Interstellar Polycyclic Aromatic Hydrocarbons Research Articles

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.

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 from 0.7 to 2.5 microns (14,000 to 4000/cm) are presented for the fifteen PAHs ranging in size from C40H18 to C50H22 whereas the spectra of the remaining twelve span the narrower range from 0.7 to 1.1 microns (14,000 to 9000/cm). The spectra of all the ionized PAHs we have studied to date have strong, broad absorption bands in the NIR arising from electronic transitions. This work shows that ionized PAHs have significant absorption bands at longer wavelengths than predicted by the current astronomical models which consider PAHs in their treatment of the radiation balance of the interstellar medium. Two implications are 1)-ionized interstellar PAHs should add weak, broad band structure to the NIR portion of the interstellar extinction curve and 2)- UV poor radiation fields can pump the PAH emission bands provided ionized PAHs are present.

Cavity ring-down spectroscopy and vibronic activity of benzo[ghi]perylene

Gas-phase cavity ring-down spectroscopy of jet-cooled benzo[ghi]perylene (C22H12) in the 26 950-28 600-cm(-1) spectral range is reported for the first time. This study is part of our extensive laboratory astrophysics program for the study of interstellar polycyclic aromatic hydrocarbons. The observed spectrum shows an intermediate level structure and significant broadening and is associated with the vibronically coupled S1(1A1)<--S0(1A1) and S2(1B1)<--S0(1A1) electronic transitions. Time-dependent density-functional calculations were performed to calculate the energetics, vibrational frequencies, and normal coordinates of the S1 and S2 states. A simple vibronic model was employed to account for the vibronic interaction between the vibronic levels of the S1 and S2 states. The calculated vibronic spectrum is found to be in good agreement with the experimental spectrum.

Hydrocarbon nanoparticles as a diffuse ISM analogue: morphology and infrared absorption in the 2000–500 cm−1 region

In this paper, we analyse the morphology and chemistry of carbonaceous dust particles previously proposed as a candidate analogue material for dust in the diffuse interstellar medium (ISM). The particles were polymerized in a low pressure capacitively coupled radiofrequency discharge in mixtures of argon/acetylene and helium/acetylene. Infrared (IR) spectra of our dust particles reveal the strong presence of aliphatic features, both stretching and bending modes, which are very similar to astronomical data from diffuse interstellar dust. A comparison between the IR spectra obtained using argon or helium as buffer gases reveals the strong presence of C = C double bonds (skeletal modes) when helium is used. The presence of these skeletal modes is clearly visible through the broad absorption feature at 1600 cm−1 (6.25 µm). The origin of this feature in our laboratory analogue is related to the change in the plasma parameters due to the variation of the buffer gas used under the same discharge conditions. Observational data of interstellar polycyclic aromatic hydrocarbons also exhibit a feature at this wavelength.

Identifying specific interstellar polycyclic aromatic hydrocarbons

Interstellar Polycyclic Aromatic Hydrocarbons (PAHs) have been thought to be ubiquitous for more than twenty years, yet no single species in this class has been identified in the Interstellar Medium (ISM) to date. The unprecedented sensitivity and resolution of present Infrared Space Observatory (ISO) and forthcoming Herschel observations in the far infrared spectral range will offer a unique way out of this embarassing impasse.

The Impact of Deuteration on the Infrared Spectra of Interstellar Polycyclic Aromatic Hydrocarbons

In this paper, we present the results of a computational study of the effects of a low absolute level of deuteration (~5%) on the observed mid-IR spectrum of the interstellar polycyclic aromatic hydrocarbon (PAH) population. The analyses are based on a global average spectrum, including all the possible singly deuterated isomers of an ensemble of partially superhydrogenated PAH species (i.e., Hn-PAHs) in both neutral and cationic forms. The distinctive spectroscopic properties considered in this paper include three classes of D-involved vibrational modes: (1) C–D in-plane bending features in the 9.5-12 μm region, (2) C–D out-of-plane bending features in the 14.5-17.9 μm region, and (3) aromatic and aliphatic C–D stretching features in the 4.4-4.7 μm region. In addition, D substitution also has the potential to alter the appearance of the C–H out-of-plane bending features in the 11-14 μm region by disrupting the adjacency of the H atoms on the PAH edge rings. The spectroscopic impacts of a low level of deuteration in the interstellar PAH population are considered, and their implications for the detailed appearance of the astronomical mid-IR PAH emission spectrum is explored. Overall, the results of this study indicate that the C–D stretching features offer the greatest prospects for the detection of deuterated interstellar PAHs. Since these features are effectively obscured by telluric CO2 absorptions, the information presented herein should be useful in the planning and design of future space-based IR observing platforms.

