IR Spectral Fingerprint of Carbon Monoxide in Interstellar Water Ice Models

TL;DR

This study uses quantum mechanical simulations to analyze the IR spectral features of CO in interstellar water ice, explaining the absence of the blueshifted peak in astronomical observations and elucidating environmental effects on spectral signatures.

Contribution

It introduces detailed quantum models of CO in water ice, clarifies the origins of IR spectral shifts, and explains the missing blueshifted peak in space observations.

Findings

The blueshifted peak is due to CO interacting with dangling H atoms on water ice.

The redshifted peak results from CO interactions within the water matrix.

The absence of the blueshifted peak in space is explained by specific interaction environments.

Abstract

Carbon monoxide (CO) is the second most abundant molecule in the gas-phase of the interstellar medium. In dense molecular clouds, it is also present in the solid-phase as a constituent of the mixed water-dominated ices covering dust grains. Its presence in the solid-phase is inferred from its infrared (IR) signals. In experimental observations of solid CO/water mixed samples, its IR frequency splits into two components, giving rise to a blue- and a redshifted band. However, in astronomical observations, the former has never been observed. Several attempts have been carried out to explain this peculiar behaviour, but the question still remains open. In this work, we resorted to pure quantum mechanical simulations in order to shed some light on this problem. We adopted different periodic models simulating the CO/H$_2$O ice system, such as single and multiple CO adsorption on water ice surfaces, CO entrapped into water cages and proper CO:H$_2$O mixed ices. We also simulated pure solid CO. The detailed analysis of our data revealed how the quadrupolar character of CO and the dispersive forces with water ice determine the energetic of the CO/H$_2$O ice interaction, as well as the CO spectroscopic behaviour. Our data suggest that the blueshifted peak can be assigned to CO interacting {\it via} the C atom with dangling H atoms of the water ice, while the redshifted one can actually be the result of CO involved in different reciprocal interactions with the water matrix. We also provide a possible explanation for the lack of the blueshifted peak in astronomical spectra. Our aim is not to provide a full account of the various interstellar ices, but rather to elucidate the sensitivity of the CO spectral features to different water ice environments.