L. J. Allamandola

Polycyclic aromatic hydrocarbons and the unidentified infrared emission bands - Auto exhaust along the Milky Way

The unidentified infrared emission features (UIR bands) are attributed to a collection of partially hydrogenated, positively charged polycyclic aromatic hydrocarbons (PAHs). This assignment is based on a spectroscopic analysis of the UIR bands. Comparison of the observed interstellar 6.2 and 7.7-micron bands with the laboratory measured Raman spectrum of a collection of carbon-based particulates (auto exhaust) shows a very good agreement, supporting this identification. The infrared emission is due to relaxation from highly vibrationally and electronically excited states. The excitation is probably caused by UV photon absorption. The infrared fluorescence of one particular, highly vibrationally excited PAH (chrysene) is modeled. In this analysis the species is treated as a molecule rather than bulk material and the non-thermodynamic equilibrium nature of the emission is fully taken into account. From a comparison of the observed ratio of the 3.3 to 11.3-micron UIR bands with the model calculations, the average number of carbon atoms per molecule is estimated to be about 20. The abundance of interstellar PAHs is calculated to be about 2 x 10 to the -7th with respect to hydrogen.

Mid- and far-infrared spectroscopy of ices: optical constants and integrated absorbances.

Laboratory spectra through the mid-infrared (4000 to 500 cm-1 [2.5-20 micrometers]) have been used to calculate the optical constants (n and k) and integrated absorption coefficients (A) for a variety of pure and mixed molecular ices of relevance to astrophysics. The ices studied were H2O, CH3OH, CO2, OCS, CH4, CO2 + CH4, CO2 + OCS, CO + CH4, CO + OCS, O2 + CH4, O2 + OCS, N2 + CH4, N2 + OCS, H2O + CH4, H2O + OCS, and H2O + CH3OH + CO + NH3. In addition, the measurements have been extended through the far-infrared (500 to 50 cm-1 [20-200 micrometers]) for the H2O, CH3OH, and H2O + CH3OH + CO + NH3 ices.

Photochemical and thermal evolution of interstellar/precometary ice analogs

An experimental investigation of the photochemical and thermal evolution of ices deposited at 10 K which contain primarily H2O, CH3OH, NH3, and CO mixed in relative proportions consistent with the proposed composition of interstellar ices is presented. These experiments, which are relevant to both interstellar and cometary ices, are the first described in which CH3OH (methanol) is a major constituent of the ice. Ultraviolet photolysis of these ice analogs invariably produces H2CO, CO2, CO, CH4, and HCO, largely at the expense of photofragmented CH3OH. In addition, photolysis produces a mixture of more complex molecules, some of which contain nitrile or isonitrile (ue5f8Cue5fcN or Cue5fcNue5f8) and carbonyl (ue5fbCue5fbO) groups. Most of the CO and CH4 leaves the sample upon warm-up to 100 K. Most of the parent ice molecules sublime away by 200 K leaving behind a mixture of more refractory substances. Warm-up to 250 K removes a component rich in CH3 groups which may correlate with the carrier(s) of the Cue5fbO and Cue5fcN bonds. A residue rich in CH2 groups remains even after warm-up to 300 K. The relevance of these results to questions concerning the composition of interstellar and cometary ices, and the scale heights of photofragments in cometary comae is discussed. The results may have some bearing on the recent suggestion that polyoxymethylene is present in Comet Halley.

Near-infrared absorption spectroscopy of interstellar hydrocarbon grains

We present new 3600 - 2700/cm (2.8 - 3.7 micrometer) spectra of objects whose extinction is dominated by dust in the diffuse interstellar medium. The observations presented here augment an ongoing study of the organic component of the diffuse interstellar medium. These spectra contain a broad feature centered near 3300/cm (3.0 micrometers) and/or a feature with a more complex profile near 2950/cm (3.4 micrometers), the latter of which is attributed to saturated aliphatic hydrocarbons in interstellar grains and is the primary interest of this paper. As in our earlier work, the similarity of the absorption bands near 2950/cm (3.4 micrometers) along different lines of sight and the correlation of these features with interstellar extinction reveal that the carrier of this band lies in the dust in the diffuse interstellar medium (DISM). At least 2.5% of the cosmic carbon in the local interstellar medium and 4% toward the Galactic center is tied up in the carrier of the 2950/cm (3.4 micrometer) band. The spectral structure of the diffuse dust hydrocarbon C-H stretch absorption features is reasonably similar to UV photolyzed laboratory ice residues and is quite similar to the carbonaceous component of the Murchison meteorite. The similarity between the DISM and the meteoritic spectrum suggests that some of the interstellar material originally incorporated into the solar nebula may have survived relatively untouched in primitive solar system bodies. Comparisons of the DISM spectrum to hydrogenated amorphous carbon and quenched carbonaceous composite are also presented. The A(sub V)/tau ratio for the 2950/cm (3.4 micrometer) feature is lower toward the Galactic center than toward sources in the local solar neighborhood (approximately 150 for the Galactic center sources vs. approximately 250 for the local ISM sources). A similar trend has been observed previously for silicates in the diffuse medium by Roche & Aitken, suggesting that (1) the silicate and carbonaceous materials in the DISM may be physically correlated and (2) there is either dust compositional variation in the galaxy or galactic variation in the grain population density distribution. We also note a possible absorption feature near 3050/cm (3.28 micrometers), a wavelength position that is characteristic of polycyclic aromatic hydrocarbons (PAHs).

