The analysis of nitrogen rotational and vibrational bands in a helium microhollow gas discharge

Abstract

Emission spectroscopy is applied to measure the gas temperature T g and the vibrational distribution of N2 (C 3Πu) and N2+(B 2Σu+) excited states from a helium microhollow gas discharge (MHGD) at atmospheric pressure. The rotational temperature T rot of N2+ is determined from relative intensity of the R-branch lines of the N2+(B 2Σu+–X 2Σg+) bands at 427.81 and 419.91 nm and the well-known Boltzmann plot (BP). Using the same diagnostic technique, the rotationally resolved N2(C 3Πu–B 3Πg) band at 380.49 nm is used to measure T rot. Under our experimental conditions, T g is equal to T rot = 550–650 K for nitrogen molecules and shows a slight increase with the discharge current in the current range 3–10 mA. From the intensity ratio of two consecutive vibrational bands of the same sequence, the N2(C 3Πu) and N2+(B 2Σu+) vibrational temperature T vib = 3,700–4,000 K is determined. It has been found that N2+(B 2Σu+) ions have non-Boltzmann distribution in the helium MHGD, while N2(C 3Πu) molecules are populated according to the Boltzmann distribution. Following the Franck–Condon principle, the vibrational distribution of the ground state of N2(X 1Σg+) molecules has been determined from the N2(C 3Πu) distribution using the inversion matrix of elements q XC(ν,ν′).