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 N 2 ( C 3 Π u ) and N 2 + ( B 2 Σ u + ) excited states from a helium microhollow gas discharge (MHGD) at atmospheric pressure. The rotational temperature T rot of N 2 + is determined from relative intensity of the R‐branch lines of the N 2 + ( B 2 Σ u + – X 2 Σ g + ) bands at 427.81 and 419.91 nm and the well‐known Boltzmann plot (BP). Using the same diagnostic technique, the rotationally resolved N 2 ( C 3 Π u – B 3 Π g ) 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 N 2 ( C 3 Π u ) and N 2 + ( B 2 Σ u + ) vibrational temperature T vib = 3,700–4,000 K is determined. It has been found that N 2 + ( B 2 Σ u + ) ions have non‐Boltzmann distribution in the helium MHGD, while N 2 ( C 3 Π u ) molecules are populated according to the Boltzmann distribution. Following the Franck–Condon principle, the vibrational distribution of the ground state of N 2 ( X 1 Σ g + ) molecules has been determined from the N 2 ( C 3 Π u ) distribution using the inversion matrix of elements q XC ( ν , ν ′).