Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C<sub>2</sub>, CN, and TiO Spectra Following Laser-induced Optical Breakdown

In this work, we present time-resolved measurements of atomic and diatomic spectra following laser-induced optical breakdown. A typical LIBS arrangement ...

University of Tennessee Space Institute

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.

Computational fluid-dynamic model of laser-induced 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.

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

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.

Balmer series Hbeta measurements in a laser-induced hydrogen plasma.

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 atomic and diatomic carbon spectra from laser ablation of graphite.

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.

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

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.

Diatomic Hönl-London factor computer program.

We report time-averaged and time-resolved emission spectra subsequent to laser-induced optical breakdown of aluminum in laboratory air and in hydrogen gas. The microplasma generated by nominal 10 ns IR laser radiation shows Stark-broadened and shifted atomic lines. An analysis of the H(alpha) and H(beta) Balmer series lines and selected aluminum lines allows one to determine electron number density in the range of 0.01 - 10 x 10(18) cm(-3) early in the plasma decay. Atomic and molecular features are investigated for diagnostic applications in laser material processing.

Measurements of aluminum and hydrogen microplasma.

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.

Spontaneous emission from the C3 radical in carbon plasma.

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).

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

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 Hund's case basis functions. In our data tables we prefer use of Hund's 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.

Computation of AlO B2Σ+ → X2Σ+ emission 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.

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

We present results including measurement and analysis of titanium monoxide. Pulsed, nanosecond Nd:YAG laser radiation is used in a typical laser-induced breakdown spectroscopy arrangement to record the spectra. This scheme provides experiments analogous to pulsed laser deposition tactics and allows for time-resolved spectroscopic analysis. The computed spectra are generated from a new, accurate line-strength file that allows us to accurately predict γ (A3Φ→X3Δ) and γ' (B3Π→X3Δ) spectral signatures. We infer temperature on the order of 3600±700  K and 4200±800  K at time delays of 52 and 72 μs, respectively. Current interest in this work includes titania (TiO2) nanoparticle generation for thin film applications.

Measurement and analysis of titanium monoxide spectra in laser-induced plasma.

Diagnostic modalities by means of optical and/or near infrared femtosecond radiation through biological media can in principle be adapted to therapeutic applications. Of specific interest are soft tissue diagnostics and subsequent therapy through hard tissue such as bone. Femto-second laser pulses are delivered to hydroxyapatite representing bone, and photo-acoustic spectroscopy is presented in order to identify the location of optical anomalies in an otherwise homogeneous medium. Imaging through bone is being considered for diagnostic, and potentially therapeutic, applications related to brain tumors. The use of mesomeric optics such as lens-axicon combinations is of interest to achieve the favorable distribution of focused radiation. Direct therapy by increasing local temperature to induce hyperthermia is one mode of brain tumor therapy. This can be enhanced by seeding the tumor with nanoparticles. Opto-acoustic imaging using femtosecond laser radiation is a further opportunity for diagnosis.

Christian G. Parigger

Department of Physics

University of Tennessee Space Institute

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C₂, CN, and TiO Spectra Following Laser-induced Optical Breakdown

Christian G. Parigger

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Department of Physics

University of Tennessee Space Institute

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C2, CN, and TiO Spectra Following Laser-induced Optical Breakdown

Department of Physics

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C₂, CN, and TiO Spectra Following Laser-induced Optical Breakdown