  Polycyclic aromatic hydrocarbon (PAH) molecules have long been suggested to
be present in the interstellar medium (ISM). Nevertheless, despite their
expected ubiquity and sustained searching efforts, identifying specific
interstellar PAH molecules from their infrared (IR) spectroscopy has so far
been unsuccessful. However, due to its unprecedented sensitivity, the advent of
the James Webb Space Telescope (JWST) may change this. Meanwhile, recent years
have witnessed breakthroughs in detecting specific PAH molecules (e.g., indene,
cyanoindene, and cyanonaphthalene) through their rotational lines in the radio
frequencies. As JWST holds great promise for identifying specific PAH molecules
in the ISM based on their vibrational spectra in the IR, in this work we model
the vibrational excitation of indene, a molecule composed of a six-membered
benzene ring fused with a five-membered cyclopentene ring, and calculate its IR
emission spectra for a number of representative astrophysical regions. This
will facilitate JWST to search for and identify indene in space through its
vibrational bands and to quantitatively determine or place an upper limit on
its abundance.