Possible molecular hydrogen formation mediated by the inner and outer carbon atoms of typical PAH cations

We have been studying the possibility of H2 formation mediated by the radical cations of gaseous polycyclic aromatic hydrocarbons (PAHs). In this study, we explored the catalytic role of both outer and inner carbon atoms of some typical PAH cations at the B3LYP/6-31G** level of theory. We presumed that H2 is formed by way of two elementary reactions: the addition of an H atom to a PAH cation and the H abstraction from the resulting arenium ion by another H atom. It was found that both reactions proceed with no or very small activation energies. Refinement of the activation energies with a spin-projected MP2 procedure confirmed a very small activation energy for the second reaction. It then follows that at least some carbon atoms of a PAH cation give sites for molecular hydrogen formation in space. Interstellar PAHs are mostly large compact ones with many inner carbon atoms. Therefore, even if such carbon atoms might be less susceptible to H addition, their catalytic role can never be overlooked in interstellar chemistry.

Deuterated Interstellar Polycyclic Aromatic Hydrocarbons

We report infrared spectral evidence of deuterated interstellar polycyclic aromatic hydrocarbons (PAHs). Two bands are detected in the infrared emission from the ionization bar in Orion at 4.4 and 4.65 � m. The one at 4.65 � m is present at the 4.4 � level, while the one at 4.4 � m is more tentative, with aequal to 1.9. An emission band at 4.65 � m is also detected in the infrared emission from M17 by Verstraete and coworkers at the 4.4 � level. These wavelengths are highly characteristic of C-D stretching modes of deuterated PAHs and fall in a featureless region oftheinterstellarspectrum.TheIRspectroscopicpropertiesofdeuteratedPAHsaresummarized,andanestimateof the deuterium fractionation between the aromatic and aliphatic carbons associated with the PAHs is given. High deuterium fractionation is implied; this is discussed in terms of interstellar PAH fractionation mechanisms. Subject headings: astrochemistry — infrared: ISM — ISM: abundances — ISM: molecules — line: identification — techniques: spectroscopic

Testing the “strong” PAHs hypothesis

The so-called strong Polycyclic Aromatic Hydrocarbons (PAHs) hypothesis postulates that isolated PAHs, which are thought to be the carriers of the Unidentified Infrared Bands, ought to be also responsible for a large number of Diffuse Interstellar Bands (DIBs). In this framework, the spectral profile of such DIBs should be due to unresolved rotational structure of vibronic absorption bands, the rotation of the molecule being by and large governed by the interaction with the interstellar radiation field. In this paper we quantitatively test the above hypothesis against the observational constraint of DIBs profile invariance, by using Monte-Carlo methods to model the photophysics of a prototypical interstellar PAH, namely the ovalene cation (C32H +). Our results show that the predicted rotational band profiles are remarkably insensitive to both the ambient conditions and the assumed values of some poorly known parameters. The present model therefore offers a quantitative link between any given PAH and the observed DIB profiles, providing a valuable tool for molecular identification.

