The profile of the bending mode band in solid CO₂

INAF-Osservatorio Astrofisico di Catania, via Santa Sofia 78, 95123 Catania, Italy
e-mail: mepalumbo@oact.inaf.it

Received: 6 April 2017
Accepted: 7 August 2017

Abstract

Context. Solid carbon dioxide (CO₂) is one of the most abundant species detected in icy grain mantles in dense molecular clouds. Its identification is based on the comparison between astronomical and laboratory spectra. In the past 30 yr the profile of solid CO₂ infrared absorption bands has been extensively studied experimentally, however, the debate on the structure (amorphous versus crystalline) of CO₂ samples obtained in laboratory by the thin-film technique is still open.

Aims. The aim of this work is to investigate if the presence of the double peak feature in the profile of the CO₂ bending mode band is related to the crystalline or amorphous structure of the sample.

Methods. We performed new laboratory experiments depositing CO₂ under ultra high vacuum (UHV) conditions at 17 K. We investigated, using infrared transmission spectroscopy, the influence of various experimental parameters on the profile of the CO₂ bands, namely deposition rate, sample thickness, annealing, and presence of H₂O, CH₃OH or CO co-deposited with CO₂.

Results. We found that, within experimental uncertainties, under UHV conditions the profile of the CO₂ bands in pure solid samples does not depend on the deposition rate or the sample thickness in the ranges investigated. In all cases the bending mode band profile shows a double peak (at 660 and 655 cm^-1). The spectra also show the Fermi resonance features that cannot be active in crystalline samples. On the other hand, when a small fraction of H₂O or CH₃OH is co-deposited with CO₂ the double peak is not observed while it is observed when a CO₂:CO mixture is considered. Furthermore, we measured the density of solid CO₂ and the refractive index (at 543.5 nm) at 17 K and at 70 K: ρ(17 K)= 1.17 g cm^-3, ρ(70K)= 1.49 g cm^-3, n(17K)= 1.285, and n(70K)= 1.372.

Conclusions. Our experimental results indicate that the presence of the double peak in the profile of the bending mode band is not an indication of a crystalline structure of the sample and they do not exclude the presence of amorphous solid CO₂ in space.