Ammonia, carbon dioxide and the non-detection of the 2152 cm$^{-1}$ CO band

TL;DR

This study investigates why the 2152 cm$^{-1}$ CO band is absent in interstellar ices by examining how NH$_3$ and CO$_2$ incorporation into amorphous solid water affects the spectral feature, revealing the role of polarity and surface interactions.

Contribution

It demonstrates that NH$_3$ more effectively blocks the 2152 cm$^{-1}$ band than CO$_2$, highlighting the impact of molecular polarity and hydrogen bonding on ice spectral features.

Findings

NH$_3$ blocks the dOH more effectively than CO$_2$

Annealing reduces the 2152 cm$^{-1}$ band

Non-detection does not exclude porous ice mantles

Abstract

CO is one of the most abundant ice components on interstellar dust grains. When it is mixed with amorphous solid water (ASW) or located on its surface, an absorption band of CO at 2152 cm$^{-1}$ is always present in laboratory measurements. This spectral feature is attributed to the interaction of CO with dangling-OH bonds (dOH) in ASW. However, this band is absent in observational spectra of interstellar ices. This raises the question whether CO forms a relatively pure layer on top of ASW or is in close contact with ASW, but not via dangling bonds. We aim to determine whether the incorporation of NH$_3$ or CO$_2$ into ASW blocks the dOH and therefore reduces the 2152 cm$^{-1}$ band. We performed laboratory experiments to simulate the layered structure of the ice mantle, that is, we grew CO ice on top of 1) pure ASW, 2) NH$_3$:H$_2$O=10:100 mixed ice, and 3) CO$_2$:H$_2$O=20:100 mixed ice. Infrared spectra were measured to quantify the strength of the 2152 cm$^{-1}$ band. In addition, a second set of experiments were performed to determine how the incorporation of NH$_3$ into ASW affects the dOH band. We found that annealing the ice reduces the 2152 cm$^{-1}$ band and that NH$_3$ blocks the dOH on ASW surface and therefore reduces the 2152 cm$^{-1}$ band more effectively than CO$_2$. We suggest that this difference between NH$_3$ and CO$_2$ can be ascribed to the polarity of the guest molecule (NH$_3$ is a polar species, whereas CO$_2$ is apolar). The polarity implies that the formation of an H-bond between the N atom of ammonia and the dOH is a barrier-less reaction. We also determined the pore surface area of the ice mixtures as a function of the annealing temperature, and found that the nondetection of 2152 cm$^{-1}$ band does not necessarily exclude the possibility of a porous ice mantle.