Unidentified Infrared Emission Research Articles

Machine learning identified molecular fragments responsible for infrared emission features of polycyclic aromatic hydrocarbons

ABSTRACT Machine learning feature importance calculations are used to determine the molecular substructures that are responsible for mid- and far-infrared (IR) emission features of neutral polycyclic aromatic hydrocarbons (PAHs). Using the extended-connectivity fingerprint as a descriptor of chemical structure, a random forest model is trained on the spectra of 14 124 PAHs to evaluate the importance of 10 632 molecular fragments for each band within the range of 2.761 to $1172.745\, \mu$m. The accuracy of the results is confirmed by comparing them with previously studied unidentified infrared emission (UIE) bands. The results are summarized in two tables available as Supplementary Data, which can be used as a reference for assessing possible UIE carriers. We demonstrate that the tables can be used to explore the relation between the PAH structure and the spectra by discussing about the IR features of nitrogen-containing PAHs and superhydrogenated PAHs.

Infrared spectra of isoquinolinium (iso-C9H7NH+) and isoquinolinyl radicals (iso-C9H7NH and 1-, 3-, 4-, 5-, 6-, 7- and 8-iso-HC9H7N) isolated in solid para-hydrogen.

Protonated polycyclic aromatic nitrogen heterocycles (H+PANH) are prospective candidates that may contribute to interstellar unidentified infrared (UIR) emission bands because protonation enhances the relative intensities of the bands near 6.2, 7.7 and 8.6 μm, and the presence of the N atom induces a blue shift of the ring-stretching modes so that the spectra of H+PANH match better with the 6.2 μm feature in class-A UIR spectra. We report the infrared (IR) spectra of protonated isoquinoline (the 2-isoquinolinium cation, iso-C9H7NH+), its neutral counterpart (the 2-isoquinolinyl radical, iso-C9H7NH), and another mono-hydrogenated product (the 6-isoquinolinyl radical, 6-iso-HC9H7N), produced on the electron-bombardment of a mixture of isoquinoline (iso-C9H7N) with excess para-hydrogen (p-H2) during matrix deposition at 3.2 K. To generate additional isomers of hydrogenated isoquinoline, we irradiated iso-C9H7N/Cl2/p-H2 matrices at 365 nm to generate Cl atoms, followed by IR irradiation to generate H atoms via Cl + H2 (v = 1) → HCl + H; the H atoms thus generated reacted with iso-C9H7N. In addition to iso-C9H7NH and 6-iso-HC9H7N observed in the electron-bombardment experiments, we identified six additional hydrogenated isoquinoline species, 1-, 3-, 4-, 5-, 7- and 8-iso-HC9H7N, via their IR spectra; hydrogenation on the N atom and all available carbon atoms except for the two sharing carbon atoms on the fused ring was observed. Spectral groupings were achieved according to their behaviors after maintenance of the matrix in darkness and on secondary photolysis at various wavelengths. The assignments were supported via comparison of the experimental results with the vibrational wavenumbers and IR intensities of possible isomers predicted using the B3LYP/6-311++G(d,p) method. The implications in the identification of the UIR band are discussed.

Infrared Spectra of 1-Quinolinium (C9H7NH+) Cation and Quinolinyl Radicals (C9H7NH and 3-, 4-, 7-, and 8-HC9H7N) Isolated in Solid para-Hydrogen.

Large protonated polycyclic aromatic hydrocarbons (H+PAH) and the corresponding nitrogen heterocycles (H+PANH) have been proposed as possible carriers of unidentified infrared (UIR) emission bands from galactic objects. The nitrogen atom in H+PANH is expected to induce a blue shift of the band associated with the CC-stretching mode of H+PAH near 6.3 μm so that their emission bands might agree better with the UIR band near 6.2 μm. We report the IR spectrum of protonated quinoline (1-quinolinium cation, C9H7NH+) and its neutral species (1-quinolinyl radical, C9H7NH) measured upon electron bombardment during the deposition of a mixture of quinoline (C9H7N) and para-hydrogen (p-H2) at 3.2 K, indicating that the protonation and hydrogenation occur mainly at the N atom site. Additional experiments on the irradiation of C9H7N/Cl2/p-H2 matrices at 365 nm to generate Cl atoms, followed by irradiation with IR light to generate H atoms via Cl + H2 (v = 1), were performed to induce the reaction H + C9H7N. This method proved to be efficient for hydrogenation reactions in solid p-H2; we identified, in addition to C9H7NH observed in electron-bombardment experiments, four radicals with hydrogenation at the C-atom site─3-, 4-, 7-, and 8-HC9H7N. Spectral assignments were achieved according to the behavior upon secondary photolysis and a comparison of experimental results with vibrational wavenumbers and IR intensities predicted with the B3LYP/6-311++G(d,p) method. The observed lines at 1641.4, 1598.4, and 1562.0 cm-1 associated with the CC-stretching mode of C9H7NH+ are blue-shifted from those at 1618.7, 1580.8, 1556.7, and 1510.0 cm-1 of the corresponding protonated naphthalene (C10H9+).