Spectroscopy of large PAHs

Polycyclic Aromatic Hydrocarbons (PAHs) are thought to be the carriers of the ubiquitous infrared emission bands (UIBs). Data from the Infrared Space Observatory (ISO) have provided new insights into the size distribution and the structure of interstellar PAH molecules pointing to a trend towards larger-size PAHs. The mid-infrared spectra of galactic and extragalactic sources have also indicated the presence of 5-ring structures and PAH structures with attached side groups. This paper reports for the first time the laboratory measurement of the UV-Vis-NIR absorption spectra of a representative set of large PAHs that have also been selected for a long duration exposure experiment on the International Space Station ISS. PAHs with sizes up to 600 amu, including 5-ring species and PAHs containing heteroatoms, have been synthesized and their spectra measured using matrix isolation spectroscopy. The spectra of the neutral species and the associated cations and anions measured in this work are also compared to astronomical spectra of Diuse Interstellar Bands (DIBs).

Theoretical Modeling of Infrared Emission from Neutral and Charged Polycyclic Aromatic Hydrocarbons. II.

The nature of the carriers of the interstellar infrared (IR) emission features between 3.3 and 12.7 μm is complex. We must consider emission from a family of polycyclic aromatic hydrocarbons (PAHs) in a multiplicity of cationic charge states (+1, +2, +3, and so on), along with neutral and anionic PAHs. The adopted intrinsic IR cross sections of the various modes of the PAHs are the key to all models. Here we make a comparison between laboratory-measured cross sections and quantum chemical calculations and find that, overall, the agreement is very good. In this paper, we consider emission from a wide variety of specific PAH molecules, which includes all available data from our laboratory and quantum chemical databases. We incorporate this into our model to produce a theoretical analysis that is more realistic, detailed, and comprehensive than prior studies. PAH molecular structures that we consider include symmetric condensed, symmetric noncondensed, aromatics containing pentagonal rings, linear, and methylated PAHs. The synthesized IR spectra show large variations in peak position for the small PAHs studied, while their spectral profile is uniquely characteristic of each different molecular structure. We also investigate the spectral variations with molecular structure of a PAH population at the surface of the Orion photodissociation region (PDR) and include an example of how the IR spectrum of our PAH population varies dramatically as a function of depth (or radiation field) through the PDR. We make a comparison of these results with Infrared Space Observatory data measured at the surface of the Orion PDR. We conclude that the charge of PAHs in a composite population has a stronger effect on its IR emission spectrum than its molecular structure. However, on the basis of the PAH samples considered in this paper, detailed studies of the interstellar IR emission features can be used effectively to identify molecular characteristics of the interstellar PAH family. In Paper III, we extend the theme of this paper by investigating the effects of hydrogenation on a wide variety of PAHs up to size 54 carbon atoms and compare our results with observational profiles for the Orion PDR.

The Aromatic Infrared Bands as seen by ISO-SWS: Probing the PAH model

We discuss the Aromatic Infrared Band (AIB) proles observed by ISO-SWS towards a number of bright interstellar regions where dense molecular gas is illuminated by stellar radiation. Our sample spans a broad range of excitation conditions (exciting radiation elds with eective temperature, Te, ranging from 23 000 to 45 000 K). The SWS spectra are decomposed coherently in our sample into Lorentz proles and a broadband continuum. We nd that the individual proles of the main AIBs at 3.3, 6.2, 8.6 and 11.3 m are well represented with at most two Lorentzians. The 7.7 m-AIB has a more complex shape and requires at least three Lorentz proles. Furthermore, we show that the positions and widths of these AIBs are remarkably stable (within a few cm 1 ) conrming, at higher spectral resolution, the results from ISOCAM-CVF and ISOPHOT-S. This spectral decomposition with a small number of Lorentz proles implicitly assumes that most of the observed bandwidth arises from a few, large carriers. Boulanger et al. (1998b) recently proposed that the AIBs are the intrinsic proles of resonances in small carbon clusters. This interpretation can be tested by comparing the AIB prole parameters (band position and width) given in this work to laboratory data on relevant species when it becomes available. Taking advantage of our decomposition, we extract the proles of individual AIBs from the data and compare them to a state-of-the-art model of Polycyclic Aromatic Hydrocarbon (PAH) cation emission. In this model, the position and width of the AIBs are rather explained by a redshift and a broadening of the PAH vibrational bands as the temperature of the molecule increases (Joblin et al. 1995). In this context, the present similarity of the AIB proles requires that the PAH temperature distribution remains roughly the same whatever the radiation eld hardness. Deriving the temperature distribution of interstellar PAHs, we show that its hot tail, which controls the AIB spectrum, sensitively depends on Nmin (the number of C-atoms in the smallest PAH) and Te. Comparing the observed proles of the individual AIBs to our model results, we can match all the AIB proles (except the 8.6 m-AIB prole) if Nmin is increased with Te. This increase is naturally explained in a picture where small PAHs are more eciently photodissociated in harsher radiation elds. The observed 8.6 m-prole, both intensity and width, is not explained by our model. We then discuss our results in the broader context of ISO observations of fainter interstellar regions where PAHs are expected to be in neutral form.

