CH3OH Ice Research Articles

Formation rates of complex organics in UV irradiated CH3OH-rich ices

(Abridged) Gas-phase complex organic molecules are commonly detected in the warm inner regions of protostellar envelopes. Recent models show that photochemistry in ices followed by desorption may explain the observed abundances. This study aims to experimentally quantify the broad-band UV-induced production rates of complex organics in CH3OH-rich ices at 20-70 K under ultra-high vacuum conditions. The reaction products are mainly identified by RAIRS and TPD experiments. Complex organics are readily formed in all experiments, both during irradiation and during a slow warm-up of the ices to 200 K after the UV lamp is turned off. The relative abundances of photoproducts depend on the UV fluence, the ice temperature, and whether pure CH3OH ice or CH3OH:CH4/CO ice mixtures are used. C2H6, CH3CHO, CH3CH2OH, CH3OCH3, HCOOCH3, HOCH2CHO and (CH2OH)2 are all detected in at least one experiment. The derived product-formation yields and their dependences on different experimental parameters, such as the initial ice composition, are used to estimate the CH3OH photodissociation branching ratios in ice and the relative diffusion barriers of the formed radicals. The experiments show that ice photochemistry in CH3OH ices is efficient enough to explain the observed abundances of complex organics around protostars and that ratios of complex molecules can be used to constrain their formation pathway.

INFRARED SPECTRA AND THERMODYNAMIC PROPERTIES OF CO2/METHANOL ICES

Ices of mixtures of carbon dioxide and methanol have been studied in a range of temperatures relevant for star-forming regions, comets, polar caps of planets and satellites, and other solar system bodies. We have performed temperature-programmed desorption measurements and recorded IR spectra of various types of samples. The presence of two slightly different structures of CO2 is manifest. A distorted CO2 structure is characterized by bandshifts between 5 cm–1 (ν3) and 10 cm–1 (ν2) with respect to normal CO2. If the samples are heated above 130 K, the distorted CO2 sublimates and only the normal structure remains. The latter can stay trapped until the sublimation of crystalline methanol (150 K). The desorption energy (E d ~ 20 kJ mol–1) of CO2 from methanol ice, and the specific adsorption surface area (6 m2 g–1) of amorphous CH3OH ice, have been determined. CO2 does not penetrate into crystalline ice. Whereas the desorption energy is similar to that of CO2/H2O samples, the specific surface of methanol is much smaller than that of amorphous solid water (ASW). The interaction of CO2 molecules with water and methanol is similar but ices of CH3OH are much less porous than ASW. The inclusion of CO2 into previously formed ices containing these two species would take place preferentially into ASW. However, in processes of simultaneous deposition, methanol ice can admit a larger amount of CO2 than water ice. CO2/CH3OH ices formed by simultaneous deposition admit two orders of magnitude more CO2 than sequentially deposited ices. These findings can have direct relevance to the interpretation of observations from protostellar environments (e.g., RAFGL7009S) and comet nuclei.

Molecular Evolution in Collapsing Prestellar Cores. II. The Effect of Grain‐Surface Reactions

The molecular evolution that occurs in collapsing prestellar cores is investigated. To model the dynamics, we adopt the Larson-Penston solution and analogs with slower rates of collapse. For the chemistry, we utilize the new standard model with the addition of deuterium fractionation and grain-surface reactions treated via the modified rate approach. The use of surface reactions distinguishes the present work from our previous model. We find that these reactions efficiently produce H2O, H2CO, CH3OH, N2, and NH3 ices. In addition, the surface chemistry influences the gas-phase abundances in a variety of ways. For example, formation of molecular nitrogen on grain surfaces followed by desorption into the gas enhances the abundance of this gas-phase species and its daughter products N2H+ and NH3. The current reaction network along with the Larson-Penston solution allows us to reproduce satisfactorily most of the molecular column densities and their radial distributions observed in L1544. The agreement tends to worsen with models that include strongly delayed collapse rates. Inferred radial distributions in terms of fractional abundances are somewhat harder to reproduce. In addition to our standard chemical model, we have also run a model with the UMIST gas-phase chemical network. The abundances of gas-phase sulphur-bearing molecules such as CS and CCS are significantly affected by uncertainties in the gas-phase chemical network. In all our models, the column density of N2H+ monotonically increases as the central density of the core increases during collapse from 3 × 104 to 3 × 107 cm-3. Thus, the abundance of this ion can be a probe of evolutionary stage. Molecular D/H ratios in assorted cores are best reproduced in the Larson-Penston picture with the conventional rate coefficients for fractionation reactions. If we adopt the newly measured and calculated rate coefficients, the D/H ratios, especially N2D+/N2H+, become significantly lower than the observed values.

