  Modeling a promising carrier of the astronomically observed polycyclic
aromatic hydrocarbon (PAH), infrared (IR) spectra of ionized molecules (C9H7)
n+ were calculated based on density functional theory (DFT). In a previous
study, it was found that void induced coronene C23H12++ could reproduce
observed spectra from 3 to 15 micron, which has carbon two pentagons connected
with five hexagons. In this paper, we tried to test the simplest model, that
is, one pentagon connected with one hexagon, which is indene like molecule
(C9H7) n+ (n=0 to 4). DFT based harmonic frequency analysis resulted that
observed spectrum could be almost reproduced by a suitable sum of ionized
C9H7n+ molecules. Typical example is C9H7++. Calculated peaks were 3.2, 7.4,
7.6, 8.4, and 12.7 micron, whereas observed one 3.3, 7.6, 7.8, 8.6 and 12.7
micron. By a combination of different degree of ionized molecules, we can
expect to reproduce total spectrum. For a comparison, hexagon-hexagon molecule
naphthalene (C10H8) n+ was studied. Unfortunately, ionized naphthalene shows
little coincidence with observed one. Carbon pentagon- hexagon molecules may
play an important role as interstellar molecular dust.