Unidentified infrared emission features in mid-infrared spectrum of comet 21P/Giacobini-Zinner

Comet 21P/Giacobini-Zinner (hereafter, comet 21P/G-Z) is a Jupiter-family comet and a parent comet of the October Draconids meteor shower. If meteoroids originating from a Jupiter-family comet contain complex organic molecules, such as amino acids, they are essential pieces of the puzzle regarding the origin of life on Earth. We observed comet 21P/G-Z in the mid-infrared wavelength region using the Cooled Mid-infrared Camera and Spectrometer (COMICS) on the 8.2 m Subaru Telescope on UT 2005 July 5. Here, we report the unidentified infrared (UIR) emission features of comet 21P/G-Z, which are likely due to complex organic molecules (both aliphatic and aromatic hydrocarbons), and the thermal emission from amorphous/crystalline silicates and amorphous carbon grains in its mid-infrared low-resolution spectrum. The UIR features at ~8.2 μm, ~8.5 μm, and ~11.2 μm found in the spectrum of comet 21P/G-Z could be attributed to polycyclic aromatic hydrocarbons (or hydrogenated amorphous carbons) contaminated by N- or O-atoms, although part of the feature at ~11.2 μm comes from crystalline olivine. The other feature at ~9.2 μm might originate from aliphatic hydrocarbons. Comet 21P/G-Z is enriched in complex organic molecules. Considering that the derived mass fraction of crystalline silicates in comet 21P/G-Z is typical of comets, we propose that the comet originated from a circumplanetary disk of giant planets (similar to Jupiter and Saturn) where was warmer than the typical comet-forming region (5–30 au from the Sun) and was suitable for the formation of complex organic molecules. Comets from circumplanetary disks might be enriched in complex organic molecules, such as comet 21P/G-Z, and may have provided pre-biotic molecules to ancient Earth by direct impact or meteor showers.

Infrared spectrum of hydrogenated corannulene rim-HC20H10 isolated in solid para-hydrogen.

Hydrogenated polycyclic aromatic hydrocarbons have been proposed to be carriers of the interstellar unidentified infrared (UIR) emission bands and the catalysts for formation of H2; spectral characterizations of these species are hence important. We report the infrared (IR) spectrum of mono-hydrogenated corannulene (HC20H10) in solid para-hydrogen (p-H2). In experiments of electron bombardment of a mixture of corannulene and p-H2 during deposition of a matrix at 3.2 K, two groups of spectral lines increased with time during maintenance of the matrix in darkness after deposition. Lines in one group were assigned to the most stable isomer of hydrogenated corannulene, rim-HC20H10, according to the expected chemistry and a comparison with scaled harmonic vibrational wavenumbers and IR intensities predicted with the B3PW91/6-311++G(2d,2p) method. The lines in the other group do not agree with predicted spectra of other HC20H10 isomers and remain unassigned. Alternative hydrogenation was achieved with H atoms produced photochemically in the infrared-induced reaction Cl + H2 (v = 1) → H + HCl in a Cl2/C20H10/p-H2 matrix. With this method, only lines attributable to rim-HC20H10 were observed, indicating that hydrogenation via a quantum-mechanical tunneling mechanism produces preferably the least-energy rim-HC20H10 regardless of similar barrier heights and widths for the formation of rim-HC20H10 and hub-HC20H10. The mechanisms of formation in both experiments are discussed. The bands near 3.3 and 3.4 µm of rim-HC20H10 agree with the UIR emission bands in position and relative intensity, but other bands do not match satisfactorily with the UIR bands.

