Vacuum-UV spectroscopy of interstellar ice analogs

II. Absorption cross-sections of nonpolar 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 Depeartment 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: 6 September 2013
Accepted: 15 December 2013

Abstract

Context. Dust grains in cold circumstellar regions and dark-cloud interiors at 10−20 K are covered by ice mantles. A nonthermal desorption mechanism is invoked to explain the presence of gas-phase molecules in these environments, such as the photodesorption induced by irradiation of ice due to secondary ultraviolet photons. To quantify the effects of ice photoprocessing, an estimate of the photon absorption in ice mantles is required. In a recent work, we reported the vacuum-ultraviolet (VUV) absorption cross sections of nonpolar molecules in the solid phase.

Aims. The aim was to estimate the VUV-absorption cross sections of nonpolar molecular ice components, including CH₄, CO₂, N₂, and O₂.

Methods. The column densities of the ice samples deposited at 8 K were 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 found that, as expected, solid N₂ has the lowest VUV-absorption cross section, which about three orders of magnitude lower than that of other species such as O₂, which is also homonuclear. Methane (CH₄) ice presents a high absorption near Ly-α (121.6 nm) and does not absorb below 148 nm. Estimating the ice absorption cross sections is essential for models of ice photoprocessing and allows estimating the ice photodesorption rates as the number of photodesorbed molecules per absorbed photon in the ice.