Photofragment Imaging of HNCO Decomposition at 210 nm: the Primary NH(a1)+CO(X1+) Channel

Abstract

The photodissociation of isocyanic acid (HNCO) on the first excited singlet state following the excitation at 210 nm was investigated with an ion velocity slice imaging technique by probing the CO fragment. It was found from the (2+1) resonance-enhanced multi-photon ionization (REMPI) spectrum that the CO fragments are rotationally hot with population up to Jmax=50. The velocity imagings of the CO fragments at JCO=30 and 35 indicate that formation of NH(a1)+CO(X1+, v=0) is the predominant dissociation channel at 210 nm. From analysis of the CO fragment translational energy distributions, the NH(a1) fragment was observed to be rotationally cold, about half of the available energy was partitioned into the translational motion of fragments after dissociation, and the NH(a1)+CO(X1+) dissociation threshold was determined at 4273830 cm1. From analysis of the CO fragment angular distributions, the dissociation anisotropy parameter was found to be negative, and increasing with the rotational quantum number of the NH fragment, i.e., from -0.75 at JNH=2-4 to -0.17 at JNH=11. Impulsive direct and vertical dissociation process of HNCO on the singlet state at 210 nm was confirmed experimentally. A classical impact dissociation model was employed to explain the dependence of the value on the rotational excitation of the NH fragment.