The CH out-of-plane bending modes of PAH molecules in astrophysical environments

We present 10 15 m spectra of as ample of Hii regions, YSOs and evolved stars that show strong unidentied infrared emission features, obtained with the ISO/SWS spectrograph on-board ISO. These spectra reveal a plethora of emission features with bands at 11.0, 11.2, 12.0, 12.7, 13.5 and 14.2 m. These features are observed to vary considerably in relative strength to each-other from source to source. In particular, the 10{15m spectra of the evolved stars are dominated by the 11.2 m band while for H ii regions the 12.7 is typically as strong as the 11.2 m band. Analysing the ISO data we nd a good correlation between the 11.2 mb and and the 3.3 m band, and between the 12.7 ma nd the 6.2m band. There is also a correlation between the ratio of the UIR bands to the total dust emission and the 12.7 over 11.2 m ratio. Bands in the 10{15 ms pectral region are due to CH out of plane (OOP) bending modes of polycyclic aromatic hydrocarbons (PAHs). We summarise existing laboratory data and theoretical quantum chemical calculations of these modes for neutral and cationic PAHs. Due to mode coupling, the exact peak position of these bands depends on the number of adjacent CH groups and hence the observed interstellar 10 15 m spectra can be used to determine the molecular structure of the interstellar PAHs emitting in the dierent regions. We conclude that evolved stars predominantly inject compact 100 200 C-atom PAHs into the ISM where they are subsequently processed, resulting in more open and uneven PAH structures.

Unidentified Infrared Emission Bands in the Diffuse Interstellar Medium

Using the mid-infrared spectrometer on board the Infrared Telescope in Space and the low-resolution grating spectrometer (PHT-S) on board the Infrared Space Observatory, we obtained 820 mid-infrared (5-12 μm) spectra of the diffuse interstellar medium (DIM) in the Galactic center, W51, and Carina Nebula regions. These spectra indicate that the emission is dominated by the unidentified infrared (UIR) emission bands at 6.2, 7.7, 8.6, and 11.2 μm. The relative band intensities (6.2/7.7 μm, 8.6/7.7 μm, and 11.2/7.7 μm) were derived from these spectra, and no systematic variation in these ratios was found in our observed regions, in spite of the fact that the incident radiation intensity differs by a factor of 1500. Comparing our results with the polycyclic aromatic hydrocarbons (PAHs) model for the UIR band carriers, we find that PAHs in the DIM have no systematic variation in their size distribution, their degree of dehydrogenation is independent of the strength of UV radiation field, and they are mostly ionized. The finding that PAHs in the DIM with low UV radiation field strength are mostly ionized is incompatible with past theoretical studies, in which a large fraction of neutral PAHs is predicted in this kind of environment. A plausible resolution of this discrepancy is that the recombination coefficients for electron and large PAH positive ion are by at least an order of magnitude less than those adopted in past theoretical studies. Because of the very low population of neutral state molecules, photoelectric emission from interstellar PAHs is probably not the dominant source of heating of the diffuse interstellar gas. The present results imply constant physical and chemical properties of the carriers of the UIR emission bands in the DIM covering the central and disk regions of the Galaxy, which could help in the identification of the carriers.

Interstellar PAH emission in the 11-14 micron region: new insights from laboratory data and a tracer of ionized PAHs.

