Polycyclic Aromatic Hydrocarbons Family Research Articles

PDRs4All

Context. Mid-infrared observations of photodissociation regions (PDRs) are dominated by strong emission features called aromatic infrared bands (AIBs). The most prominent AIBs are found at 3.3, 6.2, 7.7, 8.6, and 11.2 µm. The most sensitive, highest-resolution infrared spectral imaging data ever taken of the prototypical PDR, the Orion Bar, have been captured by JWST. These high-quality data allow for an unprecedentedly detailed view of AIBs. Aims. We provide an inventory of the AIBs found in the Orion Bar, along with mid-IR template spectra from five distinct regions in the Bar: the molecular PDR (i.e. the three H2 dissociation fronts), the atomic PDR, and the H II region. Methods. We used JWST NIRSpec IFU and MIRI MRS observations of the Orion Bar from the JWST Early Release Science Program, PDRs4All (ID: 1288). We extracted five template spectra to represent the morphology and environment of the Orion Bar PDR. We investigated and characterised the AIBs in these template spectra. We describe the variations among them here. Results. The superb sensitivity and the spectral and spatial resolution of these JWST observations reveal many details of the AIB emission and enable an improved characterization of their detailed profile shapes and sub-components. The Orion Bar spectra are dominated by the well-known AIBs at 3.3, 6.2, 7.7, 8.6, 11.2, and 12.7 µm with well-defined profiles. In addition, the spectra display a wealth of weaker features and sub-components. The widths of many AIBs show clear and systematic variations, being narrowest in the atomic PDR template, but showing a clear broadening in the H II region template while the broadest bands are found in the three dissociation front templates. In addition, the relative strengths of AIB (sub-)components vary among the template spectra as well. All AIB profiles are characteristic of class A sources as designated by Peeters (2022, A&A, 390, 1089), except for the 11.2 µm AIB profile deep in the molecular zone, which belongs to class B11.2. Furthermore, the observations show that the sub-components that contribute to the 5.75, 7.7, and 11.2 µm AIBs become much weaker in the PDR surface layers. We attribute this to the presence of small, more labile carriers in the deeper PDR layers that are photolysed away in the harsh radiation field near the surface. The 3.3/11.2 AIB intensity ratio decreases by about 40% between the dissociation fronts and the H II region, indicating a shift in the polycyclic aromatic hydrocarbon (PAH) size distribution to larger PAHs in the PDR surface layers, also likely due to the effects of photochemistry. The observed broadening of the bands in the molecular PDR is consistent with an enhanced importance of smaller PAHs since smaller PAHs attain a higher internal excitation energy at a fixed photon energy. Conclusions. Spectral-imaging observations of the Orion Bar using JWST yield key insights into the photochemical evolution of PAHs, such as the evolution responsible for the shift of 11.2 µm AIB emission from class B11.2 in the molecular PDR to class A11.2 in the PDR surface layers. This photochemical evolution is driven by the increased importance of FUV processing in the PDR surface layers, resulting in a “weeding out” of the weakest links of the PAH family in these layers. For now, these JWST observations are consistent with a model in which the underlying PAH family is composed of a few species: the so-called ‘grandPAHs’.

A C 54 B 2 Polycyclic π-System with Bilayer Assembly and Multi-Redox Activity

A C ₅₄ B ₂ Polycyclic π-System with Bilayer Assembly and Multi-Redox Activity

Toward More Efficient Organic Semiconductors: The Relationship between Morphology, Charge Transport, and Photophysical Properties

