Marla H. Moore

Far-infrared spectral studies of phase changes in water ice induced by proton irradiation

Changes in the FIR spectrum of crystalline and amorphous water ice as a function of temperature are reported. The dramatic differences between the spectra of these ices in the FIR are used to examine the effect of proton irradiation on the stability of the crystalline and amorphous ice phases from 13 to 77 K. In particular, the spectra near 13 K show interconversion between the amorphous and crystalline ice phases beginning at doses near 2 eV/molecule and continuing cyclically with increased dose. The results are used to estimate the stability of irradiated ices in astronomical environments.

Photodestruction of Relevant Interstellar Molecules in Ice Mixtures

UV photodestruction of some interstellar molecules is studied in different kinds of ices. CH4 ,C H 3OH, NH3 ,C O2, CO, and HNCO are photolyzed as pure ices, or mixed with water or molecular nitrogen, at about 10 K. The destruction cross sections of these molecules are estimated for use in photochemical models of interstellar ices. We show that the destruction rate depends on the ice in which the studied compound is embedded. Subject Headings: astrochemistry — ISM: molecules — methods: laboratory — molecular processes — ultraviolet: ISM

Infrared study of ion-irradiated N2-dominated ices relevant to Triton and Pluto: formation of HCN and HNC

Infrared spectra and radiation chemical behavior of N2-dominated ices relevant to the surfaces of Triton and Pluto are presented. This is the first systematic IR study of proton-irradiated N2-rich ices containing CH4 and CO. Experiments at 12 K show that HCN, HNC, and diazomethane (CH2N2) form in the solid phase, along with several radicals. NH3 is also identified in irradiated N2 + CH4 and N2 + CH4 + CO. We show that HCN and HNC are made in irradiated binary ice mixtures having initial N2/CH4 ratios from 100 to 4, and in three-component mixtures have an initial N2/(CH4 + CO) ratio of 50. HCN and HNC are not detected in N2-dominated ices when CH4 is replaced with C2H6, C2H2, or CH3OH. The intrinsic band strengths of HCN and HNC are measured and used to calculate G(HCN) and G(HNC) in irradiated N2 + CH4 and N2 + CH4 + CO ices. In addition, the HNC/HCN ratio is calculated to be ∼1 in both icy mixtures. These radiolysis results reveal, for the first time, solid-phase synthesis of both HCN and HNC in N2-rich ices containing CH4. We examine the evolution of spectral features due to acid–base reactions (acids such as HCN, HNC, and HNCO and a base, NH3) triggered by warming irradiated ices from 12 K to 30–35 K. We identify anions (OCN−, CN−, and N3−) in ices warmed to 35 K. These ions are expected to form and survive on the surfaces of Triton and Pluto. Our results have astrobiological implications since many of these products (HCN, HNC, HNCO, NH3, NH4OCN, and NH4CN) are involved in the syntheses of biomolecules such as amino acids and polypeptides.

Mid- and far-infrared spectroscopic studies of the influence of temperature, ultraviolet photolysis and ion irradiation on cosmic-type ices.

Infrared (IR) studies of laboratory ices can provide information on the evolution of cosmic-type ices as a function of different simulated space environments involving thermal, ultraviolet (UV), or ion processing. Laboratory radiation experiments can lead to the formation of complex organic molecules. However, because of our lack of knowledge about UV photon and ion fluxes, and exposure lifetimes, it is not certain how well our simulations represent space conditions. Appropriate laboratory experiments are also limited by the absence of knowledge about the composition, density, and temperature of ices in different regions of space. Our current understanding of expected doses due to UV photons and cosmic rays is summarized here, along with an inventory of condensed-phase molecules identified on outer solar system surfaces, comets and interstellar grains. Far-IR spectra of thermally cycled H2O are discussed since these results reflect the dramatic difference between the amorphous and crystalline phases of H2O ice, the most dominant condensed-phase molecule in cosmic ices. A comparison of mid-IR spectra of products in proton-irradiated and UV-photolyzed ices shows that few differences are observed for these two forms of processing for the simple binary mixtures studied to date. IR identification of radiation products and experiments to determine production rates of new molecules in ices during processing are discussed. A new technique for measuring intrinsic IR band strengths of several unstable molecules is presented. An example of our laboratory results applied to Europa observations is included.

Studies of proton irradiated H2O + CO2 and H2O + CO ices and analysis of synthesized molecules

Infrared spectra of H2O + CO2 and H2O + CO ices before and after proton irradiation showed that a major reaction in both mixtures was the interconversion of CO2 ⇌ CO. Radiation synthesized organic compounds such as carbonic acid were identified in the H2O + CO2 ice. Different chemical pathways dominate in the H2O + CO ice in which formaldehyde, methanol, ethanol, and methane were identified. Sublimed material was also analyzed using a mass spectrometer. Implications of these results are discussed in reference to comets.

