Christian G. Parigger

Spatial and temporal profiles of pulsed laser-induced air plasma emissions

Abstract The laser-induced breakdown and energy deposition in air were studied using a full-width at half-maximum (FWHM) 6.5-ns pulse Nd : YAG laser. For the non-resonant breakdowns, two-dimensional spontaneous emission images were obtained with nanosecond resolution from breakdown to 10 times the FWHM of the pulse of the spatial and temporal variations of the energy deposition and plasma formation processes. The spatial emission features are compared with the predictions of the laser irradiance in the focal region that results from the focus lens aberrations. The statistical variation of the absorbed energy was determined over the energy range from below-to-approximately six times the breakdown threshold. Comparisons are made of the measured plasma wave speed with one-dimensional laser-supported radiation wave predictions.

Computational fluid-dynamic model of laser-induced breakdown in air

Temperature and pressure profiles are computed by the use of a two-dimensional, axially symmetric, time-accurate computational fluid-dynamic model for nominal 10-ns optical breakdown laser pulses. The computational model includes a kinetics mechanism that implements plasma equilibrium kinetics in ionized regions and nonequilibrium, multistep, finite-rate reactions in nonionized regions. Fluid-physics phenomena following laser-induced breakdown are recorded with high-speed shadowgraph techniques. The predicted fluid phenomena are shown by direct comparison with experimental records to agree with the flow patterns that are characteristic of laser spark decay.

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.

Temperature measurements from CN spectra in a laser-induced plasma

Abstract The spontaneous-emission spectra of the CN violet system were observed following excimer-laser-induced breakdown of an atmospheric-pressure CO 2 /N 2 mixture. Using a triple monochromator and a gated linear-diode array, the spectra were acquired with spectral resolutions of 2 and 7 cm -1 within 1 μsec following the laser pulse. Comparison of the observed rotation-vibrational structure of the Δν=0 sequence and the synthetic spectra, which were calculated using direct diagonalization of the rotational and fine-structure Hamiltonians, yielded internal, molecular temperatures of approx. 8000 K. No evidence of internal nonequilibrium of the rotational and vibrational modes was observed.

Spectroscopic temperature determination of aluminum monoxide in laser ablation with 266-nm radiation

We report time-resolved measurements of diatomic aluminum monoxide spectra in the study of laser ablation by the use of frequency-quadrupled 266 nm Nd:YAG laser radiation. Spectroscopic temperatures of 3432(35) K and 3329(13) K are obtained at a delay time of 20mu, respectively, by the use of the modified diatomic Boltzmann plot and by the use of the Nelder-Mead algorithm in the fitting of the recorded spectrum.

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.

Computation of AlO B2Σ+ → X2Σ+ emission spectra.

Application of molecular spectroscopy to analytical chemistry usually requires accurate description of the particular transition of interest. In this communication we describe the creation of a list of spectral lines. Following the introduction and definition of the line strength, we present a recipe for computation of diatomic-line-strengths, including the Hönl-London factor and electric dipole line strength for each spectral line. The diatomic eigenfunction is discussed including Hunds case basis functions. In our data tables we prefer use of Hunds case (a) basis, and we apply the usual Born-Oppenheimer approximation for the electronic-vibrational strengths. This allows us to generate the table of line strengths that we frequently apply for spectroscopic temperature determination. Using these line-strength tables, we present theoretical AlO emission spectra for the B-X system of AlO. These emission spectra are computed for temperatures of 3000 and 6000 K and for typical spectroscopic resolution used in laser-induced optical breakdown studies.

Fluid dynamics effects following laser-induced optical breakdown

The fluid flow phenomena resulting from laser-spark decay is described based on a numerical simulation. The transient model involves the solution of the two-dimensional axial-symmetric transport equations of mass, momentum, and energy in nitrogen gas. Initial temperature and pressure profiles are taken upon the termination of the laser pulse, and are based on the laserinduced plasma formation processes. The specific heat and transport property models include the effects of dissociation and ionization. Temperature, pressure, and velocity profiles are reported at selected times, and experimental ultra-high speed photographs are compared with the numerical analog.

Measurement and analysis of atomic and diatomic carbon spectra from laser ablation of graphite

Spectra from plasma produced by laser-induced breakdown of graphite were recorded and analyzed to increase our understanding of the way in which carbon nanoparticles are created during Nd:YAG laser ablation of graphite. The effects of various buffer gases were studied. Electron density and temperature were determined from spectra of the first and second ions of atomic carbon. The C2 Swan spectrum was also prominent in most of the measured spectra. Temperature was inferred from each experimental Swan spectrum by determination of the temperature for which a synthetic Swan spectrum best fitted, in the least-squares sense, the measured spectrum.