In recent years, materials science has been facing an electronics revolution due to the fast progress in the development of organic semiconductors. In this context, the current study attempts to tune the photophysical properties of polycyclic aromatic hydrocarbons (PAHs) by engineering their morphology and size. Four families of PAHs─oligoacenes, circulenes, triangulenes, and sunflowers─are considered to investigate the evolution of their optoelectronic properties with size and morphology. Based on the calculated properties, several quantitative structure–property relationships have been established for the prediction of reorganization energy. It is found that morphologies like circulene and triangulene that are less aromatic and have smaller energy gaps achieve smaller reorganization energy and thus are better candidates for charge transport materials. For all considered PAHs, the excitation energies of the strongest low-lying electronic transition (β peak) span in the visible region and demonstrate the size tunability. The β peak of some oligoacene and triangulene families exhibits plasmonic behavior that is important for the fabrication of lighter optical devices compared with metallic systems. We have also explored the effect of sulfur heteroatoms on the structural, electronic, and optical properties of circulene and sunflower morphologies.

Non long-range corrected density functionals incorrectly describe the intensity of the CH stretching band in polycyclic aromatic hydrocarbons.

We present a comprehensive study of the most relevant numerical aspects influencing frequencies and intensities in the infrared spectrum of isolated polycyclic aromatic hydrocarbons (PAHs) regarding the overestimate of the IR CH-stretching bands. We use naphthalene as benchmark and show the validity of our results to different members of the PAH family. Our analysis relies on widely employed density functional theory methods and second-order vibrational perturbational theory for the computation of vibrational eigenstates. We have focused on the elucidation of the origin of the systematic overestimate of the intensities in the CH-stretching region. To rule out nonfundamental numerical errors, we have initially considered the influence of the electronic basis set and various other parameters on the different stages of the vibrational analysis. In a second stage, we have benchmarked the results of different density functional theory functionals with respect to the aforementioned overestimate taken as the ratio between the most prominent features of the spectrum, the CH-bending and the CH-stretching bands. Our results unambiguously indicate that the long-range correction plays a major role in this spurious numerical issue. More specifically, this phenomenon is due to an incorrect description of the charge distribution (and hence dipole) within the symmetrically relevant CH bonds. Long-range correction specifically remedies this issue. It improves the description of the intensities in the stretching region while at the same time it does not perturb significantly the rest of the spectrum. With respect to the frequencies, we have observed an overall improvement when compared to noncorrected functionals.

Synthesis and characterization of a twisted tri-sulfur-doped polycyclic aromatic hydrocarbon

Polycyclic aromatic hydrocarbons (PAHs) generally refer to aromatic compounds containing two or more fused benzene rings. PAHs have excellent optical, electronic and self-assembly properties, and have important applications in organic light-emitting diodes (OLED), organic field effect transistors (OFET), organic solar photovoltaic cells (OPVC) and other fields. The introduction of heteroatoms to PAHs can adjust the electronic structure, dipole moment, and the self-assembly behavior. Among them, sulfur-doped PAHs have been widely studied. Due to the proper energy level and bonding mode of sulfur atom, the sulfur-doped PAHs have stronger dipole-dipole interactions, electron donating ability, and weakened aromaticity. Sulfur-doped PAHs usually exhibit unique redox characteristics and better hole transporting, so they are widely used as p-type organic semiconductor and electronic donor in photoelectric conversion, and donation unit in donor-acceptor conjugated molecules. Doping PAHs with sulfur is mainly through two ways: post-modification and fusing the sulfur-containing heterocyclic unit. On the other hand, regarding the regulation of geometric structure of carbon skeleton, non-hexagonal ring is introduced to transform the molecular structure from planar to curved structure. The structural distortion can be achieved by the peripherial stereo-hindrance. For example, the PAHs containing helicene units have a highly distorted structure, thus showing a unique optical, electric and assembly properties. In order to construct novel PAHs, this article combines heteroatom doping and geometric control of carbon skeleton. A twisted tri-sulfur-doped PAH C36S3H10(C9H11)6 (molecule 1 ) was synthesized by oxidative cyclodehydrogenation of an oligophenylene precursor, which contains three thiophene rings. X-ray single crystal diffraction exactly characterized the structure of molecule 1 . Because the ring is not completely fused, molecule 1 contains two [5]helicene units, which leads to a twisted molecular structure. In crystalline state, molecule 1 forms dimers through C−H···π and weak π-π interactions, which further packs into one-dimensional columnar stacking through the weak interactions of C−H···π and C−H···S. Different from the planar PAHs, the twisted molecule 1 is not easy to aggregate in solution. Molecule 1 exhibits a yellow photoluminescence, but its quantum yield is low. Combined with the study of transient fluorescence spectroscopy, it is speculated that its twisted molecular structure may be the reason for its low quantum yield. The synthesis of molecule 1 enriches the family of sulfur-doped PAHs, and the derivatives with alkyl chains of molecule 1 can be synthesized in the future and be used as organic semiconductors or electron donors in solar cells and organic electronic devices, etc.

