Paul J. Wolf

Laser-Induced Breakdown Spectroscopy of Liquids: Aqueous Solutions of Nickel and Chlorinated Hydrocarbons

Spectrochemical analyses of aqueous solutions containing nickel or the chlorinated hydrocarbons (CHCs) C2Cl4, CCl4, CHCl3, and C2HCl3 were performed with the use of laser-induced breakdown spectroscopy. A Nd:YAG laser operating at 60 mJ/pulse was focused onto the surface of the liquid. Elemental line intensities were monitored in the laser-produced plume as a function of analyte concentration to determine detection limits. The limits of detection for nickel in water were 36.4 ± 5.4 mgAL and 18.0 ± 3.8 mg/L for laser irradiation at 1.06 μm and 355 nm, respectively. Ablation of pure CHCs at 355 nm produced extremely intense plasma emissions that primarily consisted of spectroscopic features attributed to CN, C3, H, N, and Cl. The spectra were structurally identical for all the CHCs except for differences in the intensities of various emission lines. With the use of emission from neutral atomic chlorine as an identifier for CHC contamination of water, no detectable traces of these elements were observed in saturated aqueous solutions. The detection limits for the CHCs were well above the saturation limits of CHC in water.

The plasma properties of laser‐ablated SiO2

The optical emission from laser‐produced plasmas generated by 1.06 μm irradiation of SiO2 targets at a flux of 7×1010 W cm−2 was recorded and analyzed between 250 and 800 nm. The ionization states of Si and O were mapped as a function of both time from the incident laser pulse and location from the front surface of the target. Electron temperatures were calculated using the relative emission intensities of Si(II) and O(II) ionization states (Te=3.4 eV), and an electron number density was determined from the Stark‐broadened linewidths of five Si(II) emission lines. The ablated material was collected on Si substrates to examine the particulate nature of the plasma. Thin films were grown in the process and properties of these films were examined using IR reflectance and transmittance spectroscopy, scanning electron microscope analyses, and Auger electron spectroscopy.

Thin film properties of germanium oxide synthesized by pulsed laser sputtering in vacuum and oxygen environments

Germanium oxide thin films were deposited on Si substrates at ambient temperature using pulsed laser sputtering of GeO2 targets in both vacuum and oxygen environments. The structure and composition of these films were deduced from infrared transmittance and Auger electron spectroscopy measurements. The film properties were strongly influenced by the pressure of the background reactive gas and the laser fluence during deposition with O2. The vacuum deposited films had a low oxygen content and the film’s stoichiometry varied from metal rich at the center to oxidelike near the edges of the film. The films deposited in oxygen appeared stoichiometric and they showed no content variation with radius. Ellipsometry studies indicated, however, that the oxygen grown films were porous. The trends observed in growing these films under various conditions are discussed with respect to potential reaction kinetics in the plume and gas expansion phenomena.

Time‐integrated optical emission studies of plumes generated from laser ablated germania glass

The optical emission from plumes induced by ArF laser irradiation of GeO2 was characterized as a function of laser fluence, distance from the target surface, and ambient O2 pressure. Dispersion of the light emitted by the plume in a vacuum revealed emission from both neutral and singly ionized Ge atoms as well as neutral O atoms. The spatial variation showed that the ion concentration decreased exponentially from the target surface while the neutral atom number density reached peak intensities at distances of ≊1.5–2.5 cm from the target surface. Interactions between the plume constituents and the ambient molecular oxygen increased the excited Ge atom and Ge ion populations in the plume and, most notably, significantly enlarged the excited O atom concentration over that produced directly from the ablation process.

Ionization mechanisms in CRRES chemical releases: 2. Strontium photoionization and Ba+/Sr+ collisional ionization cross-section calculations

Two particular collision processes involving electron transfer are investigated that may have impact on the analyses of ion flux data obtained in the CRRES G-1 and G-11b, upper atmospheric chemical release experiments. Ion pair production (σIP) and charge exchange ionization (σCE) cross sections are calculated as a function of collision energy for reactions between Ba/Sr atoms and O atoms and O+ ions, respectively, using two-state approximation theories. The cross sections for each process are calculated for both the ground and metastable states of the metal atoms. At a collision energy of 9.5 eV attained in the G-1 and G-11b CRRES releases, the calculations indicate that σIP = 1.3 × 10−16 cm2 and σCE = 2–6 × 10−16 cm2 for reactions involving Ba(1S0). The cross sections for the analogous Sr reactions are approximately 1.2 × 10−16 cm2 and 2–8 × 10−17 cm2, respectively. In addition, Sr photoionization from 3PJ metastable states is investigated. Although we calculate a 29-s photoionization time constant from these states, a kinetic analysis indicates the metastable states cannot accumulate an appreciable equilibrium population in full solar exposure. Thus photoionization from the Sr 3PJ levels does not significantly contribute to the total ion inventory.

