Search

Cyano-substituted polycyclic aromatic hydrocarbons (CN-PAHs) may contribute to the emission detected in the 7 - 9 $\mu$m (1430 - 1100 cm$^{-1}$) and 11 - 15 $\mu$m (900 - 670 cm$^{-1}$) regions of astronomical IR spectra. Anharmonic quantum chemical computations of 17 CN-PAH isomers for 4 small PAHs and Benzene reveal strong, broad absorption features across the entire 300 - 6200 cm$^{-1}$ (33 - 1.6 $\mu$m) frequency range. In particular, when a FWHM of 15 cm$^{-1}$ is applied, the composite CN-PAH spectrum is almost indistinguishable from the unsubstituted-PAH spectrum. At high resolution, however, the infrared absorption spectra reveal unique, identifiable features of CN-PAHs in the 700 - 950, 1100 - 1300, 2000 - 2500, and 3400 - 3600 cm$^{-1}$ ranges. The in-plane and out-of-plane CH bending vibrational frequencies of CN-PAHs are shifted when comparing isomers and to their unsubstituted counterparts, making their differentiation in mixed laboratory experiments possible. The overall aromatic CH stretch fundamental (2950 - 3200 cm$^{-1}$) and first overtone (5950 - 6200 cm$^{-1}$) regions are relatively unaffected by the cyano-substitution, with changes only to the breadth and intensity of the bands. Detailed spectroscopic data on the normal mode components of each state reported herein provide the means to directly assign future laboratory spectra and to guide direct IR observations of astronomical regions with, e.g., JWST., Comment: Submitted to ACS Earth and Space Chemistry

We present 5-20 micron spectral maps of the reflection nebula NGC2023 obtained with the Infrared Spectrograph SL and SH modes on board the Spitzer Space Telescope which reveal emission from polycyclic aromatic hydrocarbons (PAHs), C60, and H2 superposed on a dust continuum. We show that several PAH emission bands correlate with each other and exhibit distinct spatial distributions revealing a spatial sequence with distance from the illuminating star. We explore the distinct morphology of the 6.2, 7.7 and 8.6 micron PAH bands and find that at least two spatially distinct components contribute to the 7--9 micron PAH emission in NGC2023. We report that the PAH features behave independently of the underlying plateaus. We present spectra of compact oval PAHs ranging in size from C_66 to C_210, determined computationally using density functional theory, and investigate trends in the band positions and relative intensities as a function of PAH size, charge and geometry. Based on the NASA Ames PAH database, we discuss the 7--9 micron components in terms of band assignments and relative intensities. We assign the plateau emission to very small grains with possible contributions from PAH clusters and identify components in the 7--9 micron emission that likely originates in these structures. Based on the assignments and the observed spatial sequence, we discuss the photochemical evolution of the interstellar PAH family as they are more and more exposed to the radiation field of the central star in the evaporative flows associated with the PDRs in NGC2023., Comment: 42 pages, 25 figures, 2 animations, accepted for ApJ

We present 15-20 um spectral maps towards the reflection nebula NGC2023 obtained with the Infrared Spectrograph in short-wavelength, high-resolution mode on board the Spitzer Space Telescope. These spectra reveal emission from PAHs, C60, and H2 superposed on a dust continuum. These emission components exhibit distinct spatial distributions: with increasing distance from the illuminating star, we observe the PAH emission followed by the dust continuum emission and the H2 emission. The C60 emission is located closest to the illuminating star in the south while in the north, it seems to be associated with the H/H2 transition. Emission from PAHs and PAH-related species produce features at 15.8, 16.4, 17.4, and 17.8 um and the 15-18 um plateau. These different PAH features show distinct spatial distributions. The 15.8 um band and 15-18 um plateau correlate with the 11.2 um PAH band and with each other, and are attributed to large, neutral PAHs. Conversely, the 16.4 um feature correlates with the 12.7 um PAH band, suggesting that both arise from species that are favored by the same conditions that favor PAH cations. The PAH contribution to the 17.4 um band is displaced towards the illuminating star with respect to the 11.2 and 12.7 um emission and is assigned to doubly ionized PAHs and/or a subset of cationic PAHs. The spatial distribution of the 17.8 um band suggests it arises from both neutral and cationic PAHs. In contrast to their intensities, the profiles of the PAH bands and the 15-18 um plateau do not vary spatially. Consequently, we conclude that the carrier of the 15-18 um plateau is distinct from that of the PAH bands., Comment: 34 pages, 8 fig. To appear in ApJ (accepted)

