Reggie L. Hudson

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.

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.

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

Hydrogen atom abstraction by methyl radicals in methanol glasses at 15–100 K: evidence for a limiting rate constant below 40 K by quantum-mechanical tunneling☆

Abstract Rate constants for hydrogen atom abstraction by methyl radicals in methanol glasses have been measured from 100 to 15 K. The Arrhenius plot is nonlinear and the reaction rate constant appears to reach a limiting value below 40 K. The results are discussed in terms of simple models for quantum-mechanical tunneling in the solid state at low temperatures. Assuming that the methyl group rotation in methanol brings about a merging of the energy level distribution at the potential barrier, the observation of temperature-independent rate constants below 40 K may be attributable to a freezing out of this rotation such that tunneling occurs only from the zero-point vibrational level.

The N3 Radical as a Discriminator between Ion-irradiated And UV-photolyzed Astronomical Ices

Infrared spectroscopy has been used to show that irradiation of solid N2 and N2-rich ices with 0.8 MeV protons produces the N3 (azide) radical. In contrast, no N3 was observed after solid N2 and N2-rich ices were photolyzed by far-UV photons. Isotopic substitution experiments support the N3 identification, as does an analysis of the reactions occurring in the ices. This is the first documented difference in reaction products between the radiation chemistry and photochemistry of a nonpolar astronomical ice analog. We suggest that this difference in reaction chemistries could be used to identify ion-irradiated ices on interstellar grains and in the outer solar system. Further, N3 might be used as a tracer of solid-phase interstellar N2, which is expected to exist in dark, molecular clouds but is difficult to observe directly. Although the absolute strength of the N3 band in solid N2 is unknown, we estimate that it is at least 100 times greater than that of the fundamental vibration of N2. Subject headings: ISM: molecules — line: formation — line: identification — molecular processes

FAR-IR SPECTRAL CHANGES ACCOMPANYING PROTON IRRADIATION OF SOLIDS OF ASTROCHEMICAL INTEREST

Abstract Far-infrared spectroscopy was used to investigate crystalline-to-amorphous phase changes in irradiated solids. Radiation-induced changes at T ⪕ 77 K were studied in four solids of astrochemical interest: H2O, CH3OH, H2CO, and CO2. Rates of amorphization were compared for H2O and CH3OH. The behavior of each irradiated sample was observed on subsequent warming. Applications of the data to astronomical problems are given.

An Experimental Study of the Sublimation of Water Ice and the Release of Trapped Gases

Abstract The sublimation of Ar/H2O, CO/H2O, and CO2/H2O mixtures condensed at 23–30 K was investigated. The release of the more volatile component of the ice mixture occurred in several discrete temperature regions. Gas release, as monitored by a mass spectrometer, was correlated with changes in the infrared spectrum of the solid ice. The results are in substantial agreement with those of Bar-Nun and co-workers, who studied ice sublimation using mass spectrometry alone, and Sanford and Allamandola, who used infrared spectroscopy alone. Several different concentrations were used for all gas/H2O mixtures, and relative peak intensities during warming were found to depend on the initial gas/H2O ratio. The sublimation of H2O, CO/H2O, and CO2/H2O ices following irradiation by 1-MeV protons was also studied. Irradiation of CO or CO2 containing ices resulted in CO2 or CO release, respectively, in addition to the original species. Some implications for cometary phenomena are given.

Amino Acids from Ion-Irradiated Nitrile-Containing Ices

Solid CH(3)CN and solid H(2)O + CH(3)CN were ion irradiated near 10 K to initiate chemical reactions thought to occur in extraterrestrial ices. The infrared spectra of these samples after irradiation revealed the synthesis of new molecules. After the irradiated ices were warmed to remove volatiles, the resulting residual material was extracted and analyzed. Both unhydrolyzed and acid-hydrolyzed residues were examined by both liquid and gas chromatographic-mass spectral methods and found to contain a rich mixture of products. The unhydrolyzed samples showed HCN, NH(3), acetaldehyde (formed by reaction with background and atmospheric H(2)O), alkyamines, and numerous other compounds, but no amino acids. However, reaction products in hydrolyzed residues contained a suite of amino acids that included some found in carbonaceous chondrite meteorites. Equal amounts of D- and L-enantiomers were found for each chiral amino acid detected. Extensive use was made of (13)C-labeled CH(3)CN to confirm amino acid identifications and discriminate against possible terrestrial contaminants. The results reported here show that ices exposed to cosmic rays can yield products that, after hydrolysis, form a set of primary amino acids equal in richness to those made by other methods, such as photochemistry.

Formation of Interstellar OCS: Radiation Chemistry and IR Spectra of Precursor Ices

Extensive experimental studies have been performed on the solid-state formation of the OCS molecule in proton-irradiated water-free and water-dominated ices containing CO or CO2 as the carbon source and H2S or SO2 as the sulfur source. In each case OCS is readily formed. Production efficiency follows the trends CO > CO2 and H2S > SO2 as C,O- and S-sources, respectively. In water-dominated ices, OCS production appears to be enhanced for CO : H2S reactants. The mechanism of formation of OCS appears to be the reaction of CO with free S atoms produced by fragmentation of the sulfur parent species. While OCS is readily formed by irradiation, it is also the most easily destroyed on continued exposure. In H2O-dominated ices the half-life of H2S, SO2, and OCS is ~2 eV molecule−1, corresponding to ~7 million years in a cold dense interstellar cloud environment processed by cosmic-ray protons. The spectral profile of the ν3 band of OCS is highly dependent on temperature and ice composition, and changes with radiation processing. These effects can be used in theoretical modeling of interstellar infrared (IR) spectra; a laboratory spectrum of irradiated H2O : CO : H2S, warmed to 50 K, provides a good fit to the 2040 cm−1 feature in the W33A spectrum. The identification of OCS in CO2-dominated ices provides a further challenge, due to the overlap of the OCS band with that of CO3 formed from irradiation of the host ice. The two features can be unraveled by a curve-fitting procedure. It is the width of the 2040 cm−1 band that will help observers determine if features identified in CO2-rich ices are due to OCS or to CO3.