Influence of mercury traces on nitrogen post-discharge kinetics

Abstract

This paper presents results obtained during spectroscopic observations of dc flowing post-discharges of a pure nitrogen plasma and a nitrogen plasma containing traces of mercury. The plasmas were studied by emission spectroscopy in the range of 300–850 nm where three nitrogen spectral systems were identified: NO^β bands were observed just above the noise level at the latest decay times over 35 ms. The discharge and post-discharge were studied in nitrogen with mercury traces at about 3.7 ppb. The total gas pressure was 700 Pa and the discharge current was kept at 150 mA. The relative populations of N₂ (B ³Π_g), N₂ (C ³Π_u) and N₂⁺ (B ²Σ⁺_u) states were calculated by the dependence on the post-discharge time in the range 3–40 ms. As the uncontrolled oxygen traces were recorded in the mercury flow, the experiment with a nitrogen–oxygen mixture at the same discharge conditions was added. The results showed strong quenching of the nitrogen pink afterglow by mercury traces. The kinetic explanation can be made through a strong depopulation of the level N₂ (X ¹Σ⁺_g, v = 19) (creating the mercury ³P₁ state) and of the levels N₂ (X ¹Σ⁺_g, v = 28) or N₂ (A ³Σ⁺_u, v = 4) (creating the mercury ¹S₀ state). Both mercury states are origins of the UV light emission (254 and 185 nm, respectively) which could not be detected in our experimental setup.