  The general properties of luminous and ultraluminous infrared galaxies (LIRGs
and ULIRGs) in the local universe are well known since large samples of these
objects have been the subject of numerous spectroscopic works. There are,
however, relatively few studies of large samples of LIRGs and ULIRGs using
integral field spectroscopy (IFS). We analyze optical (3800-7200A) IFS data
taken with the Potsdam Multi-Aperture Spectrophotometer (PMAS) of the central
few kiloparsecs of 11 LIRGs. To study the stellar populations we fit the
optical stellar continuum and the hydrogen recombination lines of selected
regions. We analyze the excitation conditions of the gas using the spatially
resolved properties of the brightest optical emission lines. The optical
continua of the selected regions are well fitted with a combination of evolved
(~0.7-10Gyr) and ionizing (1-20Myr) stellar populations. The latter is more
obscured than the evolved population, and has visual extinctions in good
agreement with those obtained from the Balmer decrement. Except for NGC 7771,
there is no clear evidence for an important contribution to the optical light
from an intermediate-aged population (~100-500Myr). Even after correcting for
the presence of stellar absorption, a large fraction of spaxels with low
observed equivalent widths of Halpha in emission still show enhanced
[NII]/Halpha and [SII]/Halpha ratios. These ratios are likely to be produced by
a combination of photoionization in HII regions and diffuse emission. These
regions of enhanced ratios are generally coincident with low surface brightness
HII 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 we
find that the fraction of diffuse emission in LIRGs varies from galaxy to
galaxy, and it is generally less than 60% as found in other starburst galaxies.
(Abridged)