This white paper is submitted to the Astronomy and Astrophysics 2010 Decadal Survey (Astro2010)1 Committee on the State of the Profession to emphasize the potential of the Stratospheric Observatory for Infrared Astronomy (SOFIA) to contribute to the training of instrumentalists and observers, and to related technology developments. This potential goes beyond the primary mission of SOFIA, which is to carry out unique, high priority astronomical research. SOFIA is a Boeing 747SP aircraft with a 2.5 meter telescope. It will enable astronomical observations anywhere, any time, and at most wavelengths between 0.3 microns and 1.6 mm not accessible from ground-based observatories. These attributes, accruing from the mobility and flight altitude of SOFIA, guarantee a wealth of scientific return. Its instrument teams (nine in the first generation) and guest investigators will do suborbital astronomy in a shirt-sleeve environment. The project will invest $10M per year in science instrument development over a lifetime of 20 years. This, frequent flight opportunities, and operation that enables rapid changes of science instruments and hands-on in-flight access to the instruments, assure a unique and extensive potential - both for training young instrumentalists and for encouraging and deploying nascent technologies. Novel instruments covering optical, infrared, and submillimeter bands can be developed for and tested on SOFIA by their developers (including apprentices) for their own observations and for those of guest observers, to validate technologies and maximize observational effectiveness., Comment: 10 pages, no figures, White Paper for Astro 2010 Survey Committee on State of the Profession

The mid-IR spectra of six large, irregular PAHs with formulae (C84H24 - C120H36) have been computed using Density Functional Theory (DFT). Trends in the dominant band positions and intensities are compared to those of large, compact PAHs as a function of geometry, size and charge. Irregular edge moieties that are common in terrestrial PAHs, such as bay regions and rings with quartet hydrogens, are shown to be uncommon in astronomical PAHs. As for all PAHs comprised solely of C and H reported to date, mid-IR emission from irregular PAHs fails to produce a strong CCstr band at 6.2 um, the position characteristic of the important, class A astronomical PAH spectra. Earlier studies showed inclusion of nitrogen within a PAH shifts this to 6.2 um for PAH cations. Here we show this band shifts to 6.3 um in nitrogenated PAH anions, close to the position of the CC stretch in class B astronomical PAH spectra. Thus nitrogenated PAHs may be important in all sources and the peak position of the CC stretch near 6.2 um appears to directly reflect the PAH cation to anion ratio. Large irregular PAHs exhibit features at 7.8 um but lack them near 8.6 um. Hence, the 7.7 um astronomical feature is produced by a mixture of small and large PAHs while the 8.6 um band can only be produced by large compact PAHs. As with the CCstr, the position and profile of these bands reflect the PAH cation to anion ratio., Comment: accepted by ApJ

The mid-infrared spectra of large PAHs ranging from C54H18 to C130H28 are determined computationally using Density Functional Theory. Trends in the band positions and intensities as a function of PAH size, charge and geometry are discussed. Regarding the 3.3, 6.3 and 11.2 micron bands similar conclusions hold as with small PAHs. This does not hold for the other features. The larger PAH cations and anions produce bands at 7.8 micron and, as PAH sizes increases, a band near 8.5 micron becomes prominent and shifts slightly to the red. In addition, the average anion peak falls slightly to the red of the average cation peak. The similarity in behavior of the 7.8 and 8.6 micron bands with the astronomical observations suggests that they arise from large, cationic and anionic PAHs, with the specific peak position and profile reflecting the PAH cation to anion concentration ratio and relative intensities of PAH size. Hence, the broad astronomical 7.7 micron band is produced by a mixture of small and large PAH cations and anions, with small and large PAHs contributing more to the 7.6 and 7.8 micron component respectively. For the CH out-of-plane vibrations, the duo hydrogens couple with the solo vibrations and produce bands that fall at wavelengths slightly different than their counterparts in smaller PAHs. As a consequence, previously deduced PAH structures are altered in favor of more compact and symmetric forms. In addition, the overlap between the duo and trio bands may reproduce the blue-shaded 12.8 micron profile., Comment: ApJ, 36 pages, 9 figs

