Combined infrared and Raman study of solid CO

Context. Knowledge about the composition and structure of interstellar ices is mainly based on the comparison between astronomical and laboratory spectra of astrophysical ice analogues. Carbon monoxide is one of the main components of the icy mantles of dust grains in the interstellar medium. Because of its relevance, several authors have studied the spectral properties of solid CO both pure and in mixtures. Aims. The aim of this work is to study the profile (shape, width, peak position) of the solid CO band centered at about 2140 cm(-1) at low temperature, during warm up, and after ion irradiation to search for a structural variation of the ice sample. We also report on the appearance of the longitudinal optical-transverse optical (LO-TO) splitting in the infrared spectra of CO films to understand if this phenomenon can be related to a phase change. Methods. We studied the profile of the 2140 cm(-1) band of solid CO by means of infrared and Raman spectroscopy. We used a free web interface that we developed that allows us to calculate the refractive index of the sample to measure the thickness of the film. Results. The profile of the fundamental band of solid CO obtained with infrared and Raman spectroscopy does not show any relevant modification after warm up or ion bombardment in the dose range investigated. We explain that the LO-TO splitting is not connected to a structural variation of the film. Ion irradiation causes the formation of new molecular species. Raman spectroscopy allowed us to detect, among other bands, a band centered at 1817 cm(-1) that has been attributed to the infrared inactive species C-2 and a band centered at 1767 cm(-1) that remains unidentified.