Study on fluorescence interaction between humic acid and PAHs based on two-dimensional correlation spectroscopy

Polycyclic aromatic hydrocarbons (PAHs) pollution is an increasingly serious environmental problem, and the effective and rapid detection of PAHs is in urgent need for environment management and remediation. At present, fluorescence technology is the preferred method for fast in-situ dynamic detection of PAHs. However, the impact of humic acid on PAHs fluorescence has become a bottleneck for accurate detection of PAHs using this technology. In this work, 2D correlation analysis is used to study the interaction between the fluorescence spectroscopy of humic acid and benzo[ghi]perylene (BGP), a typical molecule in the PAHs family. First of all, the fluorescence characteristics of BGP and humic acid were analyzed and feature peaks were identified separately. Meanwhile, the fluorescence spectra of humic acids at various concentrations excited at different wavelengths were obtained to analyze the variation of fluorescence intensity with concentration for different groups of humic acid. Secondly, the conventional one-dimensional fluorescence spectra of the mixed samples of different concentrations of humic acids with fixed BGP concentration were studied to explore the effect of humic acid on the fluorescence of BGP. Finally, 2D correlation fluorescence spectra of the mixed samples of humic acid and BGP were studied with excitation wavelength as the external perturbation. The results showed that humic acid had a significant effect on the fluorescence intensity of BGP, and indicated that high concentration of humic acid could lead to fluorescence quenching of BGP. And the fluorescence characteristic information of humic acid with lower concentration could be extracted from sliced asynchronous spectrum at the signature peak of BGP. This study demonstrated a novel approach for the separation of fluorescence intensities of humic acid from PAHs, providing corrected PAHs spectra for quantitative analysis of the mixture.

Polyacenes and diffuse interstellar bands

The identification of the carriers of the diffuse interstellar bands (DIBs) remains to be established, with the exception of five bands attributed to C60+, although it is generally agreed that DIB carriers should be large carbon-based molecules (with ~10–100 atoms) in the gas phase, such as polycyclic aromatic hydrocarbons (PAHs), long carbon chains or fullerenes. The aim of this paper is to investigate more specific possible carriers among PAHs, namely elongated molecules, which could explain a correlation between the DIB wavelength and the apparent UV resilience of their carriers. More specifically, we address the case of polyacenes, C4N+2H2N+4, with N ~ 10–18 fused rectilinear aligned hexagons. Polyacenes are attractive DIB carrier candidates because their high symmetry and large linear size allow them to form regular series of bands in the visible range with strengths larger than most other PAHs, as confirmed by recent laboratory results up to undecacene (C46H26). Those with very strong bands in the DIB spectral domain are just at the limit of stability against UV photodissociation. They are part of the prominent PAH family of interstellar carbon compounds, meaning that only ~10−5 of the total PAH abundance is enough to account for a medium-strength DIB. After summarizing the limited current knowledge about the complex properties of polyacenes and recent laboratory results, the likelihood that they might meet the criteria for being carriers of some DIBs is addressed by reviewing the following properties: wavelength and strength of their series of visible bands; interstellar stability and abundances, charge state and hydrogenation; and DIB rotation profiles. No definite inconsistency has been identified that precludes polyacenes from being the carriers of some DIBs with medium or weak strength, including the so-called C2 DIBs. But, despite their many interesting properties, additional experimental data about long acenes and their visible bands are needed to make robust conclusions.