Over the past few decades, NASA missions have revealed that we live in a Universe that is not a hydrogen-dominated, physicist's paradise, but in a molecular Universe with complex molecules directly interwoven into its fabric. These missions have shown that molecules are an abundant and important component of astronomical objects at all stages of their evolution and that they play a key role in many processes that dominate the structure and evolution of galaxies. Closer to home in our galaxy, the Milky Way, they have revealed a unique and complex organic inventory of regions of star and planet formation that may well represent some of the prebiotic roots to life. Astrobiology emerges from the great interest in understanding astrochemical evolution from simple to complex molecules, especially those with biogenic potential and the roles they may play as primordial seeds in the origin of life on habitable worlds. The first part of this talk will highlight how infrared spectroscopic studies of interstellar space, combined with dedicated laboratory simulations, have revealed the widespread presence of complex organics across deep space. The remainder of the presentation will focus on the evolution of these materials and astrobiology.

Great strides have been made in our understanding of interstellar material thanks to advances in infrared astronomy and laboratory astrophysics. Ionized polycyclic aromatic hydrocarbons (PAHs), shockingly large molecules by earlier astrochemical standards, are widespread and very abundant throughout much of the cosmos. In cold molecular clouds, the birthplace of planets and stars, interstellar atoms and molecules freeze onto extremely cold dust and ice particles forming mixed molecular ices dominated by simple species such as water, methanol, ammonia, and carbon monoxide. Within these clouds, and especially in the vicinity of star and planet forming regions, these ices and PAHs are processed by ultraviolet light and cosmic rays forming hundreds of far more complex species, some of biogenic interest. Eventually, these are delivered to primordial planets by comets and meteorites. As these materials are the building blocks of comets and related to carbonaceous micrometeorites, they are likely to be important sources of complex organic materials delivered to habitable planets (including the primordial Earth) and their composition may be related to the origin of life. This talk will focus on the chemical evolution of these cosmic materials and their relevance to astrobiology.

Over 25 years of observations and laboratory work have shown that the mid-IR spectra of a majority of astronomical sources are dominated by emission features near 3.3, 6.2, 7.7, and 11.2 microns, which originate in free polycyclic aromatic hydrocarbon (PAH) molecules. PAHs dominate the mid-IR emission from many galactic and extragalactic objects. As such, this material tracks a wide variety of astronomical processes, making this spectrum a powerful probe of the cosmos Apart from bands in the mid-IR, PAHs have bands spanning the Far-IR (FIR) and emission from these FIR features should be present in astronomical sources showing the Mid-IR PAH bands. However, with one exception, the FIR spectral characteristics are known only for a few neutral small PAHs trapped in salt pellets or oils at room temperature, data which is not relevant to astrophysics. Furthermore, since most emitting PAHs responsible for the mid-IR astronomical features are ionized, the absence of any experimental or theoretical PAH ion FIR spectra will make it impossible to correctly interpret the FIR data from these objects. In view of the upcoming Herschel space telescope mission and SOFIA's FIR airborne instrumentation, which will pioneer the FIR region, it is now urgent to obtain PAH FIR spectra. This talk will present an overview recent advances in the laboratory spectroscopy of PAHs, Highlighting the FIR spectroscopy along with some quantum calculations.

Great strides have been made in our understanding of interstellar material thanks to advances in infrared astronomy and laboratory astrophysics. Ionized polycyclic aromatic hydrocarbons (PAHs), shockingly large molecules by earlier astrochemical standards, are widespread and very abundant throughout much of the Universe. In cold molecular clouds, the birthplace of planets and stars, interstellar molecules freeze onto dust and ice particles forming mixed molecular ices dominated by simple species such as water, methanol, ammonia, and carbon monoxide. Within these clouds, and especially in the vicinity of star and planet forming regions, these ices and PAHs are processed by ultraviolet light and cosmic rays forming hundreds of far more complex species, some of biogenic interest. Eventually, these are delivered to primordial planets by comets and meteorites. Astrochemical evolution, highlights of this field from a chemist's perspective, and the astronomer's infrared toolbox will be reviewed.

