Translational relaxation and electronic quenching of hot O(1D) by collisions with N2

Abstract

Translational relaxation and electronic quenching processes of translationally hot O(1D) atoms by collisions with N2 in a gas cell at room temperature are studied using a vacuum ultraviolet laser induced fluorescence technique. The initial hot O(1D) atoms which have translational energies of 18.2 and 9.8 kcal mol−1 are produced by the photodissociation of N2O at 193 nm and O2 at 157 nm, respectively. The translational relaxation processes are investigated by time resolved measurements of the Doppler profiles for the O(1D) atoms, while the quenching processes are studied by measuring both the decrease of the O(1D) concentration and the increase of the product O(3P) concentration after the photochemical formation of the hot O(1D) atoms. When the initial translational energy of O(1D) is 9.8 kcal mol−1, about 40% of the O(1D) atoms are electronically quenched before the entire thermalization of the hot O(1D) atoms takes place in a gaseous mixture with N2. This indicates that the translational relaxation rate of O(1D) by collisions with N2 is not fast enough compared with the electronic quenching by N2. It is found that the steady state distribution of the O(1D) translational energy in the upper stratosphere is superthermal and that the populations at high translational energies are higher than that estimated from an equilibrated condition with the ambient air. The cross section of the electronic quenching by N2 at the high collision energy of (8±6) kcal mol−1 is found to be (0.7±0.1) Å2, which is about 5 times smaller than that at thermal collision energy at 298 K.