Andrew Mattioda

THE NASA AMES POLYCYCLIC AROMATIC HYDROCARBON INFRARED SPECTROSCOPIC DATABASE: THE COMPUTED SPECTRA

The astronomical emission features, formerly known as the unidentified infrared bands, are now commonly ascribed to polycyclic aromatic hydrocarbons (PAHs). The laboratory experiments and computational modeling done at the NASA Ames Research Center to create a collection of PAH IR spectra relevant to test and refine the PAH hypothesis have been assembled into a spectroscopic database. This database now contains over 800 PAH spectra spanning 2-2000 μm (5000-5 cm–1). These data are now available on the World Wide Web at www.astrochem.org/pahdb. This paper presents an overview of the computational spectra in the database and the tools developed to analyze and interpret astronomical spectra using the database. A description of the online and offline user tools available on the Web site is also presented.

THE NASA AMES PAH IR SPECTROSCOPIC DATABASE VERSION 2.00: UPDATED CONTENT, WEB SITE, AND ON(OFF)LINE TOOLS

A significantly updated version of the NASA Ames PAH IR Spectroscopic Database, the first major revision since its release in 2010, is presented. The current version, version 2.00, contains 700 computational and 75 experimental spectra compared, respectively, with 583 and 60 in the initial release. The spectra span the 2.5-4000 μm (4000-2.5 cm-1) range. New tools are available on the site that allow one to analyze spectra in the database and compare them with imported astronomical spectra as well as a suite of IDL object classes (a collection of programs utilizing IDLs object-oriented programming capabilities) that permit offline analysis called the AmesPAHdbIDLSuite. Most noteworthy among the additions are the extension of the computational spectroscopic database to include a number of significantly larger polycyclic aromatic hydrocarbons (PAHs), the ability to visualize the molecular atomic motions corresponding to each vibrational mode, and a new tool that allows one to perform a non-negative least-squares fit of an imported astronomical spectrum with PAH spectra in the computational database. Finally, a methodology is described in the Appendix, and implemented using the AmesPAHdbIDLSuite, that allows the user to enforce charge balance during the fitting procedure.

Polycyclic Aromatic Hydrocarbon Emission in the 15-21 Micron Region

Observations from the Spitzer Space Telescope have drawn attention to spectroscopic structure longward of 15 μm that is associated with objects showing prominent unidentified infrared (UIR) bands in the mid-IR. If polycyclic aromatic hydrocarbons (PAHs) are indeed responsible for the UIR features, longer wavelength emission arising from out-of-plane PAH skeletal vibrations is required. Here we compare some of the Spitzer spectra with spectra from the Infrared Space Observatory and analyze these data in terms of the PAH model utilizing the spectra of neutral PAHs from the Ames PAH IR spectral database. The 14-21 μm emission spectra from the H II region S106, the young stellar object CD -42°11721, the reflection nebula NGC 7023, and the H2 ridge in LkHα 234 are presented. We show that while the emission in this region can be quite variable, the bulk of these variations can be accommodated by variations in PAH population.

Experimental Near-Infrared Spectroscopy of Polycyclic Aromatic Hydrocarbons between 0.7 and 2.5 μm

The near-infrared (NIR) spectra and absolute band strengths of 27 polycyclic aromatic hydrocarbon (PAH) cations and anions ranging in size from C14H10 to C50H22 are reported. The spectra of all the ionized PAHs we have studied to date have strong, broad absorption bands in the NIR. This work shows that ionized PAHs have significant absorption bands at wavelengths longer than predicted by the current astronomical models that consider PAHs in their treatment of the radiation balance of the interstellar medium. Two implications are (1) that ionized/open-shell interstellar PAHs should add weak, broadband structure to the NIR portion of the interstellar extinction curve and (2) that UV-poor radiation fields can pump the PAH emission bands, provided ionized/open-shell PAHs are present.

The ultraviolet to near-infrared optical properties of polycyclic aromatic hydrocarbons : A semiempirical model

Interstellar polycyclic aromatic hydrocarbon (PAH) infrared emission features represent an important and unique diagnostic tool of the chemical and physical conditions throughout the universe. However, one challenge facing the widely accepted PAH emission model has been the detection of infrared features in regions of low UV flux. Using recently published laboratory near-infrared (NIR) PAH ion absorption data measured in our laboratory, we build on previous models for PAH ion absorption in the UV-visible to extrapolate a new model that incorporates PAH ion absorption in the NIR. This model provides a basis for comparing the relative energy absorption of PAH ions in the UV-visible and NIR regions for a wide variety of stellar types. This model demonstrates that the radiation from late-type stars can pump the mid-IR PAH features.

