Spectra Of Polycyclic Aromatic Hydrocarbons Research Articles

Pollution characteristics, source apportionment and absorption spectra of size-resolved PAHs in atmospheric particles in a cold megacity of China

Polycyclic aromatic hydrocarbons (PAHs) have the capability for solar radiation absorption related to climate forcing. Herein, pollution characteristics and absorption spectra of size-resolved PAHs in atmospheric particles in a cold megacity were comprehensively investigated. The mean concentrations of Σ18PAHs in all the 11 particle size ranges were 3.95 ± 4.77 × 104 pg/m3 and 2.17 ± 1.54 × 103 pg/m3 in heating period (HP) and non-heating period (NHP), respectively. Except for most PAHs with 2 and 3 benzene rings in NHP, most other PAHs showed a unimodal distribution pattern with the peak at 0.56–1.0 µm in both periods, which was caused by PAH emission sources. The PAH-related climate forcing was mainly caused by the solar radiation absorptions at ∼325 (∼330) nm and ∼365 nm. In general, the absorption intensities were higher in HP than NHP. The absorption intensity in the particle size range of 0.56–1.0 µm was the highest, and benzo[e]pyrene was the dominant contributor. In colder periods in HP, higher PAH concentrations caused more intensive PAH-related climate forcing. This study provided new insights for pollution characteristics and absorption spectra of size-resolved PAHs in atmospheric particles, which will be useful for better understanding PAH-related climate forcing.

The infrared absorption spectrum of phenylacetylene and its deuterated isotopologue in the mid- to far-IR.

Anharmonicity strongly influences the absorption and emission spectra of polycyclic aromatic hydrocarbon (PAH) molecules. Here, IR-UV ion-dip spectroscopy experiments together with detailed anharmonic computations reveal the presence of fundamental, overtone, as well as 2- and 3-quanta combination band transitions in the far- and mid-infrared absorption spectra of phenylacetylene and its singly deuterated isotopologue. Strong absorption features in the 400-900 cm-1 range originate from CH(D) in-plane and out-of-plane wags and bends, as well as bending motions including the C≡C and CH bonds of the acetylene substituent and the aromatic ring. For phenylacetylene, every absorption feature is assigned either directly or indirectly to a single or multiple vibrational mode(s). The measured spectrum is dense, broad, and structureless in many regions but well characterized by computations. Upon deuteration, large isotopic shifts are observed. At frequencies above 1500 cm-1 for d1-phenylacetylene, a one-to-one match is seen when comparing computations and experiments with all features assigned to combination bands and overtones. The C≡C stretch observed in phenylacetylene is not observed in d1-phenylacetylene due to a computed 40-fold drop in intensity. Overall, a careful treatment of anharmonicity that includes 2- and 3-quanta modes is found to be crucial to understand the rich details of the infrared spectrum of phenylacetylene. Based on these results, it can be expected that such an all-inclusive anharmonic treatment will also be key for unraveling the infrared spectra of PAHs in general.

Fe(II) activated calcium peroxide/peroxymonosulfate: A practical system for phenanthrene degradation and upholding ecological pH

Groundwater contamination by polycyclic aromatic hydrocarbons (PAHs) can have adverse effects on both the subsurface environment and human health. This study aimed to demonstrate the degradation of PHE using a coupled oxidant system, that involves simultaneous injection of Fe(II) and oxidants, CaO2 (CP) and peroxymonosulfate (PMS). To ensure the environmental compatibility, coupled oxidant strategy was proposed to maintain the pH level between 6.0 and 8.0 following the treatment process. The influence of reaction conditions including concentration of PMS, CP, and Fe(II) concentrations, initial pH, anions, and humic acid on PHE degradation were assessed. Scavenger experiments and electron spin resonance (ESR) analysis confirmed that hydroxyl radical (HO−) and non-radical singlet oxygen (1O2) were the primary reactive oxygen species (ROS) in the reported system. PHE degradation mechanism in the coupled oxidant system was proposed based on by-products identification. In addition, the CP/PMS/Fe(II) system showed promising degradation performance for naphthalene and pyrene, indicating the proposed strategy could be a viable option for removing a broad spectrum of PAHs. Finally, the coupled oxidant system was optimized to achieve best PHE removal efficiency under natural groundwater (NGW) conditions.

Size distribution of polycyclic aromatic hydrocarbons in space: an old new light on the 11.2/3.3 μm intensity ratio.