Potential microbial remediation of pyrene polluted soil: the role of biochar

Polycyclic aromatic hydrocarbons (PAHs) are a large group of compounds composed of two or more aromatic rings. They are extremely toxic pollutants largely produced by anthropogenic activities and characterised by high persistence in the environment. Soils contaminated by PAHs could be depolluted by bioremediation techniques, an effective in-situ procedure which provides the addition of exogenous substrates able to sustain and enhance the autochthonous soil microflora and the allochthon microbial inoculum. Our research aims to study the effects of biochar, produced by slow pyrolysis of olive pomace, as a bio-stimulant of soil microflora or support for the colonisation of the allochthon Trichoderma harzianum, on degradation of pyrene used here as model molecule for the PAH family. Biochar is considered an excellent soil conditioner because of its positive effect on soil physical and chemical properties and its positive interaction with soil microorganisms. Autochthonous microbial growth, T. harzianum growth and microbial pyrene-degradation activity were surveyed in soil samples spiked with 50 ppm of pyrene and incubated for up to 28 days. Pyrene concentration was reduced by ~70% in 28 days in both bioaugmentation and biostimulation tests. Olive mill pomace biochar did not interfere with pyrene bioavailability and did not affect microbial pyrene-degrading activity. The T. harzianum did not display a distinctive ability in degrading pyrene and partially inhibited the endogenous soil microflora.

Characteristics and proteomic analysis of pyrene degradation by Brevibacillus brevis in liquid medium

Polycyclic aromatic hydrocarbons (PAHs) are widely spread in various ecosystems and are of great concern due to their potential toxicity, mutagenicity and carcinogenicity. Bioremediation has been proposed as an effective approach to remove PAHs. In this study, the physiological responses and proteome of Brevibacillus brevis under exposure to pyrene, a four-ring compound from PAHs family, were investigated. The changes of cell viability of B. brevis were observed during the degradation of pyrene by means of flow cytometry. The results indicated that pyrene stimulated superoxide dismutase (SOD) activity from 93.9 to 100.6 U mg−1 prot, whereas inhibited catalase (CAT) activity from 29.1 to 20.3 U mg−1 prot. The main compositions of B. brevis changed during pyrene degradation, with the proportion of unsaturated fatty acids increased by 13.4%. In addition, we performed a proteomic approach (two-dimensional gel electrophoresis and MALDI-TOF/TOF-MS) in order to explore how B. brevis survived upon treatment with pyrene. It was showed that the expression of 13 proteins increased whereas 10 other decreased after pyrene-treatment. The differentially expressed proteins were identified and the results indicated that they were involved in multiple biological processes including energy metabolism, biosynthesis, transmembrane transport and oxidative stress. Overall, these findings offered a new insights into the cellular response strategy developed by B. brevis to overcome the pyrene stress.

The PAH Emission Characteristics of the Reflection Nebula NGC 2023

Abstract We present 5–20 μm spectral maps of the reflection nebula NGC 2023 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 that reveal a spatial sequence with distance from the illuminating star. We explore the distinct morphology of the 6.2, 7.7, and 8.6 μm PAH bands and find that at least two spatially distinct components contribute to the 7–9 μm PAH emission in NGC 2023. We report that the PAH features behave independently of the underlying plateaus. We present spectra of compact, oval PAHs ranging in size from C66 to C210, determined computationally using density functional theory, and we 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 μm 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 μm emission that likely originate in these structures. Based on the assignments and the observed spatial sequence, we discuss the photochemical evolution of the interstellar PAH family as the PAHs are more and more exposed to the radiation field of the central star in the evaporative flows associated with the Photo-Dissociation Regions in NGC 2023.