The composition of interstellar ice and dust provides insight into the chemical history of the interstellar medium and early solar system. It is now possible to probe this unique and unusual chemistry and determine the composition of these microscopic interstellar particles which are hundreds to many thousands of light years away thanks to substantial progress in two areas: astronomical spectroscopic techniques in the middle-infrared, the spectral region most diagnostic of chemical composition, and laboratory simulations which realistically reproduce the critical conditions in various interstellar environments. High quality infrared spectra of many different astronomical sources, some associated with giant, dark molecular clouds -the birthplace of stars and planets- and others in more tenuous, UV radiation rich regions are now available. The fundamentals of IR spectroscopy and what comparisons of astronomical IR spectra with laboratory spectra of materials prepared under realistic simulated interstellar conditions tell us about the components of these materials is the subject of this talk. These observations have shown that mixed molecular ices comprised of H2O, CH3OH, CO, NH3 and H2CO contain most of the molecular material in molecular clouds and that gas phase, ionized polycyclic aromatic hydrocarbons (PAHs) are widespread and surprisingly abundant throughout most of the interstellar medium.

Tremendous strides have been made in our understanding of interstellar material over the past twenty-five years thanks to significant developments in observational IR astronomy and dedicated laboratory experiments. Twenty-five years ago the composition of interstellar dust was largely guessed at. Today the composition of interstellar dust is reasonably well understood. In the diffuse interstellar medium (ISM) the dust population is mainly comprised of small grains of silicates and amorphous carbon. In dark molecular clouds, the birthplace of stars and planets, these cold refractory dust particles are coated with mixed molecular ices whose composition is reasonably well constrained. Lastly, the signature of carbon-rich polycyclic aromatic hydrocarbons (PAHs), shockingly large molecules by early interstellar chemistry standards, is widespread throughout the Universe. This extraordinary progress has been made possible by the close collaboration of laboratory experimentalists and theoreticians with IR astronomers using groundbased, air-borne, and orbiting telescopes.

In studying various interstellar and solar system ice analogs, we have recently found that upon vacuum ultraviolet photolysis, polycyclic aromatic hydrocarbons (PAHs) frozen in water ice at low temperatures are easily ionized and indefinitely stabilized as trapped ions (Gudipati; Gudipati & Allamandola). Here we report the first experimental study that shows that PAH ionization energy is significantly lowered in PAH/H2O ices, in agreement with recent theoretical work (Woon & Park). The ionization energy (IE) of the PAH studied here, quaterrylene (C40H20, IE = 6.11 eV), is lowered by up to 2.11 eV in water ice. PAH ionization energy reduction in low-temperature water ice substantially expands the astronomical regions in which trapped ions and electrons may be important. This reduction in ionization energy should also hold for other types of trapped species in waterrich interstellar, circumstellar, and solar system ices. Subject headings: ISM: clouds - methods: laboratory - molecular processes - radiation mechanisms: nonthermal -ultraviolet: ISM - ultraviolet: solar system

Organic compounds are synthesized in the interstellar medium and can be delivered to planetary surfaces such as the early Earth, where they mix with endogenous organic mixtures. Some of these compounds are amphiphilic, having polar and non-polar groups on the same molecule. Amphiphilic compounds spontaneously self-assembly into more complex structures such as bimolecular layers, which in turn form closed membranous vesicles. The first forms of cellular life required self-assembled membranes that were likely to be available on the prebiotic Earth. Laboratory simulations show that such vesicles readily encapsulate functional macromolecules, including nucleic acids and polymerases. A goal of future investigations is to fabricate artificial cells as models of the origin of life.

If we find evidence of Life elsewhere in the Solar System it will probably be in form of chemical biomarkers, quintessentially biological molecules that indicate the presence of micro-organisms. While molecules such as amino acids and nucleo-bases might seem to be biomarkers, and alkyl substituted aromatics have been invoked as such, they are not necessarily. These molecules are present in some meteorites and are expected to be present on the surface of other planets even in the absence of life. Understanding the range of non-biological organic molecules which could act as false biomarkers in space is a prerequisite for any reasonable search for true biomarkers on other worlds. Our experiments have shown that some organic molecules in meteorites that appear biological in nature are formed by energetic processing of extraterrestrial ices can account for amino acids, quinones and other functionalized aromatic compounds. In the past, such molecules have been proposed as biomarkers. For example, alkylated aromatics were invoked as biomarkers in the Alan Hills 84001 'Martian meteorite.' When simple organics arrive at the surface of a body like Europa, either from below or from space, how long do they survive and what do they make? How can we distinguish these from real biomarkers?

