  We present a spectroscopic analysis of the central disc regions of barred
spiral galaxies, concentrating on the region that is swept by the bar but not
including the bar itself (the `Star Formation Desert' or SFD region). New
spectroscopy is presented for 34 galaxies, and the full sample analysed
comprises 48 SBa - SBcd galaxies. These data confirm the full suppression of
star formation within the SFD regions of all but the latest type (SBcd)
galaxies. However, diffuse [NII] and H alpha line emission is detected in all
galaxies. The ubiquity and homogeneous properties of this emission from SBa -
SBc galaxies favour post-Asymptotic Giant Branch (p-AGB) stars as the source of
this line excitation, rather than extreme Blue Horizontal Branch stars. The
emission-line ratios strongly exclude any contribution from recent star
formation, but are fully consistent with recent population synthesis modelling
of p-AGB emission by other authors, and favour excitation dominated by ambient
gas of approximately solar abundance, rather than ejecta from the AGB stars
themselves. The line equivalent widths are also larger than those observed in
many fully passive (e.g. elliptical) galaxies, which may also be a consequence
of a greater ambient gas density in the SFD regions.
