Distributions of rovibrational states of secondary product NO X 2Π from photodissociation of nitric acid at 193 nm

Abstract

The distributions of vibrational, rotational, spin–orbit and lambda–doublet states of nascent NO are obtained using the A 2Σ–X 2Π transition via the technique of laser-induced fluorescence. NO is produced mainly from secondary dissociation of fragment NO2 from nitric acid after photolysis at 193 nm. According to the measured distribution the partition of energy in NO X 2Π is calculated to be ∼840 cm−1 in vibration and ∼1410 cm−1 in rotation. The spin–orbit state Π21/2 of NO is populated about twice of that in Π23/2 for the vibrational levels υ=0, 1, and 2. The measured populations of vibrational states υ=0, 1, and 2 are 0.75, 0.19, and 0.07, respectively; these populations agree with results calculated with the statistical model, prior theory. The distributions of rotational states of NO X 2Π for these three vibrational levels have single maxima of Gaussian shape. The experimental data indicate that although NO2, produced photochemically from nitric acid, was proposed to be in an electronically excited state, the state distributions of NO from unstable NO2 agree with those from NO2 excited with monochromatic light in the UV-vis range after averaging for a broad distribution of internal energy. In the UV-vis range of excitation the NO fragment dissociates from the ground electronic surface of NO2. Hence, this unknown electronic state of NO2 is expected to be coupled to the ground electronic surface then leading to dissociation.