Infrared spectra of proton irradiated ices containing methanol

A set of experimental results on the spectral identification of new species synthesized in irradiated CH 3OH and H 2O+CH 3OH ices is reported. Mass spectroscopy of volatile species released during slow warming gives supporting information on identifications. H 2CO is the dominant volatile species identified in the irradiated ices; CH 4, CO and CO 2 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 H 2O+CH 3OH ice component has a decreasing concentration of CH 3OH towards its outer, most heavily irradiated layers (if independent of all other sources and sinks). The CH 4 CO and CO CO 2 ratios are calculated as a function of irradiation; after 22 eV per molecule, CH 4 CO = 1.96 and CO CO 2 = 1.45 in an H 2O+CH 3OH ice mixture. Infrared spectra of CH 3OH at T < 20 K on amorphous silicate smokes show a predominantly crystalline phase ice. Irradiation of the ice/silicate composite is compared with irradiated CH 3OH on aluminum substrates. Implication for cometary type ices are discussed.

The 3.2-3.6 micron spectra of monoceros R2/IRS-3 and Elias 16

view Abstract Citations (55) References (42) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The 3.2--3.6 Micron Spectra of Monoceros R2/IRS-3 and Elias 16 Sellgren, K. ; Smith, R. G. ; Brooke, T. Y. Abstract We have obtained 3.2-3.6 micron spectra, with a resolution lambda/delta-lambda approximately 750, of the protostar Mon R2/IRS-3 and of Elias 16, a background K giant behind the Taurus molecular cloud. A feature at 3,482 microns (2872/cm), with a full width at half-maximum of 0.09 microns (76/cm), is clearly seen in Mon R2/IRS-3. This feature is not detected in Elias 16. The 3.482 micron feature in Mon R2/IRS-3 is similar to a feature at 3.466-3.478 microns (2875-2885/cm) detected by Allamandola et al. in four protostars and attributed by these authors to a CH stretch in hydrocarbons dominated by sp3-bonded carbon. Neither Mon R2/IRS-3 nor Elias 16 shows absorption at 3.540 microns (2825/cm), which has been detected in two of the four protostars observed by Allamandola et al. and attributed by them to CH3OH ice. Our limit on CH3OH ice toward Elias 16 is compared to models of gas-grain chemistry in dark clouds. Our results confirm those of Allamandola et al. that at this resolution the 3.4 micron absorption due to dust in molecular clouds has very different spectral structure than that due to dust in the diffuse interstellar medium. Publication: The Astrophysical Journal Pub Date: September 1994 DOI: 10.1086/174634 Bibcode: 1994ApJ...433..179S Keywords: Cosmic Dust; Giant Stars; Infrared Astronomy; Infrared Spectra; Interstellar Extinction; Interstellar Matter; Pre-Main Sequence Stars; Carbon; Chemical Bonds; Hydrocarbons; Interstellar Chemistry; Line Spectra; Methyl Alcohol; Molecular Clouds; Astronomy; ISM: DUST; EXTINCTION; INFRARED: ISM: LINES AND BANDS; ISM: MOLECULES; STARS: PRE--MAIN-SEQUENCE full text sources ADS | data products SIMBAD (7)

Total ion desorption yields of H 2O, D 2O, CH 3OH, CD 3OD and water-methanol mixed ices irradiated at 584 Å

Preliminary measurements of the total ion desorption yields of several pure and mixed molecular ices have been carried out using the He I 584 Å resonance radiation as the incident photon beam. The various ice samples were prepared at 77 K in an ultrahigh vacuum system. The total ion desorption yields of H 2O ices, D 2O ices, CH 3OH ices, CD 3OD ices, H 2O-CH 3OH mixed ices and D 2O-CD 3OD mixed ices were determined. Minor isotopic effects on the ion desorption processes of the above-mentioned ices have been observed. There is a clear enhancement in the ion desorption yields of water-methanol mixed ices in comparison with pure ices. Such data are required in the interpretation of the gaseous atmospheres of comets, asteroids, icy planets, satellites and rings, and interstellar icy grains.

