  Experimental and theoretical infrared spectra, between 4000-500 cm$^{-1}$
(2.5-20 microns), and infrared band strengths of two solid phases of indene,
amorphous and crystalline, are given for the first time. The samples were
generated via vapor deposition under high vacuum conditions on a cold surface.
Density functional theory was employed for the calculations of the IR spectra.
Lacking of previous information, a monoclinic symmetry is suggested for the
theoretical crystalline phase of indene, based on the comparison of the
calculated and experimental IR spectra. Assignments, based on the calculations,
are given for the main indene IR absorptions. The infrared spectra of highly
diluted mixtures of indene in amorphous solid water at 10 K are also provided,
evidencing that the indene spectrum is not much altered by the water ice
environment. These data are expected to be useful for the search of this
species in the solid phase in astrophysical environments with the JWST. With
the band strengths obtained in this work, and applying a simple literature
model, we find that indene could represent at most 2-5 percent of the intensity
of a weak absorption feature at 3.3 microns recently reported for Elias 16. A
column density of (1.5 -0.6) 10$^{16}$ cm$^{-2}$ is estimated for indene in the
ice mantles of TMC-1. It would correspond to aprox. (2 - 0.8) 10$^{-2}$ of
cosmic carbon, which is probably too high for a single small hydrocarbon.
