Numerical modeling of the electrical breakdown and discharge properties of laser-generated plasma channels.

An extensive nonequilibrium steady-state kinetics model incorporating collisional and radiative processes is developed to study the electrical breakdown and discharge maintenance of laser-induced atmospheric plasma channels formed in externally applied electric fields. The model is based upon a self-consistent numerical solution of the Boltzmann equation for the electron energy distribution function coupled with the electron energy balance equation and the population balance equations for electrons and air species. Using the electron energy distribution function, the ionization and electron attachment rates as a function of the reduced applied electric field at different degrees of ionization are calculated. We find that the ionization rate as a function of applied electric field in a laser-induced plasma channel is orders of magnitude larger than that obtained for a natural atmospheric air discharge. Therefore, the electrical breakdown of these plasma channels may occur at significantly lower applied electric fields. The present model predicts a breakdown electric field of 10kVcm , while the experimentally determined breakdown field strength is approximately 5.7kVcm [A. P. Baronavski, NRL Memorandum Report No. NRL/MR/6110-02-8642, 2002 (unpublished)], a reduction of about a factor of 5 from the natural Paschen electrical breakdown field of approximately 30kVcm .

Scanning ultraviolet two-step laser mass spectroscopy of polycyclic aromatic hydrocarbon distributions on creosote-contaminated soil particles.

The distribution of polycyclic aromatic hydrocarbons (PAHs) on creosote-contaminated soil has been examined with scanning ultraviolet two-step laser desorption/laser ionization mass spectroscopy (UV-L2MS). The instrument has been constructed in-house by modifying a reflectron time-of-flight mass spectrometer. Two-dimensional chemical maps were accurately generated from model patterned PAH distributions. From examination of three-dimensional substrates, the depth of field of the experiment allows surfaces with roughness of up to 120 microm to be treated as a two-dimensional system and still achieve an accurate representation of the surface deposits. Soil was obtained from a former wood treatment facility. Individual particles of 100-1000 microm were mounted on indexed sample plates and examined by reflectance infrared microscopy, optical microscopy, and imaging UV-L2MS. The most intense PAH signals were associated with regions on the particles where clay/organic carbon deposits were found.

Application of saturation spectroscopy to the measurement of C(2), (3)II(u) concentrations in oxy-acetylene flames.

The technique of saturation spectroscopy is applied to measure the concentration of C(2) in the (3)II(u) state in oxyacetylene flames. The two level model is further developed and extended for use in intensity regions slightly lower than that required to saturate the resonances completely. As a result, it has proved feasible to measure both the C(2) number density in the (3)II(u) state of ~10(16)/cm(3) and an excited electronic state lifetime of ~10(-12) sec, both of which depend only on the Einstein A coefficient. The experimental setup is described in detail, and possible extensions of the technique to other atomic and molecular systems are discussed.