Laser-Plasma Spatiotemporal Cyanide Spectroscopy and Applications

Abstract

This article reports new measurements of laser-induced plasma hypersonic expansion measurements of diatomic molecular cyanide (CN). Focused, high-peak-power 1064 nm Q-switched radiation of the order of 1 TW/cm 2 generated optical breakdown plasma in a cell containing a 1:1 molar gas mixture of N 2 and CO 2 at a fixed pressure of 1.1 × 10 5 Pascal and in a 100 mL/min flow of the mixture. Line-of-sight (LOS) analysis of recorded molecular spectra indicated the outgoing shockwave at expansion speeds well in excess of Mach 5. Spectra of atomic carbon confirmed increased electron density near the shockwave, and, equally, molecular CN spectra revealed higher excitation temperature near the shockwave. Results were consistent with corresponding high-speed shadowgraphs obtained by visualization with an effective shutter speed of 5 nanoseconds. In addition, LOS analysis and the application of integral inversion techniques allow inferences about the spatiotemporal plasma distribution.

Laser-Plasma Spectroscopy of Hydroxyl with Applications

“…However, simultaneous recording of shadowgraphs and spectra would be necessary for establishment of expansion characteristics following laser-induced breakdown in air. Such an association of shadowgraphs for time-delays of the order of 1 µs has been reported recently using CN emission spectroscopy [91]. Radon inverse transforms or computed tomography would appear necessary, but possibly Abel inverse transforms may be sufficient as one realizes the symmetry of the vortex indicated in Fig.…”

“…Standard experimental components are used for laser-induced breakdown spectroscopy, summarized previously, but are included for completeness, e.g., see Ref. [91] or for general laser-induced breakdown spectroscopy Refs. [92,93].…”

“…However, it is important to obtain shadowgraphs for plasma excitation energies that were employed for time-resolved spectroscopy. Shadowgraphs reported recently [91] are captured using two separate laser devices (Continuum Surelite model SL I-10, USA) that can be externally operated to deliver laser pulses with a well-defined time delay showing less than ± 1 ns trigger-jitter between the pulses. The experiments reported in Ref.…”

“…The experiments reported in Ref. [91] are extended to time delays of the order of 100 µs for the purpose of visualizing the breakdown site for time delays that correspond to reasonable OH signals.…”

“…Time-resolved spectroscopy of laser-induced plasma in laboratory air at a humidity level of 25% reveal characteristic uv OH data due to molecular recombination. Optical breakdown was generated at a rate of 10 Hz, with the laser beam focused with f/5 optics from the top, or parallel to the slit, analogous to recently reported CN laser spectroscopy [91]. The detector pixels are binned in 4-pixel tracks along the slit direction, resulting in obtaining 256 spectra for each time delay.…”