Investigation of physical and spectral characteristics of laser-induced plasmas: Applications to laser-induced breakdown spectroscopy for analysis of aerosols and single particles.

摘要

The physical and optical characteristics of laser-induced plasmas (LIP) and the precision of measurements using laser-induced breakdown spectroscopy (LIBS) for aerosol and gas phase species are explored. In a series of coherent experiments properties of the LIP and the temporal, spatial, and energetic variations of interactions with entrained particles (aerosols) are evaluated from both a standpoint of basic plasma science and applied atomic spectroscopy.; First, the evolution of a LIP is characterized in terms of its temporally-resolved spectral absorptivity, spectral emissivity, and free electron density during the first 500 nanoseconds. Transmission measurements reveal near opacity of the LIP at early times (10-50 ns) and essential transparency at longer times (500 ns). The fundamental change from an absorbing plasma to a non-absorbing plasma during this period is important with respect to radiative energy transfer.; Second, spectral and temporal effects of laser cavity seeding and aerosol presence in the LIP volume are investigated. Improvements in the temporal stability of laser-induced breakdown initiation were observed with laser cavity seeding. 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.; Third, the effects of analyte phase on the calibration response for LIBS were investigated. Significant differences in the atomic emission signal from carbon were observed when comparing calibration streams of gas-phase and submicron-sized solid-phase species. The resulting calibration curves demonstrated large inter-species variations over a comparable range of atomic carbon concentrations, challenging a widely held assumption that dissociation of constituent species within a LIP results in independence of the analyte atomic emission signal and analyte source.; Finally, the LIBS technique is used in conjunction with imaging to measure a rate of dissociation for aerosols entrained in a LIP. Additionally, the time scales of background plasma emission, spatial distribution of atomic emission, and diffusion of atomic species within a LIP are explored. Atomic emission from the plasma volume is demonstrated to occur in spatially localized bursts. The total amount of analyte detected is shown to plateau at times coinciding with significant energy decay of the plasma.