David H. Plemmons

Balmer series H β measurements in a laser-induced hydrogen plasma

Stark-broadened emission profiles of the Balmer series Hbeta lines are measured subsequent to nanosecond laser-induced optical breakdown in gaseous hydrogen. Electron number densities are found from time-resolved spectra from Hbeta emissions to be in the range 10(15)-10(18) cm(-3). These results are compared with Halpha measurements for which number densities as high as 10(19) cm(-3) are determined from Stark widths and Stark shifts. Good agreement is reported for number densities inferred from Halpha and Hbeta emissions, down to an electron number density 3 x 10(16) cm(-3), by accurate treatment of ion dynamics in the theory.

Spectroscopic temperature measurements in a decaying laser-induced plasma using the C2 Swan system

Abstract Spontaneous emission spectra of C2 Swan bands were recorded well after i.r. 1064 nm Nd: YAG laser-induced optical breakdown of carbon monoxide. Temperatures in excess of 6000 K were determined using fits to synthetic diatomic molecular spectra.

Electron number density and temperature measurement in a laser-induced hydrogen plasma

Abstract A Nd:YAG laser operated at 1064 nm and 7.5 nsec pulse duration is used to create optical breakdown in gaseous hydrogen. Time-resolved spectral measurements of the hydrogen Balmer series are reported and analyzed to characterized the electron number density and excitation temperature of the decaying plasma. The electron density is inferred from the Stark broadened H α linewidths; excitation temperatures are estimated using Boltzmann plots of the Balmer series. In the first few microseconds following laser breakdown, electron densities are found to be in the range of 10 19 -10 16 cc -1 , with corresponding excitation temperatures in the range of approx. 100,000–6600 K.

Spherical aberration effects in lens–axicon doublets: theoretical study

Effects of spherical aberrations in converging and diverging lens-axicon doublets are investigated. Intensity profiles are obtained in the line and ring focal regions by numerically solving the Fresnel-Kirchhoff integral. Comparisons with aberration-free computations show that higher peak irradiances occur for the converging doublet when spherical aberrations are included. Results are presented for axicons with small apex angles in lens-axicon combinations illuminated by collimated Gaussian 1.064-mum laser beams.

TEMPERATURE MEASUREMENTS FROM FIRST-NEGATIVE N2+ SPECTRA PRODUCED BY LASER-INDUCED MULTIPHOTON IONIZATION AND OPTICAL BREAKDOWN OF NITROGEN

A 248-nm excimer laser was used to produce ionized nitrogen by the process of multiphoton excitation in gaseous nitrogen at room temperature. First-negative N(2)(+) emission spectra were analyzed to yield rotational temperatures of typically 600 to 1200 K. Rotational Raman scattering of H(2) in gaseous mixtures of N(2) and H(2) was used to determine if laser heating of the gas produced the observed increase in temperature, but the room temperature value of 295 K was inferred from the H(2) Raman data. Therefore the use of N(2)(+) spectra produced by multiphoton excitation at 248 nm does not appear to be acceptable for air-temperature diagnostics. N(2)(+) emission spectra were also recorded subsequent to optical breakdown in air induced by Nd:YAG 1064-nm radiation, and temperatures were determined to be greater than 5000 K in the decaying plasma.

Spatially and temporally resolved electron number density measurements in a decaying laser-induced plasma using hydrogen-alpha line profiles

A Nd:YAG laser was operated at 1064 nm and with 6-ns pulse duration to achieve optical breakdown in gaseous hydrogen at pressures of 150 and 810 Torr. Spatially and temporally resolved laser-induced emission spectra were measured early in the plasma decay. With hydrogen-alpha line profiles, electron number density values were determined along the laser beam plasma in the range 10(19) to 10(16) cc(-1).

Exciplex liquid-phase thermometer using time-resolved laser-induced fluorescence.

Pulsed photoexcitation of hydrocarbon fuels doped with organic molecules exhibits a temperature-dependent fluorescence spectrum that is used as the basis for a weakly intrusive optical thermometer. By use of pulsed excitation from a 308-nm 8-ns XeCl excimer laser with gated detection of the fluorescence emissions from doped n -heptane, we demonstrate that time-resolved measurement of the excited monomer and the redshifted excited-state complex (exciplex) fluorescence emissions can yield sub-1 degrees accuracy for temperatures ranging from 440 K to the vicinity of the critical temperature (540 K). The experiments also show that the exciplex fluorescence spectrum is pressure independent below and above supercritical pressure.

Analysis of Combined Spectra of NH and N(2).

Gas phase diagnostics with multispecies diatomic spectra isdiscussed. Analyses of spectra from the A(3)?(i) ? X(3)?(-) system of NH and the C(3)?(u) ? B(3)?(g) second-positive system of N(2) are presented. Multispecies spectroscopy is applied to experimental spectra obtained from laser-induced breakdown plasmas in anhydrous ammonia gas and a low-pressure discharge lamp.