Photodissociation of nitric acid and water in the vacuum ultraviolet; vibrational and rotational distributions of OH 2Σ+

Abstract

The absorption cross sections of nitric acid have been measured in the 1100–1900 Å region. The process HONO2 → OH(2Σ)+NO2 occurs below 1475 Å, much shorter than the thermochemical threshold at 2040 Å. The OH(2Σ) fluorescence yield is less than 2%. The vibrational and rotational distributions of OH(2Σ) from HONO2 photolysis at 1236 Å have been measured and compared with those from H2O and H2O2 photolysis. The excess energy beyond that required to dissociate the molecule and to excite ground state OH to OH(2Σ) is converted to rotation and much less to vibration of OH(2Σ) in contrast with linear cyanogen molecules where the excess energy appears as vibration and much less as rotation of a CN product. The results of internal energy partitioning have been compared with calculations based on a simple quasidiatomic impulsive model. The deviation from the model is attributed to either a process involving a large change in bond angle or in bond length. The rotational distributions of OH(2Σ) at v′=0 show a narrow peak at N′=20 for H2O photolysis and a broad peak at N′=10 for HONO2 photolysis both at 1236 Å. The extent of rotational excitation is expressed in terms of an impact parameter. The large impact parameter is found for H2O and H2O2 photolysis while for HONO2 photolysis the impact parameter is small. The rotational distributions of OH(2Σ) from H2O and H2O2 photolysis at 1236 Å deviate completely from Boltzmann behavior while that from HONO2 approaches the Boltzmann distribution. The process to yield the electronically excited NO2 from HONO2 photolysis at 1236 Å is less than 0.5%.