The intensity ratio of the 11.2/3.3 μm emission bands is considered to be a reliable tracer of the size distribution of polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium (ISM). This paper describes the validation of the calculated intrinsic infrared (IR) spectra of PAHs that underlie the interpretation of the observed ratio. The comparison of harmonic calculations from the NASA Ames PAH IR spectroscopic database to gas-phase experimental absorption IR spectra reveals a consistent underestimation of the 11.2/3.3 μm intensity ratio by 34%. IR spectra based on higher level anharmonic calculations, on the other hand, are in very good agreement with the experiments. While there are indications that the 11.2/3.3 μm ratio increases systematically for PAHs in the relevant size range when using a larger basis set, it is unfortunately not yet possible to reliably calculate anharmonic spectra for large PAHs. Based on these considerations, we have adjusted the intrinsic ratio of these modes and incorporated this in an interstellar PAH emission model. This corrected model implies that typical PAH sizes in reflection nebulae such as NGC 7023 - previously inferred to be in the range of 50 to 70 carbon atoms per PAH are actually in the range of 40 to 55 carbon atoms. The higher limit of this range is close to the size of the C60 fullerene (also detected in reflection nebulae), which would be in line with the hypothesis that, under appropriate conditions, large PAHs are converted into the more stable fullerenes in the ISM.

Revisiting the Absorption Spectra of Polycyclic Aromatic Hydrocarbons over Porto (Portugal) by TD-DFT Calculations

Brown carbon is a type of strong light-absorbing carbonaceous aerosol associated with radiative forcing. Nevertheless, the difficulty in correlating the chemical composition of brown carbon with its light absorption properties impairs the proper elucidation of its role in radiative forcing. Here, we have used a time-dependent density functional theory (TD-DFT)-based procedure to revisit the “real-world” absorption spectra of polycyclic aromatic hydrocarbons (PAHs) over the city of Porto, in Portugal, while correcting the spectra for their quantity in PM10 particulate matter. Our aim is to, by comparing these new results with those obtained previously regarding PM2.5 data, evaluate the role of different groupings of particulate matter in the light absorption of brown carbon. The results indicate that irrespective of the absorption spectra corresponding to their PM10 or PM2.5 data, the studied PAHs should contribute to radiative forcing by light absorption at UVA and (sub)visible wavelengths. However, the identity of the individual PAH species that contribute the most for the considered wavelengths can be quite different. Thus, different groupings of particulate matter appear to provide distinct contributions to light absorption and radiative forcing over the same location, even when considering the same class of molecular compounds.

Study of vibrational spectra of polycyclic aromatic hydrocarbons with phenyl side group

Computational study of polycyclic aromatic hydrocarbons (PAHs) with phenyl side group substituted at different positions is reported. The infrared spectral variations due to the position of phenyl substitution, ionization state and the size of the molecules are discussed and possible contribution of phenyl-PAHs to the mid-infrared emission features from astrophysical objects is analysed. Structurally phenyl group substitution at 2nd position gives more stable species compared to substitution at other positions. Phenyl-PAHs exhibit new aromatic bands near 695 and 741 cm−1 (14.4 and 13.5μm), due to contribution from quintet CH wag, that compare well with minor features at 14.2 and 13.5μm observed in several astrophysical objects. Just as in plain PAHs, the C–C stretch vibrational modes (∼1600 cm−1) have negligible intensity in neutrals, but the cations of all phenyl-PAHs exhibit significantly strong phenyl group C–C stretch peak close to class B type 6.2μm astrophysical band. In 2-phenylpyrene, it is the neutral molecule that exhibits this strong feature in the 6.2μm range along with other features that match with sub-features at 6.66 and 6.9μm, observed in astronomical spectra of some late type objects. The substitution of phenyl side group at solo position shifts the C–C stretch mode of parent PAH close to the region of 6.2μm astrophysical band. The results indicate possibility of phenyl-PAHs in space and the bottom-up formation of medium sized compact PAHs with phenyl side group in carbon rich cool circumstellar shells. Phenyl-PAHs need to be considered in modelling mid-infrared emission spectra of various astrophysical objects.

