  This paper presents optical long-slit spectroscopic observations of 105
barred Markarian IRAS galaxies. These observations are used to determine the
spectral type of emission-line regions in the nucleus and along the bar of the
galaxies, in order to define a homogeneous sample of Starburst Nucleus Galaxies
(SBNGs). Our selection criteria have been very efficient for selecting
star-forming galaxies, since our sample of 221 emission-line regions includes
82% nuclear or extranuclear starbursts. The contamination by Seyferts is low
(9%). The remaining galaxies (9%) are objects with ambiguous classification
(HII or LINER). The dust content and Halpha luminosity increase towards the
nuclei of the galaxies. No significant variation of the electron density is
found between nuclear and bar HII regions. However, the mean Halpha luminosity
and electron density in the bar are higher than in typical disk HII regions. We
investigate different mechanisms for explaining the excess of nitrogen emission
observed in our starburst nuclei. There is no evidence for the presence of a
weak hidden active galactic nucleus in our starburst galaxies. The cause of
this excess is probably a selective enrichment of nitrogen in the nuclei of the
galaxies, following a succession of short and intense bursts of star formation.
Our sample of SBNGs, located at a mean redshift of 0.015, has moderate Halpha
(10^41 erg/s) and far infrared (10^10 Lsun) luminosities. The types are
distributed equally among early- and late-type giant spirals with a slight
preference for Sbc/Sc types because of their barred morphology. The majority
(62%) of SBNGs are isolated with no sign of gravitational interaction. In terms
of distance, luminosity and level of interaction, SBNGs are intermediate
between HII galaxies and luminous infrared galaxies.
