Comparison of the Active Species in the RF and Microwave Flowing Discharges of N2 and Ar–20 %N2

We report a detailed comparison between RF and microwave (HF) plasmas of N 2 and Ar–20 %N 2 as well as in the corresponding afterglows by comparing densities of active species at nearly the same discharge conditions of tube diameter (5–6 mm), gas pressure (6–8 Torr), flow rate (0.6–1.0 slm) and applied power (50–150 W). The analysis reveals an interesting difference between the two cases; the length of the RF plasma (~25 cm) is measured to be much longer than that of HF (6 cm). This ensures a much longer residence time (10 −2 s) of the active species in the N 2 RF plasma [compared to that (10 −3 s) of HF], providing a condition for an efficient vibrational excitation of N 2 (X, v ) by (V–V) climbing-up processes, making the RF plasma more vibrationally excited than the HF one. As a result of high V–V plasma excitation in RF, the densities of the vibrationally excited N 2 (X, v > 13) molecules are higher in the RF afterglow than in the HF afterglow. Destruction of N 2 (X, v ) due to the tube wall is estimated to be very similar between the two system as can be inferred from the γ v destruction probability of N 2 (X, v > 3–13) on the tube wall (2–3 × 10 −3 for both cases) obtained from a comparison between the density of N 2 (X, v > 3–9) in the plasmas to that of the N 2 (X, v > 13) in the long afterglows. Interestingly enough, densities of N-atoms and N 2 (A) metastable molecules in the afterglow regions, however, are measured to be very similar with each other. The measured lower density of N 2 + ions than expected in the HF afterglow is rationalized from a high oxygen impurity in our HF setup since N 2 + ions are very sensitive to oxygen impurity .