Modeling the nocturnal/diurnal and seasonal real world absorption spectra of polycyclic aromatic hydrocarbons and their derivatives in two Chinese polluted cities

Brown carbon (BrC), an organic aerosol, plays an important role in radiative forcing. Polycyclic aromatic hydrocarbons (PAHs) and their derivatives such as oxygenated and nitrated polycyclic aromatic hydrocarbons (OPAHs & NPAHs) are the major constituents of BrC and are persistent environmental pollutants. Our strategy here is to utilize time dependent-density functional theory (TD-DFT) to model the absorption spectra of PAHs and their derivatives in two Chinese industrial sites: Qingcheng district (site A) and Longtang town of Qingyuan (site B). These data are corrected for “Real-world” experimental concentrations of PAHs over these cities. For the first time, nocturnal/diurnal and seasonal variations of PAHs are being simultaneously studied under a theoretical framework. These findings show that the absorptions at site A and B take place mainly due to PAHs while OPAHs and NPAHs have negligible contribution. The site A is highly affected by climate forcing caused by these PAHs. The absorption in winter is higher as compared to that of in summer. Our theoretical modeling approach remarkably identifies the most relevant PAHs for climate forcing in both Chinese regions.

A time-dependent density functional theory investigation of the atmospheric absorption spectra of polycyclic aromatic hydrocarbons (PAHs) and their derivatives (Alkyl-PAHs, oxygenated-PAHs, and Nitrated-PAHs) over an urban area in China

Polycyclic aromatic hydrocarbons (PAHs) and their oxygenated (OPAHs) and nitrated (NPAHs) derivatives are main chromophores of the carbonaceous aerosol brown carbon (BrC), which is linked with radiative forcing. Here, we investigated the atmospheric absorption spectra of 64 PAHs, OPAHs, and NPAHs directly over the Chinese megacity of Xi’an, by employing a time-dependent density functional theory (TD-DFT) computational approach and correcting the results for the experimentally determined atmospheric concentration of the studied molecules. The obtained data showed that these molecules contribute more to radiative forcing by absorbing light in the UVA and (sub)visible region of the spectrum. Investigating “daily” absorption spectra revealed major seasonal variation in the intensity of light absorption, but little changes in the shape of the absorption spectra. The observed light absorption can be explained mainly by contributions from PAHs and to a lesser extent by carbonyl-OPAHs, with relatively low contributions of the other OPAHs and NPAHs. Among them, benzo[b+j+k]fluoranthenes, benzo[e]pyrene, benzo[a]pyrene, benzo[ghi]perylene, indeno[1,2,3-cd]pyrene, 6H-benzo[cd]pyren-6-one, 7H-benz[de]anthracen-7-one, and benz[a]anthracene-7,12-dione are highlighted as potentially problematic contributors for radiative forcing over Xi’an.

Single-particle characterization of polycyclic aromatic hydrocarbons in background air in northern Europe

Abstract. We investigated the distribution of polycyclic aromatic hydrocarbons (PAHs) on individual ambient aerosol particles at the Swedish western coast in a pristine environment for 10 d in October 2019. The measurements were carried out using new technology with single-particle mass spectrometry (SPMS) that reveals both the inorganic particle composition as well as the particle-bound PAHs (Schade et al., 2019). More than 290 000 particles were characterized; 4412 of them reveal PAH signatures. Most of the PAH-containing particles were internal mixtures of carbonaceous material, secondary nitrate and metals from distant sources in central and eastern Europe. We characterize the aerosol with respect to the inorganic composition, comparable to conventional SPMS, before we discuss the distribution of PAHs within this particle ensemble. Vice versa, we analyze the single-particle PAH spectra for characteristic patterns and discuss the inorganic composition, origin and atmospheric processing of the respective particles. The study period comprised different meteorological situations: clean air conditions with winds from the North Sea/Kattegat and little terrestrial air pollution, long-range transport from eastern Europe and southern Sweden, and transport of aerosols from central Europe over the sea. For all meteorological conditions, PAHs were detected in particles whose inorganic content indicates traffic emissions, such as combinations of soot, iron and calcium as well as in particles with biomass-burning signatures. However, there were variations in their amounts, dependent on the geographic origin. Because of strong mixing, rapid degradation and speciation limits, e.g., for PAHs of the same nominal mass, the application of diagnostic ratios for source apportionment is limited under the conditions of our study. Nevertheless, the combination with the inorganic content and meteorological data provides unique insights into the particles' origin, aging and mixing state. We exemplarily show how the observation of PAH profiles and inorganic secondary components on a single-particle level can open a new door to investigate aerosol aging processes. To our best knowledge, we herewith present the first comprehensive study on the single-particle distribution of PAHs in ambient air as well as the first set of combined data on PAHs and inorganic composition on a single-particle level.

