Vacuum-UV spectroscopy of interstellar ice analogs

I. Absorption cross-sections of polar-ice molecules^⋆

¹ Centro de Astrobiología, INTA-CSIC, Carretera de Ajalvir, km 4, Torrejón de Ardoz, 28850 Madrid, Spain
e-mail: cruzdga@cab.inta-csic.es; munozcg@cab.inta-csic.es
² Space Sciences Center and Department of Physics and Astronomy, University of Southern California, Los Angeles, CA 90089-1341, USA
³ Department of Physics, National Central University, Jhongli City, 32054 Taoyuan Country, Taiwan

Received: 25 June 2013
Accepted: 15 December 2013

Abstract

Context. The vacuum-UV (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.

Aims. We aim to estimate the VUV absorption cross section of molecular ice components.

Methods. 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.

Results. 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.

Conclusions. 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.