Polycyclic aromatic hydrocarbons (PAHs) are believed to be the most abundant and widespread class of organic compounds in the universe, having been observed in emission towards energetic regions and absorption towards colder ones.We will present IR spectra of PAHs and their cations in H20 ice measured in the laboratory in the hopes that this will facilitate the detection of these features in the interstellar medium.

Over the past fifteen years, thanks to significant, parallel advancements in observational, experimental, and theoretical techniques, tremendous strides have been made in our understanding of the role aromatic materials play in the interstellar medium (ISM). Twenty years ago, the possible existence of an abundant population of large, carbon-rich molecules in the ISM was unthinkable. Today, the unmistakable spectroscopic signatures of polycyclic aromatic hydrocarbon molecules (PAHs) - shockingly large molecules by the standards of traditional interstellar chemistry - are recognized throughout the Universe. In this paper, we will examine the current state of the interstellar PAH model and its utility as a diagnostic tool to derive insight into the nature of the interstellar PAH population. As an example of this application, we will examine the results of our recent spectroscopic studies of polycyclic aromatic nitrogen heterocycles (PANHs)-PAHs with an atom of nitrogen substituted into the aromatic skeleton-and discuss a possible tracer of such species amongst the interstellar PAH emission bands in the latest observational data.

In situ ultraviolet-visible absorption and emission studies of vacuum ultraviolet (VUV) irradiated water-rich, cosmic ice analogs containing polycyclic aromatic hydrocarbons (PAHs) are described. W V irradiation of 12 K water ices containing the PAHs naphthalene (H2O/C10H8 = 200) and 4-methylpyrene (H2O/C17H12 > 500) readily converts the PAHs into their cation form (PAH(+)). Under these conditions, PAH photoionization is the predominant reaction. These ions are trapped and stored in the ices at temperatures between 10 and 50 K, a temperature domain common to ices throughout interstellar clouds and the solar system. Unlike the approx.15% ionization typical after W V irradiation of PAHs isolated in rare-gas matrices, in water ice, PAH photoionization and storage proceed efficiently and almost quantitatively with a greater than 70% ionization yield. As the temperature is increased from 50 to 150 K, the PAH ion bands slowly diminish as the PAH ions ultimately react to form more complex organic species involving the water host. The chemical, spectroscopic, and physical properties of these ion-rich ices can be important in icy objects such as molecular clouds, comets, and planets. Several astrophysical applications are presented.

The NASA Astrobiology Roadmap provides guidance for research and technology development across the NASA enterprises that encompass the space, Earth, and biological sciences. The ongoing development of astrobiology roadmaps embodies the contributions of diverse scientists and technologists from government, universities, and private institutions. The Roadmap addresses three basic questions: How does life begin and evolve, does life exist elsewhere in the universe, and what is the future of life on Earth and beyond? Seven Science Goals outline the following key domains of investigation: understanding the nature and distribution of habitable environments in the universe, exploring for habitable environments and life in our own solar system, understanding the emergence of life, determining how early life on Earth interacted and evolved with its changing environment, understanding the evolutionary mechanisms and environmental limits of life, determining the principles that will shape life in the future, and recognizing signatures of life on other worlds and on early Earth. For each of these goals, Science Objectives outline more specific high-priority efforts for the next 3-5 years. These 18 objectives are being integrated with NASA strategic planning.

Here we report a study of the optical luminescent properties for a variety of vacuum-ultraviolet (VUV)- irradiated cosmic ice analogs and the complex organic residues produced. Detailed results are presented for the irradiated, mixed molecular ice: H2O:CH3OH:NH3:CO (100:50:1:1), a realistic representation for an interstellar/precometary ice that reproduces all the salient infrared spectral features associated with interstellar ices. The irradiated ices and the room-temperature residues resulting from this energetic processing have remarkable photoluminescent properties in the visible (520-570 nm). The luminescence dependence on temperature, thermal cycling, and VUV exposure is described. It is suggested that this type of luminescent behavior might be applicable to solar system and interstellar observations and processes for various astronomical objects with an ice heritage. Some examples include grain temperature determination and vaporization rates, nebula radiation balance, albedo values, color analysis, and biomarker identification.

