A Systematic Study on the Absorption Features of Interstellar Ices in
  Presence of Impurities
  Spectroscopic studies play a key role in the identification and analysis of
interstellar ices and their structure. Some molecules have been identified
within the interstellar ices either as pure, mixed, or even as layered
structures. Absorption band features of water ice can significantly change with
the presence of different types of impurities (CO, CO2, CH3OH, H2CO, etc.). In
this work, we carried out a theoretical investigation to understand the
behavior of water band frequency, and strength in the presence of impurities.
The computational study has been supported and complemented by some infrared
spectroscopy experiments aimed at verifying the effect of HCOOH, NH3 , and CH3
OH on the band profiles of pure H2O ice. Specifically, we explored the effect
on the band strength of libration, bending, bulk stretching, and free-OH
stretching modes. Computed band strength profiles have been compared with our
new and existing experimental results, thus pointing out that vibrational modes
of H2O and their intensities can change considerably in the presence of
impurities at different concentrations. In most cases, the bulk stretching mode
is the most affected vibration, while the bending is the least affected mode.
HCOOH was found to have a strong influence on the libration, bending, and bulk
stretching band profiles. In the case of NH3, the free-OH stretching band
disappears when the impurity concentration becomes 50%. This work will
ultimately aid a correct interpretation of future detailed spaceborne
observations of interstellar ices by means of the upcoming JWST mission.