Condensation and vaporization studies of CH3OH and NH3 ices: major implications for astrochemistry.

In an extension of previously reported work on ices containing H2O, CO, CO2, SO2, H2S, and H2, we present measurements of the physical and infrared spectral properties of ices containing CH3OH and NH3. The condensation and sublimation behavior of these ice systems is discussed and surface binding energies are presented for all of these molecules. The surface binding energies can be used to calculate the residence times of the molecules on grain surfaces as a function of temperature. It is demonstrated that many of the molecules used to generate radio maps of and probe conditions in dense clouds, for example CO and NH3, will be significantly depleted from the gas phase by condensation onto dust grains. Attempts to derive total column densities solely from radio maps that do not take condensation effects into account may vastly underestimate the true column densities of any given species. Simple CO condensation onto and vaporization off of grains appears to be capable of explaining the observed depletion of gas phase CO in cold, dense molecular cores. This is not the case for NH3, however, where thermal considerations alone predict that all of the NH3 should be condensed onto grains. The fact that some gas phase NH3 is observed indicates that additional desorption processes must be involved. The surface binding energies of CH3OH, in conjunction with this molecule's observed behavior during warm up in H2O-rich ices, is shown to provide an explanation of the large excess of CH3OH seen in many warm, dense molecular cores. The near-infrared spectrum and associated integrated band strengths of CH3OH-containing ice are given, as are middle infrared absorption band strengths for both CH3OH and NH3.

Secondary-electron yields of solar system ICES

The charging dynamics of ice particles in plasmas (e.g. for planetary ring dynamics) is highly dependent upon the secondary-electron yields of the ices. To this end, the secondary-electron yields of H 2O, CO 2, NH 3 (ammonia) and CH 3OH (methanol) ices have been measured in a scanning electron microscope as a function of the electron-beam energy in the 2–30 keV energy range. Estimates are given for the maximum secondary-electron yield Y max of each ice and the energy E max at which this maximum yield occurs and the implications of these estimates are discussed in terms of solar system ices. Based on these results and a general literature review of Y max values for nonconducting materials, it is suggested that a typical range to quote for the secondary-electron yield of ices should be about 1–10. This is much lower than the range of 1–30 that is presently quoted in the space physics community.

Secondary electron yields of solar system ices

The secondary electron yields of H2O, CO2, NH3 (ammonia), and CH3OH (methanol) ices have been measured as a function of electron beam energy in the 2‐ to 30‐keV energy range. The ices were produced on a liquid‐nitrogen‐cooled cold finger and transferred under vacuum to a scanning electron microscope where the yield measurements were made. The imaging capabilities of the scanning electron microscope provide a means of correlating the yield measurements with the morphology of the ices and are also used to monitor charging effects. The yields were determined by measuring the amplified current from a secondary electron detector and calibrating this current signal with the amplified current signal from samples of metals with known secondary electron yields. Each of the measured yields is found to decrease with an increase in energy in the 2‐ to 30‐keV range. Estimates are given for the maximum secondary electron yield Ymax of each ice and the energy at which this maximum yield occurs. Implications for the charging of solar system ice grains are discussed.

Type II Clathrate Hydrate Formation in Cometary Ice Analogs in Vacuo

Clathrate Hydrates can be formed under high vacuum conditions by annealing vapor-deposited amorphous ices of the appropriate composition. When astrophysically significant H2O:CH3OH ices are deposited and annealed, Type II Clathrate Hydrates are formed which can hold up to 6 mole % large guest molecules such as methanol and 12 mole % small guest molecules such as CO2 and CO. The solid state transformation of amorphous mixed molecular ice into crystalline clathrate hydrate and its sublimation at higher temperatures may serve to explain heretofore anomalous mechanical and gas release properties observed in cometary ices and laboratory ice analog experiments.