Near- and Mid-Infrared Laboratory Spectra of PAH Cations in Solid H2O

Polycyclic aromatic hydrocarbons (PAHs) have been observed in absorption along lines of sight toward embedded protostars. In such cold space environments PAHs should condense into H2O-rich ice mantles at low temperature and be exposed to ionizing radiation. In this paper we present the first infrared spectra of PAH cations in solid H2O generated under conditions that resemble dense molecular clouds. After exposing PAHs in solid H2O at 15 K to low doses of UV radiation, we have observed both the vibrational absorptions of PAH cations in the mid-IR and electronic transitions in the near-IR. The PAHs observed as ions in solid H2O were naphthalene, anthracene, phenanthrene, benzo[k]fluoranthene, and benzo[ghi]perylene. Peak positions, strengths, and temperature dependence are reported for the detected ion bands, and their astrophysical significance is discussed. These laboratory measurements suggest that absorption bands of PAH cations in H2O ice may be observable by astronomers in the near- and mid-infrared.

Laboratory Infrared Spectra of Polycyclic Aromatic Nitrogen Heterocycles: Quinoline and Phenanthridine in Solid Argon and H2O

Polycyclic aromatic hydrocarbons (PAHs) are common throughout the universe. Their detection and identification are based on the comparison of IR observations with laboratory spectra. Polycyclic aromatic nitrogen heterocycles(PANHs)areheterocyclicaromatics,i.e.,PAHswith carbonatoms replacedby anitrogenatom.These moleculesshouldbepresentintheinterstellarmedium, buthavereceivedrelativelylittleattention.WepresentmidIR spectra of two PANHs, quinoline (C9H7N) and phenanthridine (C13H9N), isolated in solid argon and frozen in solid H2O at 15 K, conditions yielding data directly comparable to astronomical observations. Quinoline and phenanthridine have been detected in meteorite extracts, and in general these nitrogen heterocycles are of astrobiological interest, since this class of molecules includes nucleobases, basic components of our nucleic acids. In contrast to simple PAHs, which do not interact strongly with solid H2O, the nitrogen atoms in PANHs are potentiallycapableofhydrogenbonding withH2O.WhereastheIRspectrumofphenanthridineinH2Oissimilarto that of the same compound isolated in an argon matrix, quinoline absorptions shift up to 16 cm � 1 (0.072 � m) between argon and H2O. Thus, astronomers will not always be able to rely on IR band positions of matrix-isolated PANHstocorrectlyinterprettheabsorptionsofPANHsfrozeninH2Oicegrains.Furthermore,ourdata suggestthat relative band areas also vary, so determining column densities to better than a factor of 3 will require knowledge of the matrix in which the PANH is embedded and laboratory studies of relevant samples. Subject headingg infrared:ISM — ISM:clouds — ISM:linesandbands — ISM:molecules — line:formation — molecular data

Photochemistry of polycyclic aromatic hydrocarbons in cosmic water ice I. Mid-IR spectroscopy and photoproducts

Context. Polycyclic aromatic hydrocarbons (PAHs) are known to be abundantly present in photon-dominated regions (PDRs), as evidenced by their ubiquitous mid-IR emission bands. Towards dense clouds, however, their IR emission bands are strongly suppressed. It is here where molecules are known to reside on very cold grains (T ≤ 30 K) in the form of interstellar ices. Therefore, it is likely that non-volatile species, such as PAHs, also freeze out on grains. Such icy grains act as catalytic sites and, upon vacuum ultraviolet (VUV) irradiation, chemical reactions are initiated. In the study presented here, these reactions and the resulting photoproducts are investigated for PAH containing water ices. Aims. The aim of this work is to monitor vacuum ultraviolet induced chemical reactions of PAHs in cosmic ice through their IR signatures, to characterize the families of species formed in these reactions, and to apply the results to astronomical observations. Methods. Mid-infrared Fourier transform absorption spectroscopic measurements ranging from 6500 to 450 cm −1 are performed on freshly deposited and vacuum ultraviolet processed PAH containing cosmic H2O ices at low temperatures. Results. The mid-IR spectroscopy of anthracene, pyrene and benzo[ghi]perylene containing H2O ice is reported. Band strengths of the neutral PAH modes in H2O ice are derived. Additionally, spectra of vacuum ultraviolet processed PAH containing H2O ices are presented. These spectra are compared to spectra measured in VUV processed PAH:argon matrix isolation studies. It is concluded that the parent PAH species is ionized in H2O ice and that other photoproducts, mainly more complex PAH derivatives, also form. The importance of PAHs and their PAH:H2O photoproducts in astronomical mid-infrared spectroscopic studies, in particular in the 5−8 μm region, is discussed. As a test-case, the VUV photolyzed PAH:H2O laboratory spectra are compared to a high resolution ISOSWS spectrum of the high-mass embedded protostar W33A and to a Spitzer spectrum of the low-mass Young Stellar Object (YSO) RNO 91. For these objects, an upper limit of 2–3% with respect to H2O ice is derived for the contribution of PAHs and PAH:H2O photoproducts to the absorbance in the 5−8 μm region towards these objects.