Pentynyl Ether of β-Cyclodextrin Polymer and Silica Micro-Particles: A New Hybrid Material for Adsorption of Phenanthrene from Water.

A new hybrid material for the removal of polycyclic aromatic hydrocarbons (PAH) from water was prepared by the polymerization of pentynyl beta-cyclodextrin (PyβCD) and silica micro-particles (SMP). Phenanthrene, being one of the important members of the PAH family and a potential risk for environmental pollution, was selected for this study. Results show that phenanthrene removal efficiency of the SMP was improved significantly after hybridization with PyβCD-polymer. Approximately 50% of the phenanthrene was removed in the first 60 min and more than 95% was removed in less than 7 h when 25 mL of the 2 ppm aqueous phenanthrene solution was incubated with the 100 mg of SMP-PyβCD-polymer material. Infrared spectroscopy and thermal gravimetric analysis show that the enhanced efficiency of the SMP-PyβCD-polymer compared to the unmodified SMP was due to the formation of the inclusion complexation of phenanthrene with the PyβCD. These results indicate that SMP-PyβCD polymers have a potential to be applied as molecular filters in water purification systems and also for waste water treatment.

Impact of the 0.1% fuel sulfur content limit in SECA on particle and gaseous emissions from marine vessels

Emissions were measured on-board a ship in the Baltic Sea, which is a sulfur emission control area (SECA), before and after the implementation of the strict fuel sulfur content (FSC) limit of 0.1 m/m% S on the 1st of January 2015. Prior to January 2015, the ship used a heavy fuel oil (HFO) but switched to a low-sulfur residual marine fuel oil (RMB30) after the implementation of the new FSC limit. The emitted particulate matter (PM) was measured in terms of mass, number, size distribution, volatility, elemental composition, content of organics, black and elemental carbon, polycyclic aromatic hydrocarbons (PAHs), microstructure and micro-composition, along with the gaseous emissions at different operating conditions. The fuel change reduced emissions of PM mass up to 67%. The number of particles emitted remained unchanged and were dominated by nanoparticles. Furthermore, the fuel change resulted in an 80% reduction of SO2 emissions and decreased emissions of total volatile organic compounds (VOCs). The emissions of both monoaromatic and lighter polyaromatic hydrocarbon compounds increased with RMB30, while the heavy, PM-bound PAH species that belong to the carcinogenic PAH family were reduced. Emissions of BC remained similar between the two fuels. This study indicates that the use of low-sulfur residual marine fuel oil is a way to comply with the new FSC regulation and will reduce the anthropogenic load of SO2 emissions and secondary PM formed from SO2. Emissions of primary particles, however, remain unchanged and do not decrease as much as would be expected if distilled fuel was used. This applies both to the number of particles emitted and some toxic components, such as heavy metals, PAHs or elemental carbon (EC). The micro-composition analyses showed that the soot particles emitted from RMB30 combustion often do not have any trace of sulfur compared with particles from HFO combustion, which always have a sulfur content over 1%m/m. The soot sulfur content can impact aging and cloud condensation properties. This study is an in-depth comparison of the impact of these two fuels on the emissions of particles as well as their composition and microstructure. To evaluate the impact of the use of low-sulfur residual marine fuel oils on emissions from ships, additional research is needed to investigate the varied fuel types and compositions as well as the wide range of engine conditions and properties.

Assessment of petroleum streams for thyroid toxicity.