TD-DFT Monitoring of the Absorption Spectra of Polycyclic Aromatic Hydrocarbons over the Basque Country, Spain

Brown carbon is a type of carbonaceous aerosol with strong light absorption in the ultraviolet and visible wavelengths that leads to radiative forcing. However, it is difficult to correlate the chemical composition of brown carbon with its atmospheric light absorption properties, which translates into significant uncertainty. Thus, a time-dependent density functional theory (TD-DFT) approach was used to model the real-world absorption properties of 14 polycyclic aromatic hydrocarbons (PAHs) over three regions of the Basque Country (Spain): Bilbao, Urretxu, and Azpeitia. The data were corrected for atmospheric concentration. The results show that the absorption spectra over each region are qualitatively identical, with the absorption intensities being significantly higher over Bilbao than over Azpeitia and Urretxu. Furthermore, it was found that the light absorption by PAHs should be more relevant for radiative forcing when it occurs at UVA and (sub)visible wavelengths. Finally, among the 14 studied PAHs, benzo[b]fluoranthene, pyrene, fluoranthene, benzo[a]pyrene, and benzo[k]fluoranthene and benzoperylene were identified as the molecules with larger contributions to radiative forcing.

Excitation, emission, and synchronous fluorescence for astrochemical applications: Experiments and computer simulations of synchronous spectra of polycyclic aromatic hydrocarbons and their mixtures

Synchronous fluorescence spectroscopy is a variation of the fluorescence technique for detection and characterization of many single ring aromatic organic molecules, polycyclic aromatic hydrocarbons, and fluorescent amino acids among other molecules. To our knowledge this technique has not been used in space exploration. In order to demonstrate the utility of this technique for planetary and astrochemical applications, we present several laboratory experiments and computer simulations with pure samples and mixtures. Computer deconvolution of experimental excitation and emission fluorescence bands is presented and used to generate synchronous spectra. The computer simulation successfully predicts the number of synchronous fluorescence (SF) bands, band shapes, and band maximum wavelengths for any constant wavelength difference (Δλ). To test the simulation, emission, excitation, and synchronous spectra were obtained for anthracene in n-hexane. Excellent agreement is obtained reproducing and finding the origin of the experimental SF bands for values of Δλ between 2 and 100. The excitation, emission, and synchronous (Δλ = 10) spectra of toluene, aniline, naphthalene, acenaphthene, pyrene, and anthracene are obtained. Excitation of a mixture of the six compunds at three wavelengths (249, 266, and 355 nm) produce fluorescence emission spectra identifying some compounds. The synchronous spectrum (Δλ = 10) of the same mixture is presented and assigned based on the synchronous bands of the individual compounds. The synchronous fluorescence technique and the computer simulation method are proposed to complement other techniques in the analysis of fluorescent samples from comets, as well as in missions to planets and satellites of the solar system.

Modeling the infrared cascade spectra of small PAHs: the 11.2\xa0\u03bcm band

The profile of the 11.2 μm feature of the infrared (IR) cascade emission spectra of polycyclic aromatic hydrocarbon (PAH) molecules is investigated using a vibrational anharmonic method. Several factors are found to affect the profile including: the energy of the initially absorbed ultraviolet (UV) photon, the density of vibrational states, the anharmonic nature of the vibrational modes, the relative intensities of the vibrational modes, the rotational temperature of the molecule, and blending with nearby features. Each of these factors is explored independently and influence either the red or blue wing of the 11.2 μm feature. The majority impact solely the red wing, with the only factor altering the blue wing being the rotational temperature.

Deuterated Polycyclic Aromatic Hydrocarbons in the Interstellar Medium: The C–D Band Strengths of Multideuterated Species

Abstract Observationally, the interstellar gas-phase abundance of deuterium (D) is considerably depleted and the missing D atoms are often postulated to have been locked up into carbonaceous solids and polycyclic aromatic hydrocarbon (PAH) molecules. An accurate knowledge of the fractional amount of D (relative to H) tied up in carbon dust and PAHs has important cosmological implications since D originated exclusively from the Big Bang and the present-day D abundance, after accounting for the astration it has experienced during Galactic evolution, provides essential clues to the primordial nucleosynthesis and the cosmological parameters. To quantitatively explore the extent to which PAHs could possibly accommodate the observed D depletion, we have previously quantum-chemically computed the infrared vibrational spectra of monodeuterated PAHs and derived the mean intrinsic band strengths of the 3.3 μm C–H stretch (A 3.3) and the 4.4 μm C–D stretch (A 4.4). Here we extend our previous work to multideuterated PAH species of different deuterations, sizes, and structures. We find that both the intrinsic band strengths A 3.3 and A 4.4 (on a per-unit-bond basis) and their ratios A 4.4/A 3.3 not only show little variation among PAHs of different deuterations, sizes, and structures, they are also closely similar to that of monodeuterated PAHs. Therefore, a PAH deuteration level (i.e., the fraction of peripheral atoms attached to C atoms in the form of D) of ∼2.4% previously estimated from the observed 4.4 to 3.3 μm band ratio based on the A 4.4/A 3.3 ratio of monodeuterated PAHs is robust.

