Robert F. Ferrante

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.

Infrared Spectra and Optical Constants of Nitrile Ices Relevant to Titan's Atmosphere

Spectra and optical constants of nitrile ices known or suspected to be in Titans atmosphere are presented from 2.0 to 333.3 μm (~5000-30 cm–1). These results are relevant to the ongoing modeling of Cassini CIRS observations of Titans winter pole. Ices studied are: HCN, hydrogen cyanide; C2N2, cyanogen; CH3CN, acetonitrile; C2H5CN, propionitrile; and HC3N, cyanoacetylene. For each of these molecules, we also report new cryogenic measurements of the real refractive index, n, determined in both the amorphous and crystalline phases at 670 nm. These new values have been incorporated into our optical constant calculations. Spectra were measured and optical constants were calculated for each nitrile at a variety of temperatures, including, but not limited to, 20, 35, 50, 75, 95, and 110 K, in both the amorphous phase and the crystalline phase. This laboratory effort used a dedicated FTIR spectrometer to record transmission spectra of thin-film ice samples. Laser interference was used to measure film thickness during condensation onto a transparent cold window attached to the tail section of a closed-cycle helium cryostat. Optical constants, real (n) and imaginary (k) refractive indices, were determined using Kramers-Kronig analysis. Our calculation reproduces the complete spectrum, including all interference effects.

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.

RADIATION PRODUCTS IN PROCESSED ICES RELEVANT TO EDGEWORTH-KUIPER-BELT OBJECTS

Near the inner edge of the Edgeworth-Kuiper Belt (EKB) are Pluto and Charon, which are known to have N2- and H2O-dominated surface ices, respectively. Such non-polar and polar ices, and perhaps mixtures of them, also may be present on other trans-Neptunian objects. Pluto, Charon, and all EKB objects reside in a weak, but constant UV-photon and energetic ion radiation environment that drives chemical reactions in their surface ices. Effects of photon and ion processing include changes in ice composition, volatility, spectra, and albedo, and these have been studied in a number of laboratories. This paper focuses on ice processing by ion irradiation and is aimed at understanding the volatiles, ions, and residues that may exist on outer solar system objects. We summarize radiation chemical products of N2-rich and H2O-rich ices containing CO or CH4, including possible volatiles such as alcohols, acids, and bases. Less-volatile products that could accumulate on EKB objects are observed to form in the laboratory from acid-base reactions, reactions promoted by warming, or reactions due to radiation processing of a relatively pure ice (e.g., CO → C3O2). New IR spectra are reported for the 1–5 µm region, along with band strengths for the stronger features of carbon suboxide, carbonic acid, the ammonium and cyanate ions, polyoxymethylene, and ethylene glycol. These six materials are possible contributors to EKB surfaces, and will be of interest to observers and future missions.

Infrared spectra of crystalline phase ices condensed on silicate smokes at T less than 20 K

Infrared spectra of H2O, CH3OH, and NH3 condensed at T less than 20 K on amorphous silicate smokes reveal that predominantly crystalline phase ice forms directly on deposit. Spectra of these molecules condensed on aluminum substrates at T less than 20 K indicate that amorphous phase ice forms. On aluminum, crystalline phase H2O and CH3OH are formed by annealing amorphous deposits to 155 K and 130 K, respectively (or by direct deposit at these temperatures); crystalline NH3 is formed by direct deposit at 88 K. Silicate smokes are deposited onto aluminum substrates by evaporation of SiO solid or by combustion of SiH4 with O2 in flowing H2 followed by vapor phase nucleation and growth. Silicate smokes which are oxygen-deficient may contain active surface sites which facilitate the amorphous-to-crystalline phase transition during condensation. Detailed experiments to understand the mechanism are currently in progress. The assumption that amorphous phase ice forms routinely on grains at T less than 80 K is often used in models describing the volatile content of comets or in interpretations of interstellar cloud temperatures. This assumption needs to be reexamined in view of these results.

Laboratory IR Studies and Astrophysical Implications of C2H2-Containing Binary Ices

Studies of molecular hot cores and protostellar environments have shown that the observed abundance of gas-phase acetylene (C2H2) cannot be matched by chemical models without the inclusion of C2H2 molecules subliming from icy grain mantles. Searches for infrared (IR) spectral features of solid-phase acetylene are under way, but few laboratory reference spectra of C2H2 in icy mixtures, which are needed for spectral fits to observational data, have been published. Here, we report a systematic study of the IR spectra of condensed-phase pure acetylene and acetylene in ices dominated by carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), and water (H2O). We present new spectral data for these ices, including band positions and intrinsic band strengths. For each ice mixture and concentration, we also explore the dependence of acetylenes ν5-band position (743 cm–1, 13.46 μm) and FWHM on temperature. Our results show that the ν5 feature is much more cleanly resolved in ices dominated by non-polar and low-polarity molecules, specifically CO, CO2, and CH4, than in mixtures dominated by H2O-ice. We compare our laboratory ice spectra with observations of a quiescent region in Serpens.

IR spectra and properties of solid acetone, an interstellar and cometary molecule

Mid-infrared spectra of amorphous and crystalline acetone are presented along with measurements of the refractive index and density for both forms of the compound. Infrared band strengths are reported for the first time for amorphous and crystalline acetone, along with IR optical constants. Vapor pressures and a sublimation enthalpy for crystalline acetone also are reported. Positions of 13C-labeled acetone are measured. Band strengths are compared to gas-phase values and to the results of a density-functional calculation. A 73% error in previous work is identified and corrected.