Carbamic acid: molecular structure and IR spectra

Infrared absorption spectra of mixed H2O, NH3 and 12CO2/13CO2 ices subjected to 1 MeV proton irradiation were investigated. The results of analyses of the spectra suggest formation of carbamic acid at low temperatures. The stability of this compound in the solid phase is attributed to intermolecular hydrogen bonding of the zwitter-ion (NH3+ COO-) structure.

THE FORMATION OF CYANATE ION (OCN~) IN INTERSTELLAR ICE ANALOGS

Although the 2165 cm~1 (4.62 km) ii XCN ˇˇ IR absorption in interstellar ices was —rst detected over 20 years ago, its assignment has remained controversial, and its mode of formation has seldom been studied. Here we report an extensive laboratory investigation of this bands formation in interstellar ice analogs. Ices with known or suspected interstellar molecules were proton-irradiated at 15¨25 K to simu- late interstellar energetic processing, and their IR spectra were recorded. Reactions for irradiated mix- tures showing an XCN spectral band have been developed based on results with chemically related systems and with over 60 ices examined here. Combined with previous work, our new results leave no doubt that the band produced in the laboratory is due to OCN~, the cyanate anion. Tests of the reac- tions leading to OCN~ are described, and independent methods of producing OCN~ are reported. The results of all of these new experiments help reveal the chemistry underlying this ions formation and establish some of the conditions under which OCN~ might be found in interstellar ices. They also show that energetic processing is an efficient way to produce OCN~ in interstellar ices and that temperature increases to promote acid-base chemistry are unnecessary. Subject headings: ISM: moleculesline: formationline: identi—cationmolecular processes

Studies of proton-irradiated SO2 at low temperatures: Implications for lo

Abstract The infrared absorption spectrum from 3.3 to 27 μm (3030-370 cm − ) of SO 2 ice films has been measured at 20 and 88°K before and after 1-MeV proton irradiation. The radiation flux was chosen to simulate the estimated flux of Jovian magnetospheric 1-MeV protons incident on Io. After irradiation, SO 3 is identified as the dominant molecule synthesized in the SO 2 ice. This is also the case after irradiation of composite samples of SO 2 with sulfur, or disulfites. Darkening was observed in irradiated SO 2 ice and in irradiated S 8 pellets. Photometric and spectral measurements of the thermoluminescence of irradiated SO 2 have been made during warming. The spectrum appears as a broad band with a maximum at 4450 A. Analysis of the luminescence data suggests that, at Ionian temperatures, irradiated SO 2 ice would not be a dominant contributor to posteclipse brightening phenomena. After warming to room temperature, a form of SO 3 remains along with a sulfate and S 8 . Based on these experiments, it is reasonable to propose that small amounts of SO 3 may exist on the surface of Io as a result of irradiation synthesis in SO 2 frosts.

Infrared spectra of proton irradiated ices containing methanol

A set of experimental results on the spectral identification of new species synthesized in irradiated CH3OH and H2O+CH3OH ices is reported. Mass spectroscopy of volatile species released during slow warming gives supporting information on identifications. H2CO is the dominant volatile species identified in the irradiated ices; CH4, CO and CO2 are also formed. During warming the ice evolves into a residual film near 200 K whose features are similar to those of ethylene glycol along with a CO bonded molecular group. Irradiation simulates expected cosmic ray processing of ices in comets stored in the Oort cloud region for 4.6 billion years. Results support the idea that a comet originally containing an H2O+CH3OH ice component has a decreasing concentration of CH3OH towards its outer, most heavily irradiated layers (if independent of all other sources and sinks). The CH4CO and COCO2 ratios are calculated as a function of irradiation; after 22 eV per molecule, CH4CO = 1.96 and COCO2 = 1.45 in an H2O+CH3OH ice mixture. Infrared spectra of CH3OH at T < 20 K on amorphous silicate smokes show a predominantly crystalline phase ice. Irradiation of the ice/silicate composite is compared with irradiated CH3OH on aluminum substrates. Implication for cometary type ices are discussed.

Energetic processing of laboratory ice analogs: UV photolysis versus ion bombardment

We have the ability to perform both ultraviolet (UV) photolysis (primarily Lyman-a photons, average E ≃ 10.2 eV per photon) and ion irradiation (protons, E = 0.8 MeV) in the same experimental setup, with ices created under identical conditions. Here we present recent results on the UV and ion processing of ice mixtures at 18 K of the composition H 2 O + CO 2 + CH 3 OH (1:1:1) and H 2 O + CO 2 + CH 4 (1:1:1). H 2 O, CH 3 OH, CH 4 , and CO 2 are all major components of ices in most astrophysical environments (whether interstellar, cometary, or planetary). Identifications and formation rates of products were measured. Results for photolyzed and irradiated ices are contrasted. We find that similar chemical products are observed in both cases and that rates of formation are equivalent for most of the major products.