Time-resolved spectra and measurements of temperature and electron density of laser induced nitrogen plasma

Abstract

As an important analytical tool, laser-induced breakdown spectroscopy (LIBS) has been widely used in material analysis, environmental monitoring, and other fields. In recent years, due to increasingly serious air pollution, various LIBS-based on-line air pollution detection techniques are being developed. The temporal evolution of nitrogen plasma characteristics is of great importance for investigating the atmospheric plasma dynamics and developing the LIBS-based air pollution monitoring techniques. Temperature and electron density, which are the most important parameters of a plasma state, directly influence the kinetic behaviors of plasma formation, expansion and degradation processes, as well as the energy transfer efficiency in plasma. In this paper, the temporal evolutions of continuous background radiation, molecular spectral strength, and signal-to-background ratio (SBR) are studied based on time-resolved spectra. The results show that the lifetime of the continuous background radiation is about 700 ns, the N2+(B2u+-X2g+, v: 0-0) transition line strength reaches a maximum value within 12-15 s, the SBR first increases and then stabilizes. Accordingly, the optimal observation period for N2+(B2u+-X2g+) band system based plasma temperature investigation should be selected to be between 10 and 25 s. The temporal evolution of plasma temperature is determined by fitting experimental spectra to theoretical ones simulated by LIFBASE (a spectral simulation program). As the radiation loss is less than the loss due to the collision cooling, the plasma temperature decays exponentially from ~10000 K to ~6000 K within 10-28 s. By taking into account the instrumental broadening lineshape (Voigt lineshape), the temporal evolutions of Stark broadening and Stark shift of N 746.831 nm atomic line are obtained via Nelder-Mead simplex algorithm, and then the electron density is calculated accordingly. The results show that the electron density decays between 1017 and 1016 cm-3 in magnitude. By comparing the experimental electron decay rate with theoretical values calculated from different mechanisms, it is concluded that a three-body collision recombination is the main mechanism of electron decay.