Nitriles are one of the most common classes of molecules observed in the gas phase in space, with over a dozen having been positively identified in interstellar and circumstellar environments through the detection of their rotational transitions. Acids, in contrast, are much less common. In this paper we present laboratory data comparing the stability of two structurally related acid-nitrile pairs to ultraviolet (UV) photolytic destruction: acetic acid vs. acetonitrile (CH3-COOH vs. CH3-CN) and glycine vs. aminoacetonitrile (H2N-CH2-COOH vs. H2N-CH2-CN). We find that the nitriles are destroyed ten and five times more slowly (respectively) by UV photolysis than are the corresponding acids. This suggests that whatever their relative formation rates, acids may be less abundant than nitriles in interstellar environments in part because they are more rapidly destroyed by photolysis. The results of this infrared (IR) spectral matrix isolation study indicate that during the lifetime of a typical interstellar cloud, even in its darkest regions, a population of acids in the gas phase will likely be diminished by at least half. Since aminoacetonitrile is a precursor to the amino acid glycine, and far more stable, presolar aminoacetonitrile may be a contributor to the deuterium enriched glycine detected in meteorites. It would clearly be informative to search for aminoacetonitrile (the nitrile corresponding to glycine) in the regions where the amino acid glycine has been reported.

Water-rich, mixed molecular ices and polycyclic aromatic hydrocarbons (PAHs) are common throughout interstellar molecular clouds and the Solar System. Vacuum ultraviolet (VUV) irradiation and particle bombardment of these abiotic ices produces complex organic species, including important biogenic molecules such as amino acids and functionalized PAHs which may have played a role in the origin of life. This ability of such water-rich, oxygen dominated ices to promote production of complex organic species is surprising and points to an important, unusual, but previously overlooked mechanism at play within the ice. Here we report the nature of this mechanism using electronic spectroscopy. VUV-irradiation of PAH/H2O ices leads to an unprecedented and efficient (greater than 70 %) conversion of the neutral PAHs to their cation form (PAH+). Further, these H2O/PAH+ ices are stabile at temperatures below 50 K, a temperature domain common throughout interstellar clouds and the Solar System. Between 50 and 125 K they react to form the complex organics. In view of this, we conclude that charged PAHs and other molecular ions should be common and abundant in many cosmic ices. The chemical, spectroscopic and physical properties of these ion-rich ices can be of fundamental importance for objects as diverse as comets, planets, and molecular clouds and may account for several poorly understood phenomena associated with each of these object classes.

The cosmic deuterium to hydrogen (D/H) ratio is of key importance from a cosmological and stellar evolution perspective since deuterium originates from big-bang nucleosynthesis and is destroyed by stellar thermonuclear reactions. Further, from the interstellar perspective, the galactic distribution of deuterium and the D/H ratio among various molecular species also traces interstellar chemical evolution. Over the past few decades, radio observations have enabled the study of a handful of small, deuterated interstellar species. However, the number of deuterated species detected and environments probed are limited, raising issues of selection effects that hamper generalization and applications to other environments. Infrared spectroscopy of the interstellar medium offers a distinct advantage in this regard as the extent of deuteration of entire chemical families, rather than one species, can be probed. These observations require spaceborne telescopes because the molecular vibrations involving D which produce the strongest IR bands fall in spectral regions which are obscured by terrestrial CO2 absorption. Here we report the tentative detection of the C-D stretching vibration from deuterated interstellar polycyclic aromatic hydrocarbons in the Orion nebula. Since the PAH emission features are widespread and probe many different types of cosmic environments, follow up observations of deuterated PAHs will provide fundamental, far reaching new insight and perspective into galactic and extragalactic processes.

