Unidentified IR Research Articles

Paving the way to the synthesis of PAHs in dark molecular clouds: The formation of cyclopentadienyl radical (c-C5H5)

Context. The interest of astrophysicists and astrochemists in studying polycyclic aromatic hydrocarbons (PAHs) has grown since their assignment to previously unidentified IR emission bands of dust grains. Although observations show that PAHs are present in the interstellar medium (ISM), there is still no consensus about their formation. PAH formation has been studied following two approaches: bottom-up (i.e. through the association of smaller hydrocarbons) and top-down (via the photo-dissociation or hydrogenation of larger-structures). The recent detection of simple aromatic rings in the TMC-1 molecular cloud seems to suggest the bottom-up scenario is favoured. Aims. We study the reaction between the propargyl (C3H3) radical and the vinyl radical (C2H3) under interstellar conditions, providing new kinetic parameters for the formation of PAH precursors. Methods. We used high-level quantum-chemical calculations to describe the reaction mechanism between these two radicals. We calculated the rate constant of the individual product channels in the temperature range 10–400 K and at 1 × 10−7 atm by solving the one-dimensional master equation to quantitatively understand whether this reaction is viable in the ISM. Results. Our results show that this reaction is likely to occur even in the low-density (~104 cm−3) and low-temperature (~10 K) conditions of molecular dark clouds. The main product is the cyclopentadienyl radical (C5H5). The unsaturated open-chain hydrocarbon C5H5 in the trans (trans-C5H5) and cis conformations (cis-C5H5) are also formed, but to a lesser extent. Conclusions. Our results show that the reaction of vinyl radical with propargyl radical is efficient under interstellar conditions and yields the cyclopentadienyl radical, which could be an important precursor for the formation of more complex polycyclic hydrocarbons (e.g. indene) and simple PAHs (e.g. naphthalene) in molecular dark clouds.

A chemical link between methylamine and methylene imine and implications for interstellar glycine formation

Methylamine CH3NH2 is considered to be an important precursor of interstellar amino acid because hydrogen abstraction might lead to the aminomethyl radical •CH2NH2 that can react with •HOCO to form glycine, but direct evidence of the formation and spectral identification of •CH2NH2 remains unreported. We performed the reaction H + CH3NH2 in solid p-H2 at 3.2 K and observed IR spectra of •CH2NH2 and CH2NH upon irradiation and when the matrix was maintained in darkness. Previously unidentified IR spectrum of •CH2NH2 clearly indicates that •CH2NH2 can be formed from the reaction H + CH3NH2 in dark interstellar clouds. The observed dual-cycle mechanism containing two consecutive H-abstraction and two H-addition steps chemically connects CH3NH2 and CH2NH in interstellar media and explains their quasi-equilibrium. Experiments on CD3NH2 produced CD2HNH2, in addition to •CD2NH2 and CD2NH, confirming the occurrence of H addition to •CD2NH2.

Fullerene Oligomers and Polymers as Carriers of Unidentified IR Emission Bands

Abstract Several unidentified infrared emission bands (UIBs) have been assigned to neutral C60 molecules present in circumstellar and interstellar environments. However, due to the similarity of the infrared (IR) spectra of C60 in the solid state and in the gas phase, as of yet there is no consensus on the aggregation state of C60. In this article, we show that even strong covalent chemical bonding might have very little influence on the IR spectrum of C60, and that therefore such chemically bonded C60 could be the carrier of the same UIBs. It would best explain observations like the missing emission from C60 ions and a large variation of relative band intensities between different sources. We demonstrate that such a chemically bonded C60 can be produced by the co-condensation of C atoms together with C60 molecules, which leads to the formation of a three-dimensional C60 polymer film. Such polymerized C60 molecules cannot easily desorb, while their spectral properties in the visible and IR spectral ranges are almost undisturbed by polymerization.

Computational Study of the Malonic Acid Tautomerization Products in Highly Concentrated Particles.

Knowing the tautomeric form of malonic acid (MA) in concentrated particles is critical to understanding its effect on the atmosphere. Energies and vibrational modes of hydrated MA particles were calculated using density functional theory (DFT) at the B3LYP/6-31G(d,p) level and the effective fragment potential (EFP) method. Visualization of the keto and enol isomer vibrational modes enabled the assignment of keto isomer peaks in the 1710-1750 cm-1 range, and previously unidentified experimental IR peaks in the 1690-1710 cm-1 can now be attributed to the enol isomer. Comparison of calculated spectra of pure hydrated enol or keto isomers confirm recent experimental evidence, presented by Ghorai et al. ( J. Phys. Chem. A 2011 , 115 , 4373 - 4380 ) of a shift in the keto-enol tautomer equilibrium when MA exists as concentrated particles.

