Interstellar ice analogs: band strengths of H$_2$O, CO$_2$, CH$_3$OH, and NH$_3$ in the far-infrared region

TL;DR

This study measures and calculates the far-infrared band strengths of key interstellar ice analogs, providing essential data for interpreting space telescope observations and understanding ice compositions in space.

Contribution

It offers new experimental measurements and theoretical calculations of far-infrared band strengths for H₂O, CO₂, CH₃OH, and NH₃ ices, filling gaps in existing data.

Findings

Far-infrared band strengths were determined for the ices.

Significant differences were found between experimental and theoretical values.

The results serve as benchmarks for future space observations.

Abstract

We measure the band strengths in the far-infrared region of interstellar ice analogs of astrophysically relevant species, such as H$_2$O, CO$_2$, CH$_3$OH, and NH$_3$, deposited at low temperature (8-10 $\mathrm{K}$), followed by warm-up, to induce amorphous-crystalline phase transitions when relevant. The spectra of pure H$_2$O, NH$_3$, and CH$_3$OH ices have been measured in the near-, mid- and far-infrared spectroscopic regions using the Interstellar Astrochemistry Chamber (ISAC) ultra-high-vacuum setup. In addition, far-infrared spectra of NH$_3$ and CO$_2$ were measured using a different set-up equipped with a bolometer detector. Band strengths in the far-infrared region were estimated using the corresponding near- and mid-infrared values as a reference. We also performed theoretical calculations of the amorphous and crystalline structures of these molecules using solid state computational programs at density functional theory (DFT) level. Vibrational assignment and mode intensities for these ices were predicted. Infrared band strengths in the 25-500 ${\mu}$m range have been determined for the considered ice samples by direct comparison in the near- and mid-infrared regions. Our values were compared to those we calculated from the literature complex index of refraction. We found differences of a factor of two between the two sets of values. The calculated far-infrared band strengths provide a benchmark for interpreting the observational data from future space telescope missions, allowing the estimation of the ice column densities.