An evaluation of equilibrium conditions and temperature-dependent speciation in a laser-produced air plasma

Abstract

A laser-produced air plasma is a dynamical system with fundamental physical parameters that change significantly during its lifespan. The evolution of the spectral features from an air plasma shows broadband radiation at early times followed by discrete electronic transitions from ions, atoms, and rotational and vibration bands from molecules. The molecular band emission from an air plasma typically appears at times ⪞5 μs and persists for 100's of μs. In this article, an evaluation is made on the temperature evolution and speciation in a laser-produced air plasma. The air plasmas are generated by focusing 1064 nm, 6 ns pulses from an Nd:YAG laser. High-resolution emission spectra of various molecules such as N2, N2+, CN, OH, NH, and NO are acquired and fit using spectral models. Fitting of atomic and molecular emission features permits tracking of the air spark temperature evolution from 1 to 200 μs. Though the excitation and molecular temperatures show a good overlap at times ⪝10 μs, the molecular temperatures obtained from different species show a discontinuity at times ∼30 μs and this is related to shock collapse and subsequent changes in hydrodynamics and chemistry of the plume. The fitting of multiple species in broadband spectra has permitted calculation of the relative concentrations of various molecules as a function of temperature that provides insight into the air spark speciation. The measured relative mole fractions from laser-induced air plasma are within factors of 2–6 of the values estimated by speciation model under the assumption of thermal and chemical equilibrium across the temperature ranges studied. Probable reasons for the observed deviation in the relative fractions are discussed.