MODELING THE UNIDENTIFIED INFRARED EMISSION WITH COMBINATIONS OF POLYCYCLIC AROMATIC HYDROCARBONS

The infrared emission band spectrum associated with many different interstellar objects can be modeled successfully by using combined laboratory spectra of neutral and positively charged polycyclic aromatic hydrocarbons (PAHs). These model spectra, shown here for the first time, alleviate the principal spectroscopic criticisms previously leveled at the PAH hypothesis and demonstrate that mixtures of free molecular PAHs can indeed account for the overall appearance of the widespread interstellar infrared emission spectrum. Furthermore, these models give us insight into the structures, stabilities, abundances, and ionization balance of the interstellar PAH population. These, in turn, reflect conditions in the emission zones and shed light on the microscopic processes involved in the carbon nucleation, growth, and evolution in circumstellar shells and the interstellar medium.

Infrared spectroscopy of dense clouds in the C-H stretch region: methanol and "diamonds."

High spectral resolution (nu/delta nu = 900) studies in the 3100-2600 cm-1 (3.2-3.9 microns) range are presented of the protostars NGC 7538 IRS 9, W33A, W3 IRS 5, and S140 IRS 1. This is the spectral region in which the fundamental C-H stretching vibrations of aliphatic hydrocarbons fall. Well-resolved absorption bands at about 2825 cm-1 (3.54 microns) and 2880 cm-1 (3.47 microns) were found superposed on the low-frequency wing of the strong O-H stretch feature. The 2880 cm-1 (3.47 microns) band, a new interstellar feature, is moderately strong in the spectra of all four objects studied. The 2825 cm-1 (3.54 microns) band, previously detected toward W33A, is also in the spectrum of NGC 7538 IRS 9. The relative strength of these two bands varies, showing that they are associated with two different carriers. On the basis of comparisons with laboratory spectra, the 2825 cm-1 (3.54 microns) band is assigned to methanol (CH3OH), in agreement with the earlier work of Grim et al. (1991). This assignment is further supported by a pair of weak absorptions centered at 2600 and 2540 cm-1 (3.85 and 3.94 microns) in the spectrum of W33A recently reported by Geballe (1991). These features compare very well with laboratory spectra of CH3OH/H2O ice mixtures. The CH3OH/H2O ratio derived from the 2825 cm-1 methanol band and the 3250 cm-1 (3.08 microns) H2O feature are 0.13 and 0.40 for NGC 7538 IRS 9 and W33A, respectively. These values are smaller than the ratios of 0.61 and 0.54 derived using the 1460 cm-1 (6.85 microns) band assigned to CH3OH and the 1665 cm-1 (6.00 microns) H2O band. These apparent discrepancies may be due to a combination of scattering effects within the molecular cloud, uncertainties associated with the baselines for the 2825 cm-1 feature, and the presence of other interstellar grain materials that absorb at 1460 cm-1 (6.85 microns). Nonetheless, after H2O, CH3OH is the most abundant known interstellar ice constituent. The new band at about 2880 cm-1 (3.47 microns) falls near the position for C-H stretching vibrations in tertiary carbon atoms. The strength of this feature, in combination with the lack of strong features associated with primary (-CH3) and secondary (-CH2-) carbon atoms, suggests that the carrier of the new feature has a diamond-like structure. We therefore tentatively attribute this new feature to interstellar diamonds. The detection of this band in the spectra of all four dense molecular clouds suggests that the carrier is ubiquitous in dense clouds. Band-strength analysis indicates that a minimum of a few percent of the available cosmic carbon is tied up in this material.

