James O. Hornkohl

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 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.

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.

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.

Measurement and analysis of OH emission spectra following laser-induced optical breakdown in air

The measured emission spectra of the OH radical subsequent to laser-induced optical breakdown in air are analyzed to infer spectroscopic temperature and species number density. Emissions from the UV A2sigma+ --> X2IIi transition dominate the spectra in the wavelength range of 306-322 nm and for time delays from the optical breakdown of 30-300 micros. Contributions from other species to the recorded OH emission spectra were also investigated for spectroscopic temperature measurements in the range of 2000-6000 K and for OH number densities in the range of 10(14) - 2 x 10(16) cm(-3). Monte Carlo simulations are applied to estimate errors in the analysis of the hydroxyl spectra.

Laser-induced carbon plasma emission spectroscopic measurements on solid targets and in gas-phase optical breakdown

We report measurements of time- and spatially averaged spontaneous-emission spectra following laser-induced breakdown on a solid graphite/ambient gas interface and on solid graphite in vacuum, and also emission spectra from gas-phase optical breakdown in allene C3H4 and helium, and in CO2 and helium mixtures. These emission spectra were dominated by CII (singly ionized carbon), CIII (doubly ionized carbon), hydrogen Balmer beta (Hbeta), and Swan C2 band features. Using the local thermodynamic equilibrium and thin plasma assumptions, we derived electron number density and electron temperature estimates. The former was in the 10(16) cm(-3) range, while the latter was found to be near 20000 K. In addition, the vibration-rotation temperature of the Swan bands of the C2 radical was determined to be between 4500 and 7000 K, using an exact theoretical model for simulating diatomic emission spectra. This temperature range is probably caused by the spatial inhomogeneity of the laser-induced plasma plume. Differences are pointed out in the role of ambient CO2 in a solid graphite target and in gas-phase breakdown plasma.

Spontaneous emission from the C3 radical in carbon plasma

Spontaneous emission measurements are discussed for the Swings transitions of the C(3) radical in laser-generated graphite plasma, and the spectroscopy of the C(3) radical in carbon vapor and plasma is summarized. A review is given of some theoretical calculations and emission spectroscopic investigations are presented. Time-averaged, laser-induced optical breakdown spectra are reported from Nd:YAG laser generated graphite microplasma. In 200-300 Torr of argon and helium, and depending on the specific experimental configuration, a weak emission continuum is observed centered at 400 nm when using a laser fluence of typically 1 J/cm(2). Such continua were not detected in our previous experiments using focused laser radiation. The possibilities for the origin of this continuum are considered.

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

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).

Diatomic Hönl-London factor computer program

A new method is presented for computation of diatomic rotational line strengths, or Hönl-London factors. The traditional approach includes separately calculating line positions and Hönl-London factors and assigning parity labels. The present approach shows that one merely computes the line strength for all possible term differences and discards those differences for which the strength vanishes. Numerical diagonalization of the upper and lower Hamiltonians is used, which directly obtains the line positions, Hönl-London factors, total parities, and e/f parities for both heteronuclear and homonuclear diatomic molecules. The FORTRAN computer program discussed is also applicable for calculating n-photon diatomic spectra.

Analysis of time-resolved superposed atomic hydrogen Balmer lines and molecular diatomic carbon spectra

We present analysis of superposition spectra following laser-induced breakdown (LIB) of methane. Both hydrogen-beta and hydrogen-gamma lines contain discernible contributions from diatomic carbon emissions for time delays of 1 to 2 μs from pulsed, 8 ns, infrared Nd:YAG laser radiation LIB. Analysis of the atomic lines and molecular C(2) spectra reveal electron and molecular excitation temperatures of typically 13,000 and 5000 K, respectively.