OH product state distribution and absolute reactive cross section for the reaction H(2.57 eV)+NO→OH+N

Abstract

Photolysis of HBr with an ArF excimer laser at 193 nm yields H-atoms with high translational energy (2.57 eV), which can react with NO to form OH and N. The nascent OH rotational and fine structure state distributions were measured by laser-induced fluorescence probing the (A-X) system of OH in the near UV around 308 nm. A high degree of rotational excitation in the OH radicals formed was found, with 45% of the available excess energy being distributed among the rotational degrees of freedom of the OH products. The A-doublet distributions show higher population of the symmetrical OH(A') component, suggesting preference of a planar reaction mechanism with in-plane rotation of the HNO intermediate. Spinorbit states Π 1/2 and Π 3/2 are found to be statistically populated. From Doppler profiles, OH translational energies were obtained and found to be in agreement with the values expected from conservation of energy and momentum. Vibrational excitation of OH radicals, barely accessible at the collision energy used in our experiments, could not be observed. With a calibration method using H 2 O 2 photolysis as a well defined source of OH radicals, an absolute reactive cross section of σ=(0.01±0.005) 2 could be determined for the reaction H+NO→OH+N at 2.57 eV collision energy.