The thyroid gland, and its associated endocrine hormones, is a growing area of interest in regulatory toxicology due to its important role in metabolism, growth and development. This report presents a review of the toxicology data on chemically complex petroleum streams for thyroid hormone effects. Toxicological summaries and studies from all available published and un-published sources were considered, drawing upon the European REACH regulatory submissions for 19 petroleum streams, with in depth review of 11 individual study reports and 31 published papers on related products or environmental settings. Findings relevant to thyroid pathology or thyroid hormone homeostasis were specifically sought, summarized, and discussed. A total of 349 studies of 28-days or longer duration were considered in the review, including data on mice, rats, rabbits, dogs, humans, and fish. The thyroid was almost invariably not a target organ in these studies. Three rodent studies did find thyroid effects; one on a jet fuel product (JP-8), and two studies on a heavy fuel oil product (F-179). The JP-8 product differs from other fuels due to the presence of additives, and the finding of reduced T4 levels in mice in the study occurred at a dose that is above that expected to occur in environmental settings (e.g. 2000mg/kg). The finding for F-179 involved thyroid inflammation at 10-55mg/kg that co-occurred with liver pathology in rats, indicating a possible secondary effect with questionable relevance to humans. In the few cases where findings did occur, the polycyclic aromatic hydrocarbon (PAH) content was higher than in related substances, and, in support of one possible adverse outcome pathway, one in-vitro study reported reduced thyroid peroxidase (TPO) activity with exposure to some PAH compounds (pyrene, benzo(k)fluoranthene, and benzo(e)pyrene). However, it could not be determined from the data available for this review, whether these specific PAH compounds were substantially higher in the JP-8 or F-179 products than in studies in which thyroid effects were not observed. Thus, a few products may carry a weak potential to affect the thyroid at high doses in rodents, possibly through secondary effects on the rodent liver or possibly through a pathway involving the inhibition of TPO by specific members of the PAH family. Human epidemiology evidence found weak and inconsistent effects on the thyroid but without identification of specific chemicals involved. Two studies in petroleum workers, which found a lower rate of morbidity and mortality overall, reported a statistically significant increase in thyroid cancer, but the small number of cases could not exclude confounding variables as possible explanations for the statistical findings. Overall, the available data indicates a low potential for thyroid hormone effects from exposure to petroleum streams, especially when the aromatic content is low. Because regulatory studies for most chemicals do not include detailed thyroid function or receptor studies, it remains possible that subclinical effects on this system may exist that were not detectable using conventional pathology or hormone measurements.

The response of thyroid hormones, biochemical and enzymological biomarkers to pyrene exposure in common carp (Cyprinus carpio)

Polycyclic Aromatic Hydrocarbons (PAHs) are discharged into aquatic environments through anthropogenic activities mainly industrial and municipal effluents. There is little information on the adverse effects of pyrene, a member of the PAH family which is classified as a priority pollutant by the USEPA, on fish biochemical and physiological endpoints, particularly thyroid hormones. The present study investigated the effects of subacute semi-static pyrene exposure on biochemical, enzymological and ionoregulatory responses as well as thyroid hormones in common carp (Cyprinus carpio). The fish (140±10g, 1+ year) were exposed to 10, 50 and 100µg/l nominal concentrations of pyrene for 35 days. The results revealed that pyrene at these concentrations significantly altered plasma levels of glucose, cholesterol, triglyceride, total protein, albumin, alanine aminotransferase (ALT), aspartate aminotransferase (AST) and alkaline phosphatase (ALP). Moreover, plasma thyroid hormones (T3 and T4) were significantly decreased in fish exposed to pyrene. In contrast, plasma electrolytes (sodium, potassium and calcium) levels remained statistically unchanged after exposure to the various pyrene concentrations. In conclusion, the studied biomarkers may be used as monitoring tools to evaluate pyrene toxicity. Pyrene induced diverse effects on the physiological endpoints of common carp, thus this chemical should be considered in toxicity studies concerning PAHs. Furthermore, this study confirmed that there was an interaction between pyrene and the thyroid system in fish. Therefore, the thyroid system may be used to assess the impact of pyrene on fish.

