Vacuum-UV spectroscopy of interstellar ice analogs (original) (raw)
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Photoprocessing of astrophysical ice analogs using the Interstellar Astrochemistry Chamber
UV-photodesorption is a plausible non-thermal desorption process in dark clouds, which is required to explain the presence of molecules in the gas phase. Models of ice photoprocessing depend on the vacuum ultraviolet (VUV) absorption cross section of the ice. In the past, gas phase cross section values were used as an approximation due to the lack of reported VUV-absorption cross sections of most molecules present in interstellar ice mantles (with the exception of H 2 O, CO 2 , and NH 3). ISAC is an ultra-high-vacuum (UHV) setup where pure ices composed of H 2 O, CO, CO 2 , CH 3 OH, NH 3 , CH 4 , H 2 S, N 2 , and O 2 were deposited at 8 K. The column density of the ice samples was measured in situ by infrared spectroscopy in transmittance. VUV-absorption 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. H 2 S presents the highest absorption in the 120-160 nm range, while solid N 2 has the lowest VUV-absorption cross section, which is about three orders of magnitude lower than that of other species. Isotopic effects were studied for D 2 O, 13 CO 2 , CD 3 OD, and 15 N 2. Our method allows fast and readily available VUV spectroscopy of ices without the need of using a synchrotron beamline. Photodesorption rates of pure ices, expressed in molecules per absorbed photon, can be derived from our data.
Vacuum-UV absorption spectroscopy of interstellar ice analogues. III. Isotopic effects
Monthly Notices of the Royal Astronomical Society, 2014
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 D 2 O, CD 3 OD, 13 CO 2 , and 15 N 15 N 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 et al. 2013a; Cruz-Diaz et al. 2013b). 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.
The efficiency of photodissociation for molecules in interstellar ices
Monthly Notices of the Royal Astronomical Society, 2018
Processing by interstellar photons affects the composition of the icy mantles on interstellar grains. The rate of photodissociation in solids differs from that of molecules in the gas phase. The aim of this work was to determine an average, general ratio between the photodissociation coefficients for molecules in ice and gas. A 1D astrochemical model was utilized to simulate the chemical composition for a line of sight through a collapsing interstellar cloud core, whose interstellar extinction changes with time. At different extinctions, the calculated column densities of icy carbon oxides and ammonia (relative to water ice) were compared to observations. The latter were taken from literature data of background stars sampling ices in molecular clouds. The best-fitting value for the solid/gas photodissociation coefficient ratio was found to be ≈0.3. In other words, gas-phase photodissociation rate coefficients have to be reduced by a factor of 0.3 before applying them to icy species. A crucial part of the model is the proper inclusion of cosmic ray induced desorption. Observations sampling gas with total extinctions in excess of ≈22 mag were found to be uncorrelated to modelling results, possibly because of grains being covered with non-polar molecules.
Faraday Discussions, 2023
Being a potential process that could explain gas phase abundances of so-called Complex Organic Molecules (COMs) in the cold interstellar medium (ISM), the UV photon-induced desorption from organics-containing molecular ices has been experimentally studied. In this work, we focused on the observation of the photodesorbed products and the measurement of the associated photodesorption yields from pure and mixed molecular ices, each containing organic molecules whose detection has been achieved in the gas phase of the cold ISM, namely formic acid HCOOH and methyl formate HCOOCH 3. Each molecule, in pure ice or in ice mixed with CO or water, was irradiated at 15 K with monochromatic vacuum UV photons in the 7-14 eV range using synchrotron radiation from the SOLEIL synchrotron facility, DESIRS beamline. Photodesorption yields of the intact molecules and of the photoproducts were derived as a function of the incident photon energy. Experiments have revealed that the desorbing species match the photodissociation pattern of each isolated molecule, with little inuence of the kind of ice (pure or mixed in CO or H 2 O-rich environment). For both species, the photodesorption of the intact organics is found negligible in our experimental conditions, resulting in yields typically below 10 −5 ejected molecules per incident photon. The results obtained on HCOOH and HCOOCH 3-containing ices are similar to what has already been found for methanolcontaining ices, but contrast with the case of another complex molecule, CH 3 CN, photodesorption of which has been recently studied. Such experimental results may be linked to the observation of COMs in protoplanetary disks, in which CH 3 CN is commonly observed whereas HCOOH or methanol are detected only in some sources, HCOOCH 3 not being detected at all.
