NH and CN radical emission from corona post discharge region in high density nitrogen plasma

Abstract

The corona post discharge region in high-density nitrogen plasma has been investigated. We observed the emission band (0-0) of NH located at 336 nm corresponding to the transition A3Π-X3Π− which is particularly intense at low pressure. The possibility would be that NH is formed from a radiative recombination between a hydrogenated compound present in trace amounts (as CmHn) and nitrogen. Outside the (0-0) NH band, only the band (1-1) at 337.00 nm (i.e. 1 nm from the previous one) was detected. Since its transmission probability is lower than that of the band (0-0) and is located very close to the intense 2S+ band (0-0) (337.13 nm), detection is not possible. A very weak emission of the CN violet system ((B2Σ+-X2Σ) has been identified in the discharge zone with excitation threshold in the order of 3.37 eV. The most intense bands are between 385 nm and 388.34 nm. This radical can only come from a reaction in the discharge between the nitrogen and an organic compound of the CnHm type. Although the purification system is very sophisticated, traces of hydrocarbons can be introduced either by contamination by oil vapours from the pumping system or by their presence in the starting gas (0.01 ppm). The radical CN is observed only with a discharge in negative polarity.