Chronic Toxicity Thresholds for Sediment-Associated Benzo[a]pyrene in the Midge (Chironomus dilutus)

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in aquatic ecosystems and have been shown to be one of the causes of sediment toxicity to benthic invertebrates. Benzo[a]pyrene (BaP) was selected as a representative for the PAH family of compounds for developing chronic sediment toxicity thresholds for Chironomus dilutus. Life-cycle toxicity testing was initiated using newly hatched midge larvae and terminated until hatch of the second generation. Median lethal concentrations were 92.5±19.6 and 56.9±1.76μg/g organic carbon (OC) after exposing midges to sediment-associated BaP for 20days (before pupation) and 43days (end of test), respectively. Sublethal toxicity was described as 5 and 50% effect concentrations (EC5 and EC50), and these were 6.63±0.82 and 41.1±1.61μg/g OC for growth reduction at 20days, respectively. Impairments of emergence and reproduction of C. dilutus were also assessed at the end of the testing, and the EC5 and EC50 values were 3.41±0.53 and 26.9±1.43μg/g OC for emergence and 2.18±0.34 and 13.4±1.13μg/g OC for reproduction, respectively. In addition, bioavailability-based chronic toxicity thresholds were also established using Tenax-extractable BaP concentrations. Although more environmentally relevant, data regarding chronic toxicity are less available than those regarding acute toxicity. Therefore, establishing numeric chronic toxicity thresholds for sediment-associated BaP with the consideration of the bioavailability would improve the accuracy of assessing PAH-related sediment toxicity.

THE INFRARED SPECTROSCOPY OF COMPACT POLYCYCLIC AROMATIC HYDROCARBONS CONTAINING UP TO 384 CARBONS

The mid- and the far-infrared spectra of polycyclic aromatic hydrocarbons (PAHs) have been computed using density functional theory. This study has focused on PAHs in the highly symmetric, compact, coronene family with sizes up to 384 carbons. We have identified trends in the peak position and intrinsic strength of the vibrational modes of these species and compared these to trends previously reported for less symmetric and smaller PAHs. The computed spectral modes have been used to calculate the IR emission spectrum of PAHs pumped by UV photons. The results have been compared to observed interstellar spectra to elucidate the characteristics of the interstellar PAH family. The calculations show that highly symmetric PAHs are very stable and, hence, might be favored under the harsh conditions of interstellar space. Our calculated vibrational properties confirm and extend previous studies for small PAHs to the large compact PAHs studied here, specifically in terms of the dependence of the spectral characteristics on ionization and on H-adjacency. The calculations show that for PAHs larger than 150 carbons, the 6.3 {mu}m feature becomes very broad and shifts to longer wavelengths, the 8.6 {mu}m band becomes stronger than the '7.7' {mu}m band, and the 11.0/12.7 band strength ratiomore » gets too large compared with observations. Thus, PAHs with 150 carbons or more are unlikely to be the dominant species in interstellar space. The simplicity of the observed spectra in the 15-20 {mu}m range points toward a preponderance of compact PAHs in the interstellar PAH family.« less

Properties of the high-mass star forming region IRAS 22475+5939

IRAS 22475+5939 has been well studied by previous astronomers, but we can still discover new characteristics about it, using the first observations of the lines from CO J=2−1, 13CO J=2−1 and 13CO J=3−2 by the KOSMA 3m telescope. The mapping of the intensity ratio of 13CO J=3−2 and 13CO J=2−1 shows that the distribution of the temperature has two peaks, neither of which coincide with the IRAS 22475+5939 source or the center of the HII region, but rather are located at the edge of the HII region. Overlays of the Spitzer IRAC 8 μm with CO contours indicate that they are associated with each other and that the strongest polycyclic aromatic hydrocarbon (PAH) emission is at the IRAS 22475+5939 source position. The IRAS LRS spectrum at 7 μm ∼ 23 μm and the PHT-s spectrum at 2 μm ∼ 12 μm of the IRAS 22475+5939 source also exhibit strong PAH emission characteristics in the main PAH bands. The diversity of the PAH family should be responsible for the plateaus in the PAH emission in the PHT-s and IRAS-LRS spectra. Analysis and modeling in the infrared bands suggest that IRAS 22475+5939 is more likely to be a class I young stellar object. Where this is the case, the star is likely to have a temperature of Teff ∼ 9995.8K, mass ∼ 15.34 M⊙, luminosity ∼ 1.54 × 104 L⊙ and age ∼ 1.54 × 104 yr. The model shows that circumstellar disc emission is important for wavelengths between 1 and 10 μm, otherwise, the envelope fluctuates for λ > 10μm. Bipolar outflow is confirmed in the molecular cloud, and the excited star of the HII region could be the driving source of the outflow. High resolution is required for better results.

