The ultrasensitivity of the cavity ring down spectroscopy technique has been used to detect the minor CN species in a premixed rich (φ=1.2) methane/air flame stabilized at 53<space>hPa. The CN radical has been probed around 387<space>nm on the B-X (0-0) band. A<space>peak concentration of 4.7.10 9 <space>cm - 3 has been measured which corresponds to a mole fraction of 0.022<space>ppm. A<space>detection limit of a few ppb has been reached despite the presence of an important off-resonance background due to the flame.
L'extreme sensibilite de la technique d'absorption 'cavity ring-down spectroscopy' a ete mise a profit pour detecter l'espece minoritaire CN formee dans une flamme de premelange methane/air de richesse 1,2<space>stabilisee a 53<space>hPa. Les experiences ont ete realisees sur la bande B-X (0-0) du radical CN autour de 387<space>nm. La concentration maximale mesuree est de 4,7.10 9 <space>cm - 3 , ce qui correspond a une fraction molaire de 0,022<space>ppm. Une detection limite de l'ordre de quelques ppb a ete atteinte malgre la perturbation induite par des phenomenes non resonnants dus a la flamme.