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The general properties of luminous and ultraluminous infrared galaxies (LIRGsand ULIRGs) in the local universe are well known since large samples of theseobjects have been the subject of numerous spectroscopic works. There are,however, relatively few studies of large samples of LIRGs and ULIRGs usingintegral field spectroscopy (IFS). We analyze optical (3800-7200A) IFS datataken with the Potsdam Multi-Aperture Spectrophotometer (PMAS) of the centralfew kiloparsecs of 11 LIRGs. To study the stellar populations we fit theoptical stellar continuum and the hydrogen recombination lines of selectedregions. We analyze the excitation conditions of the gas using the spatiallyresolved properties of the brightest optical emission lines. The opticalcontinua of the selected regions are well fitted with a combination of evolved(~0.7-10Gyr) and ionizing (1-20Myr) stellar populations. The latter is moreobscured than the evolved population, and has visual extinctions in goodagreement with those obtained from the Balmer decrement. Except for NGC 7771,there is no clear evidence for an important contribution to the optical lightfrom an intermediate-aged population (~100-500Myr). Even after correcting forthe presence of stellar absorption, a large fraction of spaxels with lowobserved equivalent widths of Halpha in emission still show enhanced[NII]/Halpha and [SII]/Halpha ratios. These ratios are likely to be produced bya combination of photoionization in HII regions and diffuse emission. Theseregions of enhanced ratios are generally coincident with low surface brightnessHII regions and diffuse emission detected in the Halpha and Pa-alpha images.Using the PMAS line ratios and the NICMOS Pa-alpha photometry of HII regions wefind that the fraction of diffuse emission in LIRGs varies from galaxy togalaxy, and it is generally less than 60% as found in other starburst galaxies.(Abridged)

PMAS Optical Integral Field Spectroscopy of Luminous Infrared Galaxies. II.-- Spatially resolved stellar populations and excitation conditions