Vibrational excitations in molecular layers probed by ballistic electron microscopy

We demonstrate the information on molecular vibrational modes via the second derivative (d2IB/dV2) of the ballistic electron emission spectroscopy (BEES) current. The proposed methoddoes not create huge fields as in the case of conventional derivative spectroscopy andmaintains a zero bias across the device. BEES studies carried out on three different types oflarge polycyclic aromatic hydrocarbon (PAH) molecular layers show that thed2IB/dV2 spectra consist of uniformly spaced peaks corresponding to vibronic excitations. The peakspacing is found to be identical for molecules within the same PAH family though theBEES onset voltage varies for different molecules. In addition, injection into a particularorbital appears to correspond to a specific vibrational mode as the manifestation of thesymmetry principle.

POLYCYCLIC AROMATIC HYDROCARBON FAR-INFRARED SPECTROSCOPY

The far-IR characteristics of astrophysically relevant polycyclic aromatic hydrocarbons (PAHs) averaging in size around 100 carbon atoms have been studied using the theoretical spectra in the NASA Ames PAH IR Spectroscopic Database. These spectra were calculated using density functional theory. Selections of PAH species are made, grouped together by common characteristics or trends, such as size, shape, charge, and composition, and their far-IR spectra compared. The out-of-plane modes involving the entire molecule are explored in detail, astronomical relevance is assessed, and an observing strategy is discussed. It is shown that PAHs produce richer far-IR spectra with increasing size. PAHs also produce richer far-IR spectra with increasing number of irregularities. However, series of irregular-shaped PAHs with the same compact core have common "Jumping-Jack" modes that "pile up" at specific frequencies in their average spectrum. For the PAHs studied here, around 100 carbon atoms in size, this band falls near 50 μm. PAH charge and nitrogen inclusion affect band intensities but have little effect on far-IR band positions. Detailed analysis of the two-dimensional, out-of-plane bending "drumhead" modes in the coronene and pyrene "families" and the one-dimensional, out-of-plane bending "bar" modes in the acene "family" show that these molecular vibrations can be treated as classical vibrating sheets and bars of graphene, respectively. The analysis also shows that the peak position of these modes is very sensitive to the area of the emitting PAH and does not depend on the particular geometry. Thus, these longest wavelength PAH bands could provide a unique handle on the size of the largest species in the interstellar PAH family. However, these bands are weak. Observing highly excited regions showing the mid-IR bands in which the emission from classical dust peaks at short wavelengths offers the best chance of detecting PAH emission in the far-IR. For these regions sensitivity is not an issue, spectral contrast is maximized and the PAH population is only comprised of highly stable, compact symmetric PAHs, such as the members of the pyrene and coronene "families" discussed in detail here.

Search for far-IR PAH bands with Herschel: modelling and observational approaches

Herschel opens the possibility to detect the low-frequency vibrational bands of individual polycyclic aromatic hydrocarbon (PAH) molecules and therefore to progress in our understanding of the nature of these species and the properties of the environments from which they emit. However, unless one individual molecule dominates the PAH family, this detection will not be straightforward and it is necessary to optimise the observational search with an educated guess of the band profiles and intensities. Such educated guess can be obtained from models that include a detailed description of the molecular properties (anharmonicity, rotation...) in the modelling of the cooling cascade of the emitting species. First results are expected soon from the observation of the Orion Bar as part of the HEXOS Herschel key program.