Vacuum-UV spectroscopy of interstellar ice analogs. I. Absorption cross-sections of polar-ice molecules

TL;DR

This study measures the vacuum-UV absorption cross sections of key interstellar ice molecules, providing essential data for modeling ice photoprocessing in space, using a fast, accessible method that improves upon previous gas-phase approximations.

Contribution

The paper reports the first measurements of VUV absorption cross sections for CH3OH, CO, and H2S ices, enhancing the accuracy of interstellar ice models.

Findings

H2S has the highest absorption in 120-160 nm range.

Results agree with previous data for H2O and NH3 ices.

Provides a fast method for VUV spectroscopy of ices.

Abstract

The VUV absorption cross sections of most molecular solids present in interstellar ice mantles with the exception of H2O, NH3, and CO2 have not been reported yet. Models of ice photoprocessing depend on the VUV absorption cross section of the ice to estimate the penetration depth and radiation dose, and in the past, gas phase cross section values were used as an approximation. We aim to estimate the VUV absorption cross section of molecular ice components. Pure ices composed of CO, H2O, CH3OH, NH3, or H2S were deposited at 8 K. The column density of the ice samples was measured in situ by infrared spectroscopy in transmittance. VUV spectra of the ice samples were collected in the 120-160 nm (10.33-7.74 eV) range using a commercial microwave-discharged hydrogen flow lamp. We provide VUV absorption cross sections of the reported molecular ices. Our results agree with those previously reported for H2O and NH3 ices. Vacuum-UV absorption cross section of CH3OH, CO, and H2S in solid phase are reported for the first time. H2S presents the highest absorption in the 120-160 nm range. Our method allows fast and readily available VUV spectroscopy of ices without the need to use a synchrotron beamline. We found that the ice absorption cross sections can be very different from the gas-phase values, and therefore, our data will significantly improve models that simulate the VUV photoprocessing and photodesorption of ice mantles. Photodesorption rates of pure ices, expressed in molecules per absorbed photon, can be derived from our data.