Second-harmonic generation in planar waveguides of doped silica.

Second-harmonic generation was produced in germanium-doped silica planar waveguides prepared by simulfaneous illumination with 1064- and 532-nm laser light. During preparation using prism coupling to specific waveguiding modes, the film-generated second-harmonic intensity grew as a function of preparation time until it saturated. The growth rate and saturation level for p-polarized second-harmonic intensity was an order of magnitude greater than that observed for the s polarization. The efficiency for a 2-cm waveguide length was at least 0.5%. The comparison of experimental results indicates a mechanism for this planar geometry that is similar to that producing harmonic effects in optical fibers.

Activated reactive laser deposition of GeO2 films

Amorphous GeO2 optical thin films were grown in an oxygen ambient on heated Si substrates using the technique of pulsed laser deposition. The application of a partially ionized oxygen plasma generated by passing the plume through a ring electrode facilitated stoichiometric film growth in low O2 partial pressures. Emission spectroscopy of the plume revealed an enhancement in the ionic and neutral excited Ge species. The concentration of excited neutral and ionic oxygen atoms also significantly increased when the ring electrode was activated at P(O2)≳10 mTorr. Coupling the results of the film property measurements with the emission studies suggested that the presence of O atoms near the substrate surface during film growth was more critical in promoting oxidation than the gas phase process in the plume. The low‐pressure conditions that were utilized to deposit stoichiometric film growth identified the appropriate conditions to produce uniform films over a large area that may be suitable for waveguide fabrication.

Ionization mechanisms in CRRES chemical releases: 1. In situ measurements and model results

The quadrupole ion mass spectrometer (QIMS) measured fluxes of barium and strontium ions during the CRRES G-1 chemical release in full Sun and the G-11b release in darkness. The barium ion fluxes detected in darkness were only a factor of 10 lower than the photoionization fluxes, indicating that the rate of nonsolar-UV ionization was at least a factor of 8 higher than literature values of charge transfer and collisional ionization cross sections would suggest. Model calculations of the QIMS ion fluxes using new, state-specific, calculations of the cross sections for ion-pair formation and charge transfer [Wolf and Hunton, this issue] demonstrate that the large nonphotoionization rates are consistent with significant metastable excitation in the neutral cloud but not with a ground state population. Upper limits to the collisional ionization rates for the conditions of the releases are 4.3 × 10−3 s−1 for ion-pair production and 2.2 × 10−3 S−1 for charge transfer. Collisions of high-energy electrons with the neutrals early in the cloud expansion is suggested as a mechanism for producing the metastable metal atoms. A similar analysis of the strontium data indicates that photoionization, charge transfer, and collisional ion-pair production alone cannot explain the large fluxes of strontium ions detected in the sunlit release. Associative ionization may therefore be an important ionization mechanism for that metal in sunlight. The implications of these results for analysis of space critical ionization velocity experiments are discussed.

Pressure broadening and line shifting of atomic strontium 5s{sup 2} {sup 1}S{sub 0}{yields}5s5p {sup 3}P{sub 1} and 5s5p {sup 3}P{sub 0,1,2}{yields}5s6s {sup 3}S{sub 1} absorption transitions induced by noble-gas collisions

The broadening and shifting of spectral lines induced by collisions with the five noble gases in both the intercombination 5s{sup 2} {sup 1}S{sub 0}{yields}5s5p {sup 3}P{sub 1} system and the triplet 5s5p {sup 3}P{sub 0,1,2}{yields}5s6s {sup 3}S{sub 1} manifold of Sr are studied using tunable dye laser absorption spectroscopy. Cross sections for impact broadening and line shifting are determined from an examination of the spectral line profiles. These results are utilized in an analysis to compute difference potentials modeled by the Lennard-Jones (6-12) potential and the coefficients C{sub 6} and C{sub 12} derived from this analysis are reported.

Laser Ablation Deposition of Germanium Oxide Thin Films

Laser pulsed sputtering of GeO 2 targets in both vacuum and oxygen environments is us;ed to produce germanium oxide thin films. Infrared transmission and Auger electron spectroscopies are employed to infer compositional information on the films. The films grown in vacuum are oxygen deficient and they show a radial variation in the metal to oxide content. In contrast, the films grown in an oxygen atmosphere appear stoichiometric along the entire film.