Infrared spectra of protonated and hydrogenated corannulene (C20H10) and sumanene (C21H12) using matrix isolation in solid para-hydrogen – implications for the UIR bands

AbstractPolycyclic aromatic hydrocarbons (PAH) and their derivatives, including protonated and cationic species, are suspected to be carriers of the unidentified infrared (UIR) emission bands observed from the galactic and extragalactic sources. We investigated the infrared (IR) spectra of protonated nonplanar PAHs: corannulene (C20H10) and sumanene (C21H12), that are regarded as a fragments of a fullerene,C60. The protonated corannulene H+ C20H10 and sumanene H+ C21H12 were produced in seperate experiments by bombarding a mixture of corannulene/sumanene and para-hydrogen (p-H2) with electrons during deposition at 3.2 K. During maintenance of the electron-bombarded matrix in darkness the intensities of IR lines of protonated corannulene decreased because of neutralization by electrons that were slowly released from the trapped sites whereas the hydrogenated species were produced. The observed lines were classified into several groups according to their responses to darkness and secondary irradiation at 365 nm/385 nm LEDs. Spectral assignments were derived based on a comparison of the observed spectra with those predicted with the B3PW91/6-311+ +G(2d,2p) method. The observed IR spectrum of hub-H+ C20H10, the most stable protonated isomer, resembles several bands of the Class-A UIR bands.

Formation and infrared identification of protonated fluoranthene isomers 3-, 9-, and 10-C16H11+ in solid para-H2.

Polycyclic aromatic hydrocarbons (PAH) and their derivatives are prospective carriers of unidentified infrared (UIR) emission features observed in interstellar media. Fluoranthene (C16H10) is a simple planar PAH with five- and six-membered rings; it can be considered as a fragment of C60, which, along with its cationic counterpart, has been identified in interstellar media. Protonated fluoranthene, C16H11+, was generated upon electron bombardment during deposition at 3.2 K of p-H2 containing fluoranthene in a small proportion. The intensities of infrared features of C16H11+ decreased after maintaining the matrix in darkness because of its neutralization with trapped electrons. According to the correlations in intensities upon neutralization and secondary photolysis, observed lines were classified into three groups which are assigned to isomers 3-C16H11+, 9-C16H11+, and 10-C16H11+. Experimental vibrational wavenumbers and relative IR intensities of the features agree with corresponding calculated values predicted for these three isomers of C16H11+ with the B3PW91/6-311++G(2d,2p) method. 3-C16H11+ and 9-C16H11+ are predicted to have the lowest energy (within 5 kJ mol-1), whereas 10- and 1-C16H11+ are lying above the global minimum 3-C16H11+ by ∼20 kJ mol-1. However, definitive identification of 1-C16H11+ could not be made as only the most intense line is tentatively assigned. Although the observed spectra of these isomers match unsatisfactorily with the UIR bands, they will facilitate the potential terrestrial and extraterrestrial identification of these species.

Infrared Spectrum of Protonated Corannulene H+C20H10 in Solid para-Hydrogen and its Potential Contribution to Interstellar Unidentified Infrared Bands

Polycyclic aromatic hydrocarbons (PAH) and their derivatives, including protonated and cationic species, are suspected to be carriers of the unidentified infrared (UIR) emission bands observed from the galactic and extragalactic sources. We extended our investigations of infrared (IR) spectra of protonated planar PAH to a nonplanar PAH, corannulene (C20H10), which is regarded as a fragment of a fullerene, C60. The protonated corannulene H+C20H10 was produced on bombarding a mixture of corannulene and para-hydrogen (p-H2) with electrons during deposition at 3.2 K. During maintenance of the electron-bombarded matrix in darkness the intensities of IR lines of protonated corannulene decreased because of neutralization by electrons that were slowly released from the trapped sites. The observed lines were classified into two groups according to their responses to secondary irradiation at 365 nm. Eighteen lines in one group are assigned to the lowest-energy species among five possible isomers, hub-H+C20H10, and ...

Origin of Spectral Band Patterns in the Cosmic Unidentified Infrared Emission.

The cosmic unidentified infrared emission (UIE) band phenomenon is generally considered as indicative of free-flying polycyclic aromatic hydrocarbon molecules in space. However, a coherent explanation of emission spectral band patterns depending on astrophysical source is yet to be resolved under this attribution. Meanwhile astronomers have restored the alternative origin as due to amorphous carbon particles, but assigning spectral patterns to specific structural elements of particles is equally challenging. Here we report a physical principle in which inclusion of nonplanar structural defects in aromatic core molecular structures (π domains) induces spectral patterns typical of the phenomenon. We show that defects in model π domains modulate the electronic-vibration coupling that activates the delocalized π-electron contribution to aromatic vibrational modes. The modulation naturally disperses C=C stretch modes in band patterns that readily resemble the UIE bands in the elusive 6-9 μm range. The electron-vibration interaction mechanics governing the defect-induced band patterns underscores the importance of π delocalization in the emergence of UIE bands. We discuss the global UIE band regularity of this range as compatible with an emission from the delocalized sp^{2} phase, as π domains, confined in disordered carbon mixed-phase aggregates.

Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups: Intensity Scaling for the C–H Stretching Modes and Astrophysical Implications

Abstract The so-called unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and ubiquitously seen in a wide variety of astrophysical regions are generally attributed to polycyclic aromatic hydrocarbon (PAH) molecules. Astronomical PAHs may have an aliphatic component, as revealed by the detection in many UIE sources of the aliphatic C–H stretching feature at . The ratio of the observed intensity of the feature to that of the aromatic C–H feature allows one to estimate the aliphatic fraction of the UIE carriers. This requires knowledge of the intrinsic oscillator strengths of the aromatic C–H stretch ( ) and the aliphatic C–H stretch ( ). Lacking experimental data on and for the UIE candidate materials, one often has to rely on quantum-chemical computations. Although the second-order Møller–Plesset (MP2) perturbation theory with a large basis set is more accurate than the B3LYP density functional theory, MP2 is computationally very demanding and impractical for large molecules. Based on methylated PAHs, we show here that, by scaling the band strengths computed at an inexpensive level (e.g., B3LYP/6-31G*), we are able to obtain band strengths as accurate as those computed at far more expensive levels (e.g., MP2/6-311+G(3df,3pd)). We calculate the model spectra of methylated PAHs and their cations excited by starlight of different spectral shapes and intensities. We find that , the ratio of the model intensity of the feature to that of the feature, is insensitive to the spectral shape and intensity of the exciting starlight. We derive a straightforward relation for determining the aliphatic fraction of the UIE carriers (i.e., the ratio of the number of C atoms in aliphatic units to that in aromatic rings ) from the observed band ratios : for neutrals and for cations.

A Small Fullerene (C24) may be the Carrier of the 11.2 μm Unidentified Infrared Band

Abstract We analyze the spectrum of the 11.2 μm unidentified infrared band (UIR) from NGC 7027 and identify a small fullerene (C24) as a plausible carrier. The blurring effects of lifetime and vibrational anharmonicity broadening obscure the narrower, intrinsic spectral profiles of the UIR band carriers. We use a spectral deconvolution algorithm to remove the blurring, in order to retrieve the intrinsic profile of the UIR band. The shape of the intrinsic profile—a sharp blue peak and an extended red tail—suggests that the UIR band originates from a molecular vibration–rotation band with a blue band head. The fractional area of the band-head feature indicates a spheroidal molecule, implying a nonpolar molecule and precluding rotational emission. Its rotational temperature should be well approximated by that measured for nonpolar molecular hydrogen, ∼825 K for NGC 7027. Using this temperature, and the inferred spherical symmetry, we perform a spectral fit to the intrinsic profile, which results in a rotational constant implying C24 as the carrier. We show that the spectroscopic parameters derived for NGC 7027 are consistent with the 11.2 μm UIR bands observed for other objects. We present density functional theory (DFT) calculations for the frequencies and infrared intensities of C24. The DFT results are used to predict a spectral energy distribution (SED) originating from absorption of a 5 eV photon, and characterized by an effective vibrational temperature of 930 K. The C24 SED is consistent with the entire UIR spectrum and is the dominant contributor to the 11.2 and 12.7 μm bands.

THE 6 μm FEATURE AS A TRACER OF ALIPHATIC COMPONENTS OF INTERSTELLAR CARBONACEOUS GRAINS

ABSTRACT An unidentified infrared emission (UIE) feature at 6.0 μm is detected in a number of astronomical sources showing the UIE bands. In contrast to the previous suggestion that this band is due to C=O vibrational modes, we suggest that the 6.0 μm feature arises from olefinic double-bond functional groups. These groups are likely to be attached to aromatic rings, which are responsible for the major UIE bands. The possibility that the formation of these functional groups is related to the hydrogenation process is discussed.