The gas-phase electronic absorption spectra of the naphthalene (C10H8(+)) and acenaphthene (C12H10(+)) cations have been measured in the visible range in a free 10 jet planar expansion in an attempt to collect data in an astrophysically relevant environment. The direct absorption spectra of two out of four bands measured of the gas-phase cold naphthalene cation along with the gas-phase vibronic absorption spectrum of the cold acenaphthene cation are reported for the first time. The study has been carried out using the ultrasensitive and versatile technique of cavity ringdown spectroscopy (CRDS) coupled to a pulsed discharge slit nozzle (PDN). The new CRDS-PDN set up is described and its characteristics are evaluated. The direct-absorption spectra of the PAH ions are discussed and compared to the gas-phase and solid-phase data available in the literature. The analysis of the results show that cold, free flying PAH ions are generated in the argon discharge primarily through soft Penning ionization. This enables the intrinsic band profiles to be measured, a key requirement for astrophysical applications.

Ultraviolet photolysis of various ice mixtures at low temperature and pressure caused the addition of amino (-NH2), methyl (-CH3), methoxy (-OCH3), and cyano (-CN) functional groups to the polycyclic aromatic hydrocarbon (PAH) coronene (C22H12). The implications of these results for interstellar and meteoritic chemistry are discussed. Previously only simple PAH photo-oxidation had been reported. This work represents the first experimental evidence that ice photochemistry may have contributed to aromatics bearing carbon and nitrogen containing side groups that are detected in primitive meteorites and interplanetary dust particles. Furthermore, these results suggest a wider range of modified PAHs should be expected in interstellar lees and materials predating solar system formation.

The optical luminescent properties for a variety of vacuum-ultraviolet (VUV) irradiated cosmic ice analogs and the complex organic residues produced from irradiation might be applicable to Solar System and interstellar observations and processes for various astronomical objects with an ice heritage. Some examples include grain temperature determination and vaporization rates, nebula radiation balance, albedo values, color analysis, and biomarker identification. Detailed results are presented for the mixed molecular ice: H2O:CH3OH:NH3:CO (100:50:1:1), a realistic representation for an interstellar/precometary ice. The irradiated ices and the room-temperature residues resulting from this energetic processing have remarkable photoluminescent properties in the visible (520-570 nm). The luminescence dependence on temperature, thermal cycling, and VUV exposure of the residue is described.

Much of the volatiles in interstellar dense clouds exist in ices surrounding dust grains. Their low temperatures preclude most chemical reactions, but ionizing radiation can drive reactions that produce a suite of new species, many of which are complex organics. The Astrochemistry Lab at NASA-Ames studies the UV radiation processing of interstellar ice analogs to better identify the resulting products and establish links between interstellar chemistry, the organics in meteorites, and the origin of life on Earth. Once identified, the spectral properties of the products can be quantified to assist with the search for these species in space. Of particular interest are findings that UV irradiation of interstellar ice analogs produces molecules of importance in current living organisms, including quiriones, amphiphiles, and amino acids. Quinones play a variety of roles including electron transport and are used by all organisms. Studies show that quinones should be made wherever polycyclic aromatic hydrocarbons are irradiated in interstellar ices (Bernstein et al. 2001). Amphiphiles are also made when mixed molecular ices are irradiated. These amphiphiles self-assemble into membrane-walled vesicles when put in liquid water (Dworkin et al. 2001). Ice irradiation can also produce (Bernstein et al. 2002) and destroy amino acids (Ehrenfreund et al. 2001). Many of the same processes thought to make organics in space should yield products highly enriched in deuterium (D) (Sandford et al. 2001). The high D/H ratios in some interstellar species, and the knowledge that many of the organics in primitive meteorites are D-enriched, suggests that such linkages do exist. By using D as a tracer, we expect to learn more about the different chemical processes occurring in space and their relative importance in the production of different organic compounds and delivery to planetary surfaces.

Comets and carbonaceous micrometeorites may have been significant sources of organic compounds on the early Earth. Ices on grains in interstellar dense molecular clouds contain a variety of simple molecules as well as aromatic molecules of various sizes. While in these clouds the icy grains are processed by ultraviolet light and cosmic radiation which produces more complex organic molecules. We have run laboratory simulations to identify the types of molecules which could have been generated photolytically in pre-cometary ices. Experiments were conducted by forming various realistic interstellar mixed-molecular ices with and without polycyclic aromatic hydrocarbons (PAHs) at approx. 10 K under high vacuum irradiated with UV light from a hydrogen plasma lamp. The residue that remained after warming to room temperature was analyzed by HPLC, and by laser desorption mass spectrometry. The residue contains several classes of compounds which may be of prebiotic significance.