A diagnostic method for real-time measurements of the density of nitrogen atoms in the postglow of an Ar–N₂ discharge using a catalytic probe

We determined the density of neutral nitrogen atoms in an Ar–N₂ postglow using a fiber-optics catalytic probe. The probe, which had a catalyst made of nitrided iron, was calibrated with a NO titration. The recombination coefficient for the heterogeneous recombination of N atoms on the nitrided iron surface was determined by comparing the probe signal with the NO titration. Within the limits of experimental error the coefficient was found to be independent of the catalyst temperature between 400 and 650 K and had a value of 0.21±0.04. Real-time measurements of the N-atom density were performed at a nitrogen flow of 600 SCCM (standard cubic centimeter per minute) for several discharge powers between 80 and 300 W, and for argon flow rates between 200 and 3000 SCCM. With increasing discharge power the N-atom density increased monotonously; with increasing Ar flow the N-atom density at first increased, reaching a broad maximum at about 1.8×10²¹ m^-3 for an Ar flow of 2000 SCCM, after which the N-atom density decreased with any further increase of Ar flow.