Platinum Group Minerals and Silicate Inclusions In Chromitite From the Naga-Manipur Ophiolite Complex, Indo-Myanmar Orogenic Belt, Northeast India

Abstract This study reports the first documented occurrence of platinum group minerals (PGM) and silicate inclusions in the podiform chromitites of the Naga-Manipur ophiolite complex, Indo-Myanmar orogenic belt, northeastern India. The studied chromitites were divided into three groups, high-Cr (Cr# = 0.72 to 0.80), intermediate-Cr (Cr# = 0.66 to 0.68), and high-Al (Cr# = 0.46 to 0.47). The bulk PGE content is low and varies in the range of 57–162 ppb. The chondrite normalized PGE patterns show that the studied chromitites are enriched in IPGE (Os, Ir, Ru) over PPGE (Rh, Pt, Pd), which in general is consistent with podiform chromitites hosted in a mantle sequence of ophiolites. All the PGE patterns are characterized by a consistent positive anomaly in Ru and some samples display a slight positive anomaly in Pt, compared with Rh and Pd. Most of the PGM occur in fresh chromite crystals consisting of single or polyphase grains composed of different PGM associated with Ni and Cu sulfides and silicates. The primary PGM paragenesis consists of minerals of the laurite (RuS 2 )–erlichmanite (OsS 2 ) series with minor osmium, iridium, and ruthenium. An unidentified Ir–Ni sulfide mineral, probably kashinite, was also observed. Based on their assemblage, we can argue that the PGM discovered in the Naga-Manipur chromitites are magmatic in origin and formed at relatively high sulfur fugacity, as testified by the presence of Os-rich laurite, erlichmanite, and kashinite. However, the relatively low PGE content of the investigated chromitites suggests that sulfur saturation was not achieved in the parental magma during the early stage of their crystallization. The presence of heazlewoodite in chromite fractures and in the altered matrix reflects its secondary origin, suggesting that it formed progressively during the serpentinization process. The presence of both Cr-rich and Al-rich chromitite suggest that the Naga-Manipur chromitite formed from separate intrusions of magmas derived from different depleted mantle sources ( e.g ., MORB versus boninitic) during evolution of the oceanic lithosphere from the MOR (Al-rich chromitite) towards a supra subduction zone (SSZ). Abundant hydrous silicates inclusions were found exclusively in the Cr-rich chromitites, whereas the Al-rich chromitites contain only clinopyroxene, confirming the proposed geodynamic evolution.

Coronene and pyrene (5, 7)-member ring defects

PAHs are known to be one of the carriers of the ubiquitous aromatic IR bands. The IR spectra of many objects show IR emission features derived from PAH molecules of different size. Still some of the characteristics of the emitting population remain unclear. The emission bands show details which cannot be explained so far. These unidentified IR features require further laboratory and observational investigations. We present a theoretical study of the IR spectra of PAHs containing (5,7)-member ring defects, focusing on pyrene and coronene. Using density functional theory, we investigate the effects of such defects on the IR spectra of pyrene and coronene and their cations and anions. In addition, we explore parts of the potential energy surface of the neutral species and discuss alternative formation pathways. The addition of (5,7)-membered ring defects in pyrene and coronene results in a change of the IR spectra, both molecules lose their typical spectroscopic signature. We find shifts in the positions of the band as well as different intensities and a rise in the number of features. The boundaries in terms of the size of the PAHs exhibiting a (5,7)-membered ring defect are studied and shown. Investigation of the minimal energy pathway leads to a result of 8.21 eV for pyrene and 8.41 eV for coronene as minimum activation barriers for the transformation from ground state to defected state. Whereas pyrene retains some of its symmetry due to the symmetry exhibited by the Stone-Wales defect itself, coronene loses much more of its symmetry. The formation of these (5,7)-ring defects in PAHs may be well supported in AGB stars or PNe. Those environments strongly enable the transition from the ground state to the defect state. Therefore the knowledge of the IR spectra of these molecules will support future investigations aiming for a thorough understanding of the unidentified IR emission bands.

Synchrotron-based rotationally resolved high-resolution FTIR spectroscopy of azulene and the unidentified infrared bands of astronomy.

