Emission spectroscopy of expanding laser-induced gaseous hydrogen-nitrogen plasma

Hypersonic Imaging and Emission Spectroscopy of Hydrogen and Cyanide Following Laser-Induced Optical Breakdown

This work communicates the connection of measured shadowgraphs from optically induced air breakdown with emission spectroscopy in selected gas mixtures. Laser-induced optical breakdown is generated using 850 and 170 mJ, 6 ns pulses at a wavelength of 1064 nm, the shadowgraphs are recorded using time-delayed 5 ns pulses at a wavelength of 532 nm and a digital camera, and emission spectra are recorded for typically a dozen of discrete time-delays from optical breakdown by employing an intensified charge-coupled device. The symmetry of the breakdown event can be viewed as close-to spherical symmetry for time-delays of several 100 ns. Spectroscopic analysis explores well-above hypersonic expansion dynamics using primarily the diatomic molecule cyanide and atomic hydrogen emission spectroscopy. Analysis of the air breakdown and selected gas breakdown events permits the use of Abel inversion for inference of the expanding species distribution. Typically, species are prevalent at higher density near the hypersonically expanding shockwave, measured by tracing cyanide and a specific carbon atomic line. Overall, recorded air breakdown shadowgraphs are indicative of laser-plasma expansion in selected gas mixtures, and optical spectroscopy delivers analytical insight into plasma expansion phenomena.

Temporally and Spatially Resolved Emission Spectroscopy of Hydrogen, Cyanide and Carbon in Laser-Induced Plasma

In this study, we examine the atomic and molecular signatures in laser-induced plasma. Abel inversions of measured line-of-sight data reveal insight into the radial plasma distribution. Laser-plasma is generated with 6 ns, Q-switched Nd:YAG radiation with energies in the range of 100 to 800 mJ. Temporally- and spatially-resolved emission spectroscopy investigates expansion dynamics. Specific interests include atomic hydrogen (H) and cyanide (CN). Atomic hydrogen spectra indicate axisymmetric shell structures and isentropic expansion of the plasma kernel. The recombination radiation of CN emanates within the first 100 nanoseconds for laser-induced breakdown in a 1:1 mole ratio CO2:N2 gas mixture. CN excitation temperatures are determined from fitting recorded and computed spectra. Chemical equilibrium mole fractions of CN are computed for air and the CO2:N2 gas mixture. Measurements utilize a 0.64-m Czerny–Turner type spectrometer and an intensified charge-coupled device.