The Ames infrared spectral database of isolated, neutral and ionized polycyclic aromatic hydrocarbons (PAHS) shows that aromatic CH out-of-plane bending frequencies are significantly shifted upon ionization. For solo- and duet-CH groups, the shift is pronounced and consistently toward higher frequencies. The solo-CH modes are blueshifted by an average of 27 cm-1 and the duet-CH modes by an average of 17 cm-1. For trio- and quartet-CH groups, the ionization shifts of the out-of-plane modes are more erratic and typically more modest. As a result of these ionization shifts, the solo-CH out-of-plane modes move out of the region classically associated with these vibrations in neutral PAHS, falling instead at frequencies well above those normally attributed to out-of-plane bending, vibrations of any type. In addition, for the compact PAHs studied, the duet-CH out-of-plane modes are shifted into the frequency range traditionally associated with the solo-CH modes. These results refine our understanding of the origin of the dominant interstellar infrared emission feature near 11.2 microns, whose envelope has traditionally been attributed only to the out-of-plane bending of solo-CH groups on PAHS, and provide a natural explanation for the puzzling emission feature near 11.0 microns within the framework of the PAH model. Specifically, the prevalent but variable long-wavelength wing or shoulder that is often observed near 11.4 microns likely reflects the contributions of duet-CH units in PAH cations. Also, these results indicate that the emission between 926 and 904 cm-1 (10.8 and 11.1 microns) observed in many sources can be unambiguously attributed to the out-of-plane wagging, of solo-CH units in moderately sized (fewer than 50 carbon atom) PAH cations, making this emission an unequivocal tracer of ionized interstellar PAHS.

Photon-induced Complete Dehydrogenation of Putative Interstellar Polycyclic Aromatic Hydrocarbon Cations: Coronene and Naphtho[2,3-[CLC]a[/CLC]]pyrene

Fourier transform ion cyclotron resonance mass spectra of coronene and naphtho[2,3-a]pyrene radical cations exposed to broadband ultraviolet/visible radiation are presented. Parent ions are formed and confined in a Penning trap at 3.0 T and photolyzed with an Xe arc lamp. Each of the two radical cations is found to dehydrogenate completely to leave the C+24 bare carbon cluster cation. The extent of dehydrogenation depends directly on irradiation power. We infer that polycyclic aromatic hydrocarbon cations are inherently unstable to photolysis. We discuss some implications of hydrogen loss to yield bare carbon cluster cations. The relevance of dehydrogenation to unidentified interstellar infrared emission bands is discussed briefly.

Simulation of interstellar aromatic hydrocarbons using ion cyclotron resonance. Preliminary results

Using laser microprobe and Fourier transform ion cyclotron resonance (FTICR) mass spectrometry, we have produced and trapped aromatic species which might well be laboratory analogues of interstellar polycyclic aromatic hydrocarbons. These species are produced by laser ablation of a sample of pyrolysed coronene. They are large (up to two hundred carbon atoms) and can be highly dehydrogenated upon irradiation. More or less condensed forms have also been identified. Although still preliminary, these results open a new field of investigation: the study of the photophysics and chemistry of these large reactive species in isolation conditions close to those found in interestellar space. This will be the first objective of the PIRENEA experiment, a FTICRMS set-up devoted to astrophysics. © 1997 John Wiley & Sons, Ltd.

Inference of a 7.75 eV Lower Limit in the Ultraviolet Pumping of Interstellar Polycyclic Aromatic Hydrocarbon Cations with Resulting Unidentified Infrared Emissions

The discrete infrared features known as the unidentified infrared (UIR) bands originating in starburst regions of other galaxies, and in H II regions and planetary nebulae within the Milky Way, are widely thought to be the result of ultraviolet pumped infrared fluorescence of polycyclic aromatic hydrocarbon (PAH) molecules and ions. These UIR emissions are estimated to account for 10%-30% of the total energy emitted by galaxies. Laboratory absorption spectra including the vacuum ultraviolet region, as described in this paper, show a weakening of the intensity of absorption features as the population of cations increases, suggesting that strong pi* <-- pi transitions are absent in the spectra of PAH cations. This implies a lower energy bound for ultraviolet photons that pump infrared emissions from such ions at 7.75 eV, an amount greater than previously thought. The implications include size and structure limitations on the PAH molecules and ions which are apparent constituents of the interstellar medium. Also, this might affect estimations of the population of early-type stars in regions of rapid star formation.