The interstellar C-H stretching band near 3.4 microns : constraints on the composition of organic material in the diffuse interstellar medium

To better constrain and quantify the composition of material in the diffuse interstellar medium (ISM), absorption spectra between 3600 and 2700 cm-1 (2.8 and 3.7 microns) have been taken of objects which have widely varying amounts of visual extinction along different lines of sight. The spectra of these objects contain a broad feature centered at approximately 3300 cm-1 (approximately 3.0 microns), attributed to O-H stretching vibrations, and/or a feature near 2950 cm-1 (3.4 microns) attributed to C-H stretching vibrations. The lack of correlation between the strengths of these two bands indicates that they do not arise from the same molecular carrier. The features in the 3100-2700 cm-1 (3.2-3.7 microns) region fall into one of two classes. We attribute the first class of features to material in the diffuse ISM on the basis of the similarity between the band profiles along the very different lines of sight to Galactic center source IRS 7 and VI Cygni #12. Similar features are also reported for Galactic center source IRS 3, Ve 2-45, and AFGL 2179. Higher resolution spectra of the objects OH 01-477 and T629-5, which are known to be M stars, are dominated by a series of narrow bands in this region. These bands are largely due to OH in the stars photospheres. While the spectra of OH 01-477 and T629-5 are likely to contain C-H absorption from diffuse ISM dust, the strength of the overlapping photospheric OH features presently prevents us from quantifying the depths of the interstellar C-H feature towards these objects. The interstellar feature for Galactic center source IRS 7 has subpeaks near 2955, 2925, and 2870 cm-1 (+/- 5 cm-1), which we attribute to C-H stretching vibrations in the -CH2- and -CH3 groups of aliphatic hydrocarbons. These band positions fall within 5 cm-1 of the values normal for saturated aliphatics. The absence of a distinct band near 2855 cm-1 suggests that the material contains small amounts of electronegative groups like -O-H or -C triple bond N. The relative strengths and profiles of the 2955 and 2925 cm-1 features towards five objects suggests an average diffuse ISM line-of-sight -CH2-/-CH3 ratio of about 2.5, indicating the presence of relatively complex organic materials. The strengths of the subpeaks at 2925 and 2955 cm-1, due to -CH2- and -CH3 groups, respectively, correlate with visual extinction, strongly suggesting that the C-H stretching band is a general feature of the material along different lines of sight in the diffuse ISM. We find average ratios of A nu/tau(2925 cm-1) = 240 +/- 40 and A nu/tau(2955 cm-1) = 310 +/- 90 for the objects we have observed. We deduce that 2.6%-35% of the cosmic carbon in the ISM is tied up in the carrier of this band with the most likely value falling near 10%. The interstellar C-H band is remarkably similar to the feature in lab residues produced by irradiating analogs of dense molecular cloud ices. This is consistent with a model in which the hydrocarbon component in the diffuse interstellar medium consists of complex hydrocarbons containing aliphatic side chains and bridges which are produced in dense molecular clouds and subsequently modified in the diffuse medium.

Theoretical modeling of the infrared fluorescence from interstellar polycyclic aromatic hydrocarbons

We have modeled the family of interstellar IR emission bands at 3.3, 6.2, 7.7, 8.6, 11.3, and 12.7 microns by calculating the fluorescence from a size distribution of interstellar polycyclic aromatic hydrocarbons (PAHs) embedded in the radiation field of a hot star. It is found that the various emission bands are dominated by distinctly different PAHs, from molecules with much less than about 80 C atoms for the 3.3 micron feature, to molecules with 10 exp 2-10 exp 5 C atoms for the emission in the IRAS 12 and 25 micron bands. We quantitatively describe the influence on the emergent spectrum of various PAH properties such as the molecular structure, the amount of dehydrogenation, the intrinsic strength of the IR active modes, and the size distribution. Comparing our model results to the emission spectrum from the Orion Bar region, we conclude that interstellar PAHs are likely fully, or almost fully, hydrogenated. Moreover, it is found that the intrinsic strengths of the 6.2 and 7.7 micron C-C stretching modes, and the 8.6 micron C-H in-plane bending mode are 2-6 times larger than measured for neutral PAHs in the laboratory.