On the aliphatic versus aromatic content of the carriers of the ‘unidentified’ infrared emission features

Although it is generally accepted that the so-called "unidentified" infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 micrometer are characteristic of the stretching and bending vibrations of aromatic hydrocarbon materials, the exact nature of their carriers remains unknown: whether they are free-flying, predominantly aromatic gas-phase molecules, or amorphous solids with a mixed aromatic/aliphatic composition are being debated. Recently, the 3.3 and 3.4 micrometer features which are commonly respectively attributed to aromatic and aliphatic C-H stretches have been used to place an upper limit of ~2\% on the aliphatic fraction of the UIE carriers (i.e. the number of C atoms in aliphatic chains to that in aromatic rings). Here we further explore the aliphatic versus aromatic content of the UIE carriers by examining the ratio of the observed intensity of the 6.2 micrometer aromatic C-C feature (I6.2) to that of the 6.85 micrometer aliphatic C-H deformation feature (I6.85). To derive the intrinsic oscillator strengths of the 6.2 micrometer stretch (A6.2) and the 6.85 micrometer deformation (A6.85), we employ density functional theory to compute the vibrational spectra of seven methylated polycyclic aromatic hydrocarbon molecules and their cations. By comparing I6.85/I6.2 with A6.85/A6.2, we derive the fraction of C atoms in methyl(ene) aliphatic form to be at most ~10\%, confirming the earlier finding that the UIE emitters are predominantly aromatic. We have also computed the intrinsic strength of the 7.25 micrometer feature (A7.25), another aliphatic C-H deformation band. We find that A6.85 appreciably exceeds A7.25. This explains why the 6.85 micrometer feature is more frequently detected in space than the 7.25 micrometer feature.

THE INFRARED SPECTRUM OF PROTONATED OVALENE IN SOLID PARA-HYDROGEN AND ITS POSSIBLE CONTRIBUTION TO INTERSTELLAR UNIDENTIFIED INFRARED EMISSION

ABSTRACT The mid-infrared emission from galactic objects, including reflection nebulae, planetary nebulae, proto-planetary nebulae, molecular clouds, etc, as well as external galaxies, is dominated by the unidentified infrared (UIR) emission bands. Large protonated polycyclic aromatic hydrocarbons (H+PAHs) were proposed as possible carriers, but no spectrum of an H+PAH has been shown to exactly match the UIR bands. Here, we report the IR spectrum of protonated ovalene (7-C32H15 +) measured in a para-hydrogen (p-H2) matrix at 3.2 K, generated by bombarding a mixture of ovalene and p-H2 with electrons during matrix deposition. Spectral assignments were made based on the expected chemistry and on the spectra simulated with the wavenumbers and infrared intensities predicted with the B3PW91/6-311++G(2d,2p) method. The close resemblance of the observed spectral pattern to that of the UIR bands suggests that protonated ovalene may contribute to the UIR emission, particularly from objects that emit Class A spectra, such as the IRIS reflection nebula, NGC 7023.

Organic dust in galaxies

Recent space infrared telescopes, Infrared Space Observatory, Spitzer Space Telescope, and AKARI have made significant progress in our understanding of organic dust in the Universe. In this review, we discuss recent observations with these space telescopes of the unidentified infrared emission (UIE) features in the near to mid-infrared, which come from very small organic dust, and the absorption features from 3 to 7 µm, which characterize large organic dust. They provide us with a new view of organic dust in galaxies. We also briefly discuss latest AKARI observations of H2O and CO2 ices in 2.5-5 µm in the Large Magellanic Cloud in comparison with observations in our Galaxy, which suggests the importance of dust surface chemistry in the formation of organic matters in the Universe.

THE C–H STRETCHING FEATURES AT 3.2–3.5 μm OF POLYCYCLIC AROMATIC HYDROCARBONS WITH ALIPHATIC SIDEGROUPS

ABSTRACT The so-called “unidentified” infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 μm are ubiquitously seen in a wide variety of astrophysical regions. The UIE features are characteristic of the stretching and bending vibrations of aromatic hydrocarbon materials, e.g., polycyclic aromatic hydrocarbon (PAH) molecules. The 3.3 μm aromatic C–H stretching feature is often accompanied by a weaker feature at 3.4 μm. The latter is generally thought to result from the C–H stretch of aliphatic groups attached to the aromatic systems. The ratio of the observed intensity of the 3.3 μm aromatic C–H feature to that of the 3.4 μm aliphatic C–H feature allows one to estimate the aliphatic fraction of the UIE carriers, provided that the intrinsic oscillator strengths of the 3.3 μm aromatic C–H stretch ( ) and the 3.4 μm aliphatic C–H stretch ( ) are known. While previous studies on the aliphatic fraction of the UIE carriers were mostly based on the ratios derived from the mono-methyl derivatives of small PAH molecules, in this work we employ density functional theory to compute the infrared vibrational spectra of PAH molecules with a wide range of sidegroups including ethyl, propyl, butyl, and several unsaturated alkyl chains, as well as all the isomers of dimethyl-substituted pyrene. We find that, except for PAHs with unsaturated alkyl chains, the corresponding ratios are close to that of mono-methyl PAHs. This confirms the predominantly aromatic nature of the UIE carriers previously inferred from the ratio derived from mono-methyl PAHs.