Assessment of the polycyclic aromatic hydrocarbon-diffuse interstellar band proposal.

The potential link between neutral and/or ionized polycyclic aromatic hydrocarbons (PAHs) and the diffuse interstellar band (DIB) carriers is examined. Based on the study of the general physical and chemical properties of PAHs, an assessment is made of their possible contribution to the DIB carriers. It is found that, under the conditions reigning in the diffuse interstellar medium, PAHs can be present in the form of neutral molecules as well as positive and/or negative ions. The charge distribution of small PAHs is dominated, however, by two charge states at one time with compact PAHs present only in the neutral and cationic forms. Each PAH has a distinct spectral signature depending on its charge state. Moreover, the spectra of ionized PAHs are always clearly dominated by a single band in the DIB spectral range. In the case of compact PAH ions, the strongest absorption band is of type A (i.e., the band is broad, falls in the high-energy range of the spectrum, and possesses a large oscillator strength), and seems to correlate with strong and broad DIBs. For noncompact PAH ions, the strongest absorption band is of type I (i.e., the band is narrow, falls in the low-energy range of the spectrum, and possesses a small oscillator strength), and seems to correlate with weak and narrow DIBs. Potential molecular size and structure constraints for interstellar PAHs are derived by comparing known DIB characteristics to the spectroscopic properties of PAHs. It is found that (i) only neutral PAHs larger than about 30 carbon atoms could, if present, contribute to the DIBs. (ii) For compact PAHs, only ions with less than about 250 carbon atoms could, if present, contribute to the DIBs. (iii) The observed distribution of the DIBs between strong/moderate and broad bands on the one hand and weak and narow bands on the other can easily be interpreted in the context of the PAH proposal by a distribution of compact and noncompact PAH ions, respectively. A plausible correlation between PAH charge states and DIB "families" is thus provided by the PAH-DIB proposal. Following this proposal, DIB families would reflect conditions within a cloud which locally determine the relative importance of cations, anions, and neutral species, rather than tracers of a specific species. Observational predictions are given to establish the viability of the PAH hypothesis. It is concluded that small PAH ions are very promising candidates as DIB carriers provided their population is dominated by a finite number (100-200) of species. A key test for the PAH proposal, consisting of laboratory and astronomical investigations in the ultraviolet range, is called for.

Infrared spectroscopy of matrix-isolated polycyclic aromatic hydrocarbon cations. 3. The polyacenes anthracene, tetracene, and pentacene.

Gaseous, ionized polycyclic aromatic hydrocarbons (PAHs) are thought to be responsible for a very common family of interstellar infrared emission bands. Unfortunately, very little infrared spectroscopic data are available on ionized PAHs. Here we present the near- and mid-infrared spectra of the polyacene cations anthracene, tetracene, and pentacene. We also report the vibrational frequencies and relative intensities of the pentacene anion. The cation bands corresponding to the CC modes are typically about 10-20 times more intense than those of the CH out-of-plane bending vibrations. For the cations the CC stretching and CH in-plane bending modes give rise to bands which are an order of magnitude stronger than for the neutral species, and the CH out-of-plane bends produce bands which are 3-20 times weaker than in the neutral species. This behavior is similar to that found for most other PAH cations. The most intense PAH cation bands fall within the envelopes of the most intense interstellar features. The strongest absorptions in the polyacenes anthracene, tetracene, and pentacene tend to group around 1400 cm-1 (between about 1340 and 1500 cm-1) and near 1180 cm-1, regions of only moderate interstellar emission. These very strong polyacene bands tend to fall in gaps in the spectra of the other PAH cations studied to date suggesting that while PAHs with polyacene structures may contribute to specific regions of the interstellar emission spectra, they are not dominant members of the interstellar PAH family.