Laboratory studies of the infrared spectral propertries of CO in astrophysical ices

Analysis of laboratory spectra of numerous astrophysical ice analogs demonstrates that the exact band position, width, and profile of the solid state CO fundamental near 2137 cm-1 (4.679 microns) can provide important information on the physical conditions present during the ice accretion phase as well as during any subsequent thermal processes and radiation exposure. In the ices studied, the CO peak position varies from 2134 to 2144 cm-1 (4.686 to 4.664 microns) and the band width from 2.1 to over 20 cm-1 depending on the composition of the ice. In an ice matrix dominated by H2O, the CO peak falls at 2136.7 cm-1, has a full width at half-maximum of about 9 cm-1, and shows a prominent sideband at 2152 cm-1. This sideband and minor structure superposed on the main band arise from CO trapped in different matrix sites. These features provide information concerning the thermal and radiation history of the ice. The solid CO band in interstellar spectra often has contributions from broad (12 cm-1) and narrow (5 cm-1) components. We identify the broad component with CO intimately mixed in matrices dominated by polar molecules, of which H2O is likely to be the major component. Examination of the interstellar and laboratory band profiles shows that either the abundance of nonpoplar impurities in these ices must be less than 10% or the ices have been thermally annealed or processed by ultraviolet radiation. The narrow component is likely to originate from grain mantles dominated by nonpolar molecules such as CO2. These components reflect differences in the physical and chemical conditions in regions of the cloud along the line of sight. Laboratory determination of the absorption strength of the CO fundamental in H2O-rich ices showed that the value used in the past was approximately 60% too low and that most previously determined solid-state CO column densities have been systematically overestimated. The rich spectral behavior of the CO band observed in the laboratory studies clearly indicates that future high-quality astronomical spectra in the 2200-2100 cm-1 range can produce a wealth of new information and provide deeper insights into the nature of astrophysical ices.

Variations in the Peak Position of the 6.2 μm Interstellar Emission Feature: A Tracer of N in the Interstellar Polycyclic Aromatic Hydrocarbon Population

This paper presents the results of an investigation of the molecular characteristics that underlie the observed peak position and profile of the nominal 6.2 � m interstellar emission band generally attributed to the CC stretching vibrations of polycyclic aromatic hydrocarbons (PAHs). It begins with a summary of recent experimental and theoreticalstudiesofthespectroscopicpropertiesof large(>30carbonatoms)PAHcationsastheyrelatetothisaspect of the astrophysical problem. It then continues with an examination of the spectroscopic properties of a number of PAH variants within the context of the interstellar 6.2 � m emission, beginning with a class of compounds known as polycyclic aromatic nitrogen heterocycles (PANHs; PAHs with one or more nitrogen atoms substituted into their carbon skeleton). In this regard, we summarize the results of recent relevant experimental studies involving a limited set of small PANHs and their cations and then report the results of a comprehensive computational study that extends that work to larger PANH cations including many nitrogen-substituted variants of coronene + (C24H þ ), ovalene + (C32H þ ), circumcoronene + (C54H þ ), and circum-circumcoronene + (C96H þ ). Finally, we report the results of more focused computational studies of selected representatives from a number of other classes of PAH variants that share one or more of the key attributes of the PANH species studied. These alternative classes of PAH variants include (1) oxygen- and silicon-substituted PAH cations; (2) PAH-metal ion complexes (metallocenes) involving the cosmically abundant elements magnesium and iron; and (3) large, asymmetric PAH cations. Overall, the studies reported here demonstrate that increasing PAH size alone is insufficient to account for the position of the shortest wavelength interstellar 6.2 � m emission bands, as had been suggested by earlier studies. On the other hand, this work reveals that substitution of one or more nitrogen atoms within the interior of the carbon skeleton of a PAH cation induces a significant blueshift in the position of the dominant CC stretching feature of these compounds that is sufficient to account for the position of the interstellar bands. Subsequent studies of the effects of substitution by other heteroatoms (O and Si), metal ion complexation (Fe + ,M g + , and Mg 2+ ), and molecular symmetry variation—all of which fail to reproduce the blueshift observed in the PANH cations—indicate that N appears tobeuniqueinitsabilitytoaccommodatethepositionoftheinterstellar6.2 � mbandswhilesimultaneouslysatisfying the other constraints of the astrophysical problem. This result implies that the peak position of the interstellar feature near 6.2 � m traces the degree of nitrogen substitution in the population, that most of the PAHs responsible for the interstellarIRemissionfeaturesincorporatenitrogenwithintheiraromaticnetworks,andthatalowerlimitof1%‐2% of the cosmic nitrogen is sequestered within the interstellar PAH population. Finally, in view of the ubiquity and abundance of interstellar PAHs and the permanent dipoles and distinctive electronic structures of these nitrogen-substituted variants, this work impacts a wide range of observational phenomena outside of the infrared region of the spectrum including the forest of unidentified molecular rotational features and the anomalous Galactic foreground emission in the microwave, and the diffuse interstellar bands (DIBs) and other structure in the interstellar extinction curve in the ultraviolet/visible. These astrophysical ramifications are discussed, and the dipole moments and rotational constants are tabulated to facilitate further investigations of the astrophysical role of nitrogen-substituted aromatic compounds.