Relaxation Processes of Translationally Hot O( 1 D) by Collisions with O 2

The collisions of translationally hot O( 1 D) with O 2 result in two processes, translational energy relaxation and electronic quenching to O( 3 P). These two processes were studied in a gas cell at room temperature using the vacuum ultraviolet laser-induced fluorescence technique. The initial hot O( 1 D) atoms were produced by the photodissociation of N 2 O at 193 nm, which have average translational energies of 18.1 kcal mol -1 in the laboratory frame. Time-resolved measurements of the Doppler profiles for the hot O( 1 D) atoms revealed the translational energy relaxation process, whereas the quenching process was investigated by measuring both the decrease of the O( 1 D) concentration and the increase of the product O( 3 P) concentration at various delay times after the photochemical formation of the hot O( 1 D) atoms. From the simulation employing an elastic hard-sphere collision model with a Monte Carlo method, the hard-sphere diameter for the translational energy relaxation process of hot O( 1 D) by collisions with O 2 was found to be 2.5 ± 0.2 Å. The cross section of the electronic quenching of O( 1 D) by O 2 at the high collision energy of 8.7 ± 6 kcal mol -1 was found to be 3.3 ± 0.7 Å 2, which is a little smaller than that at the thermal collision energy at 298 K. The observed collision energy dependence is explained by a centrifugal barrier on the entrance attractive potential surface of the quenching reaction.