Mid-infrared vibrational study of deuterium-containing PAH variants

Polycyclic Aromatic Hydrocarbon (PAH) molecules have been long proposed to be a major carrier of ‘Unidentified Infrared’ (UIR) emission bands that have been observed ubiquitously in various astrophysical environments. These molecules can potentially be an efficient reservoir of deuterium. Once the infrared properties of the deuterium-containing PAHs are well understood both experimentally and theoretically, the interstellar UIR bands can be used as a valuable tool to infer the cause of the deuterium depletion in the ISM.Density Functional Theory (DFT) calculations have been carried out on deuterium-containing ovalene variants to study the infrared properties of these molecules. These include deuterated ovalene, cationic deuterated ovalene, deuteronated ovalene and deuterated-deuteronated ovalene. We present a D/H ratio calculated from our theoretical study to compare with the observationally proposed D/H ratio.

The survival of PAHs and (hydro)carbon nanoparticles in H II regions

AbstractObservations show that emission from the Unidentified Infrared (UIR) bands is strongly suppressed in H II regions. UIR bands are generally attributed to vibrational relaxation of FUV - excited Polycyclic Aromatic Hydrocarbon (PAH) molecules or hydrocarbon nanoparticles containing aromatic domains. If the strongly reduced UIR emission in H II regions is due to the suppression of the carriers, an efficient destruction mechanism is required to explain observations. The aim of this work is to clarify whether UV processing of PAHs and nanoparticles is indeed responsible for the observed lack of infrared emission. We present here our first results on the physical response to photo-processing of the proposed UIR-bands carriers.

ON THE ORIGIN OF THE 11.3 MICRON UNIDENTIFIED INFRARED EMISSION FEATURE

The 11.3 μm emission feature is a prominent member of the family of unidentified infrared emission (UIE) bands and is frequently attributed to out-of-plane bending modes of polycyclic aromatic hydrocarbon (PAH) molecules. We have performed quantum mechanical calculations of 60 neutral PAH molecules and found that it is difficult to reconcile the observed astronomical feature with any or a mix of these PAH molecules. We have further analyzed the fitting of spectra of several astronomical objects by the NASA PAH database program and found that reasonable fittings to the observed spectra are only possible by including significant contributions from oxygen- and/or magnesium-containing molecules in the mix. A mix of pure PAH molecules, even including units of different sizes, geometry, and charged states, is unable to fit the astronomical spectra. Preliminary theoretical results on the vibrational spectra of simple molecules with mixed aromatic/aliphatic structures show that these structures have consistent clusters of vibrational modes and could be viable carriers of the UIE bands.

A THEORETICAL STUDY ON THE VIBRATIONAL SPECTRA OF POLYCYCLIC AROMATIC HYDROCARBON MOLECULES WITH ALIPHATIC SIDEGROUPS

The role of aliphatic side groups on the formation of astronomical unidentified infrared emission (UIE) features is investigated by applying the density functional theory (DFT) to a series of molecules with mixed aliphatic-aromatic structures. The effects of introducing various aliphatic groups to a fixed polycyclic aromatic hydrocarbon (PAH) core (ovalene) are studied. Simulated spectra for each molecule are produced by applying a Drude profile at $T$=500 K while the molecule is kept at its electronic ground state. The vibrational normal modes are classified using a semi-quantitative method. This allows us to separate the aromatic and aliphatic vibrations and therefore provide clues to what types of vibrations are responsible for the emissions bands at different wavelengths. We find that many of the UIE bands are not pure aromatic vibrational bands but may represent coupled vibrational modes. The effects of aliphatic groups on the formation of the 8 $\mu$m plateau are qua ntitatively determined. The vibrational motions of methyl ($-$CH$_3$) and methyl ene ($-$CH$_2-$) groups can cause the merging of the vibrational bands of the pa rent PAH and the forming of broad features. These results suggest that aliphatic structures can play an important role in th e UIE phenomenon.