Time-resolved spectroscopy measurements of hydrogen-alpha, -beta, and -gamma emissions

Abstract

Hydrogen emission spectroscopy results are reported following laser-induced optical breakdown with infrared Nd:YAG laser radiation focused into a pulsed methane flow. Measurements of Stark-broadened atomic hydrogen-alpha, -beta, and -gamma lines show electron number densities of 0.3 to 4x10(17) cm(-3) for time delays of 2.1 to 0.4 micros after laser-induced optical breakdown. In methane flow, recombination molecular spectra of the Delta nu = +2 progression of the C(2) Swan system are discernable in the H(beta) and H(gamma) plasma emissions within the first few microseconds. The recorded atomic spectra indicate the occurrence of hydrogen self-absorption for pulsed CH(4) flow pressures of 2.7x10(5) Pa (25 psig) and 6.5x10(5) Pa (80 psig).

Atomic and molecular emissions in laser-induced breakdown spectroscopy

“…Applications of Stark-broadening include H-alpha, H-beta, and H-gamma (H α : 656.28 nm, H β : 486.14 nm, H γ : 434.05 nm) diagnostic of LIB in methane [74][75][76]. Time resolved spectroscopy measurements of hydrogen-alpha, -beta, and -gamma emissions were performed in an expanding flow of methane gas.…”

“…Hydrogen Stark widths were used to characterize the plasma decay during the first 2 microseconds after laser-induced optical breakdown [74]. Analysis of the recorded hydrogen spectra include measurements of full-width-half-maximum (FWHM) and full-width-half-area (FWHA) to infer electron number density N e [76].…”

“…Laser-induced micro-plasma that contains carbon usually shows spectroscopic signatures of the diatomic molecular Swan system [22,23,74]. For the LIB experiments in expanding methane, these molecular C 2 "fingerprints" are discernible for time delays on the order of 1 to 2 μs, see Fig.…”

“…Noteworthy is a recent study [71] of measured hydrogen Balmer lines and detailed elaboration on theory of Stark broadening, namely the Griem, Oks, and Gigosos et al theories, including further discussion of hydrogen H α line self-absorption in laser-induced plasma. Laser-induced breakdown of aluminum [72,73] and applications of H-alpha, H-beta, and H-gamma (H α : 656.28 nm, H β : 486.14 nm, H γ : 434.05 nm) diagnostic of LIB in methane [74][75][76] are also discussed. Recent work on hydrogen-beta further elaborates details of the Stark-broadened emissions, including asymmetries in the double-peak H β spectra [77].…”

Asymmetric hydrogen beta electron density diagnostics of laser-induced plasma

“…The laser beam composed of these pulses, generated at a repetition rate of 10 Hz, 25 is aligned parallel to the spectrometer slit by way of three mirrors. A quartz lens focuses the beam into a spot size of 26 about 50 µm in diameter, and laser-induced optical breakdown is accomplished with a peak irradiance of 0.5 to 0.75 27 TW/cm 2 . The resulting plasma is imaged onto the spectrometer slit with a 1:1 magnification by two quartz lenses 28 where the last lens optically couples with the spectrometer's f # of 5.2.…”