  This is the second paper in a series discussing the properties of
ultraluminous infrared galaxies (ULIGs: L_ir > 10^{12} L_sun; Ho = 75 km s^{-1}
Mpc^{-1} and qo = 0.0) from the 1-Jy sample of Kim (1995). This paper presents
the first results of a spectroscopic survey at optical wavelengths of a
randomly selected subset of 45 ULIGs from Kim & Sanders (1997). These new data
are combined with previous data from Veilleux et al. (1995) to determine the
spectral properties of luminous infrared galaxies (LIGs) with L_ir ~=
10^{10.5}-10^{13} L_sun. We find that the fraction of Seyfert galaxies among
LIGs increases dramatically above L_ir ~= 10^{12.3} L_sun --- nearly half of
the galaxies with L_ir > 10^{12.3} L_sun present Seyfert characteristics. Many
of the optical properties of these Seyfert galaxies are consistent with the
presence of a genuine active galactic nucleus (AGN) in the core of these
objects.
  The continuum colors and strengths of the stellar H-beta and MgIb features in
and out of the nuclei of ULIGs indicate that star formation has recently (~10^7
yr) taken place in the nuclear and circumnuclear regions of many of these
objects. As expected, photoionization by hot stars appears to be the dominant
source of ionization in the objects with H II region-like spectra. Evidence is
presented that the ionization source in infrared-selected galaxies with nuclear
LINER-like spectra (38% of the ULIGs in our sample) is likely to be shocks or
of stellar origins rather than an AGN. Shock ionization associated with
starburst-driven outflows may also explain the LINER-like emission detected
outside the nuclei of some galaxies.