Spectroscopy and processing of interstellar ice analogs
AIP Conference Proceedings, 2006
Recent results from the Raymond and Beverly Sackler Laboratory for Astrophysics on spectroscopy and processing of interstellar ice analogues are summarized. This includes thermal desorption studies of pure, layered and mixed CO, N2 and O2 ices, and infrared spectroscopy and heating of CO-CO2, CO-H2O, CO-HCOOH, CO-CH4 and CO-CH3OH layered and mixed ices. Laboratory data of CO-surface adsorbates show good agreement with the unidentified 2175 cm-1 interstellar feature. Complementary ab initio quantum chemical calculations and molecular dynamics simulations have been performed to provide insight into the gas-grain interactions and interstellar ice processing. This includes the first molecular dynamics study of the photodissociation of water ice and the corresponding photodesorption efficiencies. The relevance of these data in the analysis of astronomical data is emphasized throughout.
Desorption of Hot Molecules from Photon Irradiated Interstellar Ices
The Astrophysical Journal, 2008
We present experimental measurements of photodesorption from ices of astrophysical relevance. Layers of benzene and water ice were irradiated with a laser tuned to an electronic transition in the benzene molecule. The translational energy of desorbed molecules was measured by time-of-flight ( ToF) mass spectrometry. Three distinct photodesorption processes were identified: a direct adsorbate-mediated desorption producing benzene molecules with a translational temperature of around 1200 K, an indirect adsorbate-mediated desorption resulting in water molecules with a translational temperature of around 450 K, and a substrate-mediated desorption of both benzene and water producing molecules with translational temperatures of around 530 and 450 K, respectively. The translational temperature of each population of desorbed molecules is well above the temperature of the ice matrix. The implications for gas-phase chemistry in the interstellar medium are discussed.
Photon- and electron-stimulated desorption from laboratory models of interstellar ice grains
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2010
The nonthermal desorption of water from ice films induced by photon and low energy electron irradiation has been studied under conditions mimicking those found in dense interstellar clouds. Water desorption following photon irradiation at 250 nm relies on the presence of an absorbing species within the H 2 O ice, in this case benzene. Desorption cross sections are obtained and used to derive first order rate coefficients for the desorption processes. Kinetic modeling has been used to compare the efficiencies of these desorption mechanisms with others known to be in operation in dense clouds.
UV-photoprocessing of interstellar ice analogs: New infrared spectroscopic results
Astronomy & Astrophysics, 2003
The physical conditions governing the dense cloud environment are reproduced in a high vacuum experimental setup at low temperature T ≈ 12 K. The accretion and photoprocessing of ices on grain surfaces is simulated by depositing an ice layer on a cold finger, while it is irradiated by ultraviolet (UV) photons. After irradiation the sample is slowly warmed to room temperature; a residue remains, containing the most refractory products of photo-and thermal processing. In this paper we report on the analysis of the residues performed by means of gas chromatography-mass spectrometry (GC-MS). A number of new molecules based on hexamethylenetetramine (HMT, C 6 H 12 N 4 ), the most abundant component of the residues reported here, were detected: methyl-HMT (C 6 H 11 N 4 -CH 3 ), hydroxy-HMT (C 6 H 11 N 4 -OH), methanyl-HMT (C 6 H 11 N 4 -CH 2 OH), amin-aldehyd-HMT (C 6 H 11 N 4 -NH-CHO) and methanyl-aldehyd-HMT (C 6 H 11 N 4 -CHOH-CHO). To the best of our knowledge, this is the first reported synthesis of these molecules. Currently, these are the heaviest identified components of the residue. These species might also be present in the interstellar medium, given that the ice was submitted to high temperatures, of the order of 300 K, and form part of comets. Our work serves as preparation for the ESA-Rosetta mission, which plans to do in situ analysis of the composition of a comet nucleus with the COSAC instrumentation.