Investigation of a nitrogen post-discharge of an atmospheric-pressure microwave plasma torch by optical emission spectroscopy

The species, N2(C), N2(B), and N2+(B), in the post-discharge of a nitrogen microwave induced discharge (2.45 GHz) at atmospheric pressure are investigated by means of optical emission diagnosis of the spatial distribution of emission intensities of N2(C-B), N2(B-A), and N2+(B-X) transitions. Correspondingly, the post-discharge can be divided into two distinct regimes, the early and late afterglows. It is found that not only atomic N survives in the late afterglow regime of the post-discharge but also the N2+ ions are produced even far from the microwave launcher. This is attributed to the fact that the vibrationally excited N2(X, υ) and ground state N(4S) with a long lifetime can be conveyed at far distance and act as the precursor for generating N2+ ions locally.