Vacuum-UV absorption spectroscopy of interstellar ice analogues. III. Isotopic effects

TL;DR

This study measures the vacuum-ultraviolet absorption cross-sections of heavy isotopologues in icy dust grain analogs at 8 K, providing crucial data to improve models of interstellar ice photochemistry and photodesorption processes.

Contribution

First measurements of solid-phase VUV absorption spectra of heavy isotopologues in interstellar ice analogs, enhancing the accuracy of astrophysical models.

Findings

VUV spectra of heavy isotopologues differ from light ones and gas phase spectra.

Data significantly improve models of VUV absorption in interstellar ices.

Implications for more accurate estimation of photodesorption rates.

Abstract

This paper reports the first measurements of solid-phase vacuum-ultraviolet (VUV) absorption cross-sections of heavy isotopologues present in icy dust grain mantles of dense interstellar clouds and cold circumstellar environments. Pure ices composed of D2O, CD3OD, 13CO2, and 15N15N were deposited at 8 K, a value similar to the coldest dust temperatures in space. 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 as the VUV source. Prior to this work, we have recently submitted a similar study of the light isotopologues (Cruz-Diaz, Mu\~noz Caro and Chen). The VUV spectra are compared to those of the light isotopologues in the solid phase, and to the gas phase spectra of the same molecules. Our study is expected to improve very significantly the models that estimate the VUV absorption of ice mantles in space, which have often used the available gas phase data as an approximation of the absorption cross sections of the molecular ice components. We will show that this work has also important implications for the estimation of the photodesorption rates per absorbed photon in the ice.