.17em}+\hspace{0.17em}}\mathrm{NH}{X}^3\mathrm{\Sigma \hspace{0.17em}\to \hspace{0.17em}}\mathrm{CO}{X}^1\mathrm{\Sigma \hspace{0.17em}+\hspace{0.17em}}\mathrm{NH}{A}^3\pi$ . Primary processes in the near‐ultraviolet photolysis of HNCO are discussed on the basis of bond dissociation energies obtained in the present work.