  Reflection Nebula NGC2023 shows specific interstellar infrared spectrum due
to polycyclic aromatic hydrocarbon (PAH) in a wide wavelength range from 5 to
20 micrometer. By our previous quantum chemistry calculation, it was suggested
that a molecule group having hydrocarbon pentagon-hexagon combined skeleton
could reproduce ubiquitous interstellar infrared spectrum. In this paper,
observed NGC2023 spectrum was compared in detail with such carrier candidates.
First model molecule was di-cation (C23H12)2+ with two hydrocarbon pentagons
combined with five hexagons. Observed strong infrared bands were 6.2, 7.7, 8.6,
and 11.2 micrometer. Whereas, calculated strong peaks were 6.4, 7.5, 7.7, 8.5,
and 11.2 micrometer. Observed weaker bands from 10 to 15 micrometer were 11.0,
12.0, 12.7, 13.5, and 14.2 micrometer, which were reproduced well by computed
bands as 10.9, 12.0, 12.6, 13.6, and 14.0 micrometer. From 15 to 20 micrometer,
observed 15.8, 16.4, 17.4, 17.8, and 19.0 micrometer were correlated with
calculated 15.6, 16.5, 17.2, 18.2, and 18.8 micrometer. It should be noted that
we could successfully reproduce interstellar infrared spectrum by applying a
single molecule. Second model molecule was (C12H8)3+ with one pentagon combined
with two hexagons. Again, observed strong bands at 6.2, 7.7, 8.6, 11.2 and 12.7
micrometer were successfully computed as 6.3, 7.4, 7.7, 8.6, 11.1, and 12.8
micrometer. It was concluded that by introducing hydrocarbon pentagon-hexagon
combined ionized molecules, interstellar PAH oriented infrared spectrum could
be successfully reproduced.
