  Aims. This study was designed to examine the viability of protonated
nitrogen-substituted polycyclic aromatic hydrocarbons (H+PANHs) as candidates
for the carriers of the diffuse interstellar bands (DIBs). Methods. We obtained
the electronic spectra of two protonated PANH cations, protonated acridine and
phenanthridine, using parent ion photo-fragment spectroscopy and generated
theoretical electronic spectra using ab initio calculations. Results. We show
that the spectra of the two species studied here do not correspond to known
DIBs. However, based on the general properties derived from the spectra of
these small protonated nitrogen-substituted PAHs, we propose that larger H+PANH
cations represent good candidates for DIB carriers due to the expected
positions of their electronic transitions in the UV-visible and their narrow
spectral bands.
