RÉSUMÉ

The dissociation degree of molecular oxygen in air is measured on the axis of a plasma arcjet thanks to the actinometry technique. For this purpose, a small amount of argon is introduced into the gas of interest, and two emission lines, one of atomic oxygen (777.3 nm) and one of argon (811.5 nm), are analyzed locally, together with the corresponding electron density and energy distribution. Then, provided that a number of assumptions are satisfied, the dissociation degree of oxygen is proportional to the ratio of the line intensities. It is found to decrease from 1 at the nozzle exit down to 0.3 at a distance of 10 cm and increase again where a shock wave is expected.