Mid-infrared spectroscopic signatures of dibenzopyrene cations – The effect of symmetry on PAH IR spectroscopy

In this work, we characterize – for the first time – in the gas phase infrared spectra of three isomeric Polycyclic Aromatic Hydrocarbon (PAH) cations of C24H14 composition that belong to distinctly different symmetry groups (C2h, Cs and C1). Mid-infrared (Mid-IR) spectra are recorded by means of infrared multiple photon dissociation (IRMPD) spectroscopy at the free electron laser for infrared experiments (FELIX) laboratory. The measured infrared (IR) band positions compare reasonably well with density functional theory (DFT) calculated values. The number of IR active bands increases as the symmetry of the molecule lowers. The IRMPD spectra of irregular PAHs are found to be dense and do not resemble the sharp signatures typical of astronomical IR bands, but rather look like the broad plateau on which these are perched. This lends credit to the GrandPAH hypothesis that suggests that small and irregular PAHs are weeded out by the strong interstellar radiation field and only large regular PAHs remain.

Deuterated Polycyclic Aromatic Hydrocarbons in the Interstellar Medium: The C–D Band Strengths of Monodeuterated Species

Abstract Deuterium (D) is one of the light elements created in the Big Bang. As the Galaxy evolves, the D/H abundance in the interstellar medium (ISM) decreases from its primordial value due to astration. However, the observed gas-phase D/H abundances of some sightlines in the local Galactic ISM are substantially lower than the expected reduction by astration. The missing D could have been depleted onto polycyclic aromatic hydrocarbon (PAH) molecules which are ubiquitous and abundant in interstellar regions. To quantitatively explore the hypothesis of PAHs as a possible reservoir of interstellar D, we quantum-chemically compute the infrared vibrational spectra of monodeuterated PAHs and their cations. We find that, as expected, when H in PAHs is replaced by D, the C–H stretching and bending modes at 3.3, 8.6, and 11.3 μm shift to longer wavelengths at ∼4.4, 11.4, and 15.4 μm, respectively, by a factor of ∼ , the difference in reduced mass between the C–H and C–D oscillators. From the computed spectra we derive the mean intrinsic band strengths of the 3.3 μm C–H stretch and 4.4 μm C–D stretch to be and for neutral deuterated PAHs which would dominate the interstellar emission at 3.3 and 4.4 μm. By comparing the computationally derived mean band-strength ratio of for neutral PAHs with the mean ratio of the observed intensities of , we find that the degree of deuteration (i.e., the fraction of peripheral atoms attached to C atoms in the form of D) is ∼2.4%, corresponding to a D enrichment of a factor of ∼1200 with respect to the interstellar D/H abundance.

Kinetic Monte Carlo statistics of curvature integration by HACA growth and bay closure reactions for PAH growth in a counterflow diffusion flame

This paper uses a Kinetic Monte Carlo model that includes processes to integrate curvature due to the formation of five- and seven-member rings to simulate polycyclic aromatic hydrocarbons (PAHs) growing in lightly sooting ethylene and acetylene counterflow diffusion flames. The model includes new processes to form seven-member rings via hydrogen-abstraction-acetylene-addition and bay closure reactions on sites containing partially embedded five-member rings. The model additionally includes bay closure and HACA bay capping reactions for the integration of five-member rings. The mass spectra of PAHs predicted by the model are assessed against experimental data, and the distribution of embedded five-member rings and seven-member rings is studied as a function of spatial location, molecule size and frequency of events sampled in the simulation. The simulations show that the formation of seven-member rings and the embedding of five-member rings is a competitive process. Both types of rings are observed more frequently as the simulation proceeds from the fuel outlet towards the stagnation plane. Approximately 15% of the events that integrate curvature resulted in the formation of a seven-member ring coupled to an embedded five-member ring, and the remaining 85% of events embedded five-member rings via the formation of six-member rings. The proportion of PAHs containing embedded five-member rings and/or seven-member rings is observed to be a function of PAH size, passing through a maximum for PAHs containing 15–20 six-member rings. However, the proportion of PAHs containing both types of ring increases with PAH size, where upwards of 10% of PAHs containing at least one five-member ring and 15 or more six-member rings also contain a seven-member ring.

