Infrared spectra of solid indene pure and in water ice. Implications for observed IR absorptions in TMC-1

TL;DR

This study provides the first experimental and theoretical IR spectra of solid indene in pure and water ice environments, aiding astrophysical detection efforts and estimating its abundance in interstellar space.

Contribution

It presents new IR spectra and band strengths for amorphous and crystalline indene, including in water ice, with theoretical calculations suggesting a monoclinic crystalline phase and implications for astrophysical observations.

Findings

Indene's IR spectra are similar in water ice and pure form.

Estimated indene abundance in TMC-1 is 2-5% of the 3.3 micron feature.

Indene could account for a small but significant fraction of observed interstellar features.

Abstract

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.