Near-Infrared Spectroscopy of Nitrogenated Polycyclic Aromatic Hydrocarbon Cations from 0.7 to 2.5 μm

The near-infrared (NIR) spectra and absolute band strengths of 10 nitrogenated polycyclic aromatic hydrocarbon (PANH) radical cations isolated in an argon matrix are presented and compared with the spectra of their parent polycyclic aromatic hydrocarbon (PAH) radical cations. The 0.7Y2.5 � m (14,500Y4000 cm � 1 ) spectrum for the open-shell cation forms of two nitrogenated anthracenes (C13H9N and C 12H8N2), four isomeric nitrogenated benzanthracenes (C17H11N), and four isomeric nitrogenated dibenzanthracenes (C21H13N) are reported. These ionized PANHs have allowed electronic transitions that give rise to strong absorption bands in the NIR. Low-lying excited states for these PANH ions are computed using time-dependent density functional theory (TDDFT). The resulting vertical excitation spectrum characterizes the transitions, and leads to a simple model that predicts the qualitative trends in absorption energy. The direction of the shift depends on the position of the nitrogen atom within the PANH and the relative magnitudes of the donor and acceptor molecular orbitals involved in the transitions. As with nonnitrogenated PAHs, ionized interstellar PANHs can be expected to contribute to the mid-IR emission features from UV-rich as well as UV-poor regions, and add weak, broad band structure to the NIR region of the interstellar extinction curve. Subject headingg astrochemistry — dust, extinction — ISM: general — ISM: molecules — molecular data

THE 5.25 AND 5.7 μm ASTRONOMICAL POLYCYCLIC AROMATIC HYDROCARBON EMISSION FEATURES

Astronomical mid-IR spectra show two minor polycyclic aromatic hydrocarbon (PAH) features at 5.25 and 5.7 μm (1905 and 1754 cm–1) that hitherto have been little studied, but contain information about the astronomical PAH population that complements that of the major emission bands. Here, we report a study involving both laboratory and theoretical analysis of the fundamentals of PAH spectroscopy that produce features in this region and use these to analyze the astronomical spectra. The Infrared Space Observatory Short Wavelength Spectrograph spectra of 15 objects showing these PAH features were considered for this study, however only four (HD 44179; NGC 7027; Orion Bar, two positions) have sufficient signal-to-noise between 5 and 6 μm to allow for an in-depth analysis. All four astronomical spectra show similar peak positions and profiles. The 5.25 μm feature is peaked and asymmetric, with an FWHM of about 0.12 ± 0.01 μm (~40 ± 6.5 cm–1), while the 5.7 μm feature is broader and flatter, with an FWHM of about 0.17 ± 0.02 μm (50 ± 5.6 cm–1). Detailed analysis of the laboratory spectra and quantum-chemical calculations show that the astronomical 5.25 and 5.7 μm bands are a blend of combination, difference and overtone bands primarily involving CH stretching and CH in-plane and CH out-of-plane bending fundamental vibrations. The experimental and computational spectra show that, of all the hydrogen adjacency classes that are possible on PAHs, solo and duo hydrogens consistently produce prominent bands at the observed positions, whereas quartet hydrogens do not. In all, this study supports the picture that astronomical PAHs are large with compact, regular structures. From the coupling with primarily strong CH out-of-plane bending modes, one might surmise that the 5.25 and 5.7 μm bands track the neutral PAH population. However, theory suggests that the role of charge in these astronomical bands might also be important.