The interaction between laser-induced plasmas and individual particles controls the rate of particle dissociation and subsequent atomic diffusion and emission processes, with implications for single-particle spectroscopy, as well as materials synthesis and other plasma sources. It is demonstrated through quantitative plasma imaging studies that discrete particles dissociate on a time scale of tens of microseconds within plasmas formed by 300-mJ Nd:YAG laser pulses. Significant spatial nonhomogeneity, as measured by localized atomic emission from particle-derived calcium atoms, persists on a comparable time scale, providing a measure of their average atomic diffusion rate of 0.04 m(2)/s. In addition, the resulting calcium atomic emission is explored using image analysis as well as traditional spectroscopic analysis.

The current work demonstrates an optical technique that utilizes micron-resolution particle image velocimetry (µ-PIV) for temperature measurement. The technique is based on the premise that Brownian motion will cause width-wise broadening of the cross-correlation peak. A correlation-based PIV algorithm detects the magnitude of Brownian particle motion and can be used to determine the temperature of the fluid. Results were obtained using fluorescing (rhodamine 542/612) spherical 700 nm diameter polystyrene-latex particles in water. Temperature changes up to 25 °C were determined with an experimental accuracy of ±3 °C.

No abstract

The effects of analyte phase on the calibration response for laser-induced breakdown spectroscopy is investigated for a range of carbon species. Significant differences in the atomic emission signal from carbon were observed when comparing calibration streams of gas-phase and submicrometer-sized solid-phase carbon species. The resulting calibration curve slopes varied by a factor of 8 over a comparable range of atomic carbon concentrations for five different analyte sources, while the plasma electron density and temperature remained essentially constant. The current findings challenge a widely held assumption that complete dissociation of constituent species within a highly energetic laser-induced plasma results in independence of the analyte atomic emission signal on the analyte source. A physical model of the plasma-analyte interaction is proposed that provides a framework to account for the observed dependence on the physical state of the analyte.

No abstract

Phencyclone, 1, a potent Diels-Alder diene, reacts with a series of N-alkylmaleimides, 2, to form hindered adducts, 3. The 300 MHz 1H and 75 MHz 13C NMR studies of these adducts at ambient temperatures have demonstrated slow rotations on the nuclear magnetic resonance (NMR) timescales for the unsubstituted bridgehead phenyl groups, and have revealed substantial magnetic anisotropic shielding effects in the 1H spectra of the N-alkyl groups of the adducts. The selected N-alkyl groups for the target compounds emphasized smaller branched alkyls, including C3 (isopropyl, a); C4 (isobutyl, b; and t-butyl, c); C5 (n-pentyl, d; isopentyl [isoamyl], e; 1-ethylpropyl, f; t-amyl, g;) and a related C8 isomer (1,1,3,3-tetramethylbutyl ["t-octyl"], h). The straight-chain n-pentyl analog was included as a reference. This present work on the branched N-alkylmaleimide adducts appreciably extends our earlier compilation on the N-n-alkylmaleimide adducts. Key methods for proton assignments included "high-resolution" 1H-1H chemical shift correlation spectroscopy, COSY45. 13C NMR of the adducts, 3, verified the expected number of aryl carbons for slow exchange limit (SEL) spectra of the bridgehead phenyl groups. The synthetic routes involved reaction of the corresponding amines, 4, with maleic anhydride to give the N-alkylmaleamic acids, 5, which underwent cyclodehydration to form the maleimides, 2. Magnetic anisotropic shielding magnitudes for alkyl group protons in the adducts were calculated relative to corresponding proton chemical shifts in the maleimides. Geometry optimizations for the above adducts (and for the N-n-butylmaleimide adduct) were performed at the Hartree-Fock level with the 6-31G* basis set. The existence of different contributing conformers for the adducts is discussed with respect to their calculated energies and implications regarding experimentally observed anisotropic shielding magnitudes.

A statistical analysis of ambient air particle monitoring, namely PM2.5, is presented to elucidate the correlations between laser-induced breakdown spectroscopy (LIBS)-based speciated aerosol monitoring and non-speciated aerosol monitoring (i.e., total particle counts). LIBS was used in a real-time, conditional-processing mode to identify individual aerosol particles containing detectable quantities of either calcium or sodium, as based on the resulting atomic emission signals. Using this technique, real-time measurements of speciated aerosol particle concentrations and analyte mass concentrations were evaluated for a total of 60 1-hour sampling periods spread over a 5-week period. For each 1-hour sampling period, total aerosol counts were simultaneously monitored using a commercial light scattering-based instrument. Over the 30 sampling periods, aerosol counts (both total and LIBS-based) were found to vary by more than one order of magnitude. For aerosol particles in the 500 nm to 2.5 microm size range, significant correlations were found between the two sampling methods, resulting in correlation coefficients (r2) ranging from 0.22 to 0.93. In addition, transient fluctuations in aerosol counts on a timescale of 5 to 10 minutes were successfully observed simultaneously with the two monitoring techniques, thereby demonstrating the temporal resolution of LIBS.

The spectral and temporal effects of laser cavity seeding and aerosol presence in the laser-induced plasma volume are investigated with the goal of examining factors affecting the precision of laser-induced breakdown spectroscopy. Improvements in the temporal stability of laser-induced breakdown initiation were observed with laser cavity seeding. However, laser cavity seeding produced no significant improvement in analyte precision for a range of gas-phase atomic emission lines, including nitrogen, hydrogen, and carbon. In contrast, greater shot-to-shot analyte precision, as measured by a nearly 60% reduction in relative standard deviation, was realized with the elimination of concomitant aerosols from the analyte sample stream. Such findings are consistent with the expected interactions of ambient aerosols and the laser-induced breakdown process.