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We present the data and our analysis of MIR fine-structure emission linesdetected in Spitzer/IRS high-res spectra of 202 local LIRGs observed as part ofthe GOALS project. We detect emission lines of [SIV], [NeII], [NeV], [NeIII],[SIII]18.7, [OIV], [FeII], [SIII]33.5, and [SiII]. Over 75% of our galaxies areclassified as starburst (SB) sources in the MIR. We compare ratios of theemission line fluxes to stellar photo- and shock-ionization models to constrainthe gas properties in the SB nuclei. Comparing the [SIV]/[NeII] and[NeIII]/[NeII] ratios to the Starburst99-Mappings III models with aninstantaneous burst history, the line ratios suggest that the SB in our LIRGshave ages of 1-4.5Myr, metallicities of 1-2Z_sun, and ionization parameters of2-8e7cm/s. Based on the [SIII]/[SIII] ratios, the electron density in LIRGnuclei has a median electron density of ~300cm-3 for sources above the lowdensity limit. We also find that strong shocks are likely present in 10 SBsources. A significant fraction of the GOALS sources have resolved neon linesand 5 show velocity differences of >200km/s in [NeIII] or [NeV] relative to[NeII]. Furthermore, 6 SB and 5 AGN LIRGs show a trend of increasing line widthwith ionization potential, suggesting the possibility of a compact energysource and stratified ISM in their nuclei. We confirm a strong correlationbetween the [NeII]+[NeIII] emission, as well as [SIII]33.5, with both the IRluminosity and the 24um warm dust emission measured from the spectra. Finally,we find no correlation between the hardness of the radiation field or the linewidth and the ratio of the total IR to 8um emission (IR8). This may be becausethe IR luminosity and the MIR fine-structure lines are sensitive to differenttimescales over the SB, or that IR8 is more sensitive to the geometry of thewarm dust region than the radiation field producing the HII region emission.

Mid-Infrared Atomic Fine-Structure Emission Line Spectra of Luminous Infrared Galaxies: Spitzer/IRS Spectra of the GOALS Sample