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.

The infrared bands of polycyclic aromatic hydrocarbons in the 1.6–1.7 μm wavelength region

Context. The 3.3 μm aromatic C–H stretching band of polycyclic aromatic hydrocarbon (PAH) molecules seen in a wide variety of astrophysical regions is often accompanied by a series of weak satellite bands at ∼3.4–3.6 μm. One of these sources, IRAS 21282+5050, a planetary nebula, also exhibits a weak band at ∼1.68 μm. While the satellite features at ∼3.4–3.6 μm are often attributed to the anharmonicities of PAHs, it is not clear whether overtones or combination bands dominate the 1.68 μm feature. Aims. In this work, we examine the anharmonic spectra of eight PAH molecules, including anthracene, tetracene, pentacene, phenanthrene, chrysene, benz[a]anthracene, pyrene, and perylene, to explore the origin of the infrared bands in the 1.6–1.7 μm wavelength region. Methods. Density functional theory (DFT) in combination with the vibrational second-order perturbation theory (VPT2) was used to compute the anharmonic spectra of PAHs. To simulate the vibrational excitation process of PAHs, the Wang–Landau random walk technique was employed. Results. All the dominant bands in the 1.6–1.7 μm wavelength range and in the 3.1–3.5 μm C–H stretching region are calculated and tabulated. It is demonstrated that combination bands dominate the 1.6–1.7 μm region, while overtones are rare and weak in this region. We also calculate the intensity ratios of the 3.1–3.5 μm C–H stretching features to the bands in the 1.6–1.7 μm region, I3.1 − 3.5/I1.6 − 1.7, for both ground and vibrationally excited states. On average, we obtain ⟨I3.1 − 3.5/I1.6 − 1.7⟩≈12.6 and ⟨I3.1 − 3.5/I1.6 − 1.7⟩≈17.6 for PAHs at ground states and at vibrationally excited states, respectively.

Polycyclic Aromatic Hydrocarbons with Armchair Edges: Potential Emitters in Class B Sources

We have systematically investigated a series of polycyclic aromatic hydrocarbons (PAHs) with armchair edges that maximize the aromaticity, denoted as Clar PAHs, using density functional theory. Their structures fall into three major categories: branched, intermediate, and armchair. Branched PAHs contain up to 60 carbons, they are eroded and the least stable, and their spectra are the most complex. Armchair PAHs are the most stable and their spectra differ from the nominal PAH spectra by lacking a 11.2 μm band and having their 7.8 and 8.2 μm bands coalesce as their size increases. Only intermediate and armchair PAHs have 12.7/13.5 μm PAH band intensity ratios that are consistent with observations. The fitting results show that Clar PAH spectra are more consistent with class B than class A sources, with only a few small Clar PAHs contributing to the fit of the class A source IRAS 23133+6050 and large Clar PAHs contributing to the fit of the class B source IRAS 17347−3139, and NGC 7027. Overall, our study suggests that large Clar PAHs are potential emitters in class B sources.

Anharmonicity in the mid-infrared spectra of polycyclic aromatic hydrocarbons: molecular beam spectroscopy and calculations

Aims. In this work we determine the effects of anharmonicity on the mid-infrared spectra of the linear polycyclic aromatic hydrocarbons (PAHs) naphthalene, anthracene, tetracene and pentacene recorded using the free electron laser FELIX. Methods. Comparison of experimental spectra obtained under supersonic jet conditions with theoretically predicted spectra was used to show if anharmonicity explicitly needs to be taken into account. Results. Anharmonic spectra obtained using second-order vibrational perturbation theory agree on average within 0.5% of the experimental frequencies. Importantly, they confirm the presence of combination bands with appreciable intensity in the 5–6 μm region. These combination bands contain a significant fraction of the IR absorption, which scales linearly with the size of the PAH. Detection and assignment of the combination bands are a preliminary indication of the accuracy of far-IR modes in our anharmonic theoretical spectra. Detailed analysis of the periphery-sensitive CH out-of-plane band of naphthalene reveals that there is still room for improvement of the VPT2 approach. In addition, the implications of our findings for the analysis of the aromatic infrared bands are discussed.