Vacuum-UV spectroscopy of interstellar ice analogs

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 CH 4 , CO 2 , N 2 , and O 2 . 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 N 2 has the lowest VUV-absorption cross section, which about three orders of magnitude lower than that of other species such as O 2 , which is also homonuclear. Methane (CH 4 ) 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.

Methane ice photochemistry and kinetic study using laser desorption time-of-flight mass spectrometry at 20 K

“…37 The solid phase CH 4 absorption spectra slightly shift to shorter wavelengths compared to the gas phase, 38 and present three non resolved absorptions near Lyman-a. 37 The solid phase CH 4 absorption spectra slightly shift to shorter wavelengths compared to the gas phase, 38 and present three non resolved absorptions near Lyman-a.…”

The importance of OH radical–neutral low temperature tunnelling reactions in interstellar clouds using a new model

“…The rate constants for photodissociation on surfaces are assumed to be the same as in the gas phase. Recent work shows, however, that this assumption need not be accurate for all species [44][45][46]. We have added direct photodesorption by external UV photons and by cosmic ray generated UV photons to this network following recent laboratory measurements [47][48][49], which show that photodesorption with efficiencies near 10 −3 per UV photon can occur by a variety of mechanisms when UV photons strike ices of assorted species.…”

UV photoprocessing of NH3 ice: photon-induced desorption mechanisms

“…; Öberg et al (2009b)), probably due to their low VUV-absorption cross section (Cruz-Díaz et al (2014b)). During irradiation of a pure NH3 ice, a photodesorption yield similar to that of NH3 molecules was measured for N2 molecules, for a fluence equivalent to that experienced by ice mantles during the expected lifetime of dense clouds (Y pd (N2) = 1.7 +1.7 −0.9 x 10 −3 molecules incident photon ), although photodesorption yields up to four times higher were measured for higher fluences.…”