Chasing the unidentified IR bands: The first rotationally resolved high-resolution infrared spectrum of azulene is reported using synchrotron Fourier transform infrared spectroscopy including a rovibrational analysis of the out-of-plane fundamental ν44. Comparison of azulene, naphthalene, indole, and biphenyl infrared bands leads to coincidences with UIR bands at 12.8 μm with naphthalene and at 13.55 and 14.6 μm with biphenyl bands, but excluding azulene as a strong absorber.

Identification of the “unidentified IR features” of interstellar dust

Identification of the “unidentified IR features” of interstellar dust

UIR bands in the ISO SWS spectrum of the carbon star TU Tauri

Polycyclic Aromatic Hydrocarbon (PAH) molecules are thought to form in the ejecta of carbon-rich Asymptotic Giant Branch (AGB) stars. Fifty ISO SWS spectra of warm carbon-rich AGB stars have been investigated for the Unidentified IR (UIR) emission due to fluorescence by PAH molecules. In this sample the binary star TU Tau, which has a blue companion, shows interesting spectral structure in the appropriate wavelength regions. The profiles of the UIR bands in TU Tau have been derived by comparing to suitable carbon-star spectra. The presence of the UIR bands in TU Tau is attributed to UV photons originating from the A2 companion star which are necessary to excite PAH molecules. The absence of the UIR bands in the remainder of the sample is ascribed to the lack of UV ( or visible) photons in their environment. Hence, the absence of UIR bands does not necessarily imply the absence of circumstellar PAHs in these sources. The derived UIR band profiles have been compared to UIR band profiles from Reflection Nebulae, Planetary Nebulae (PNe), HII regions, Young Stellar Objects, evolved stars and Galaxies. The profiles of TU Tau are shown to have the most resemblance to those from PNe. Integrated band flux ratios have also been determined and compared to object type flux ratio correlations found in other studies. Here no definite match was found. TU Tau is the only binary with a blue companion in this sample of AGB stars. In line with earlier studies, we suggest that the blue photons provided by this companion are required for efficient excitation of PAH molecules in AGB ejecta. In addition, we argue that these blue photons may promote complex chemistry in the ejecta of TU Tau. The similarity in the peak profiles observed in the spectrum of TU Tau with those in the spectra of PNe indicates that PAHs are formed in the circumstellar envelope of carbon-rich AGB stars and make it largely unmodified into the PNe phase. The variations in the band strength ratios between the different objects has been linked to the ionization state of PAHs and reflects the different physical environments between this AGB star and PNe. In contrast, the variation in UIR band profiles between stellar ejecta and the Interstellar Medium ( ISM) are largely attributed to chemical modifications during the ISM phase.

Quantized Redshifts of Galaxies: Stimulated Raman Scattering in Cold Intergalactic Rydberg Matter

That the redshifts for galaxies in the local supercluster are quantizedwas recently confirmedby Guthrie and Napier(AAAmp;A310 (1996) 353). These redshifts are here proposed to be due to stimulatedStokes Raman processes in intergalactic matter in the form of Rydberg Matter (RM). Rydberg Matteris an electronically excited material, as demonstrated by its use as laser medium in a thermally excitedultra-broadband tunable IR laser (Chem. Phys. Lett. 376 (2003) 812). Its existence in interstellar andintergalactic space is demonstrated by several observational results, notably the unidentified IR bands,that agree well with the emission from Rydberg Matter. A stimulated Raman process will allow theH I 21 cm radiation to proceed without deflection, in agreement with observation. Such redshiftswill be additive during the passage through space. The process in Rydberg Matter here proposed togive rise to the Stokes Raman process is excitation of electronic translational modes in the planarclusters forming the matter. The specific cluster sizes found in laboratory experiments give rise toa few differently sized redshift quanta, which is in good agreement with the observed quanta. Anexcitation level (principal quantum number) of Rydberg Matter in intergalactic space between 175and 200 gives the correct size of the redshift quanta.

Excitation processes for the emission of the unidentified IR bands

To our knowledge, no individual polycyclic aromatic hydrocarbon (PAH) molecule has yet been shown to carry all the ubiquitous and closely correlated unidentified IR (UIR) bands. More generally, no IR space observatory (ISO) instrument was able to split these bands into the narrower signatures expected from molecules. Again, the Japanese IRTS satellite has shown that the near-IR CH-stretching band observed near the galactic plane carries an important aliphatic component alien to aromatic molecules. On the other hand, a number of solid-state, disordered, more or less hydrogenated, carbonaceous materials have been shown to carry all the UIR bands. Moreover, their emission spectrum, computed under thermal equilibrium with typical circumstellar radiation fields, was shown to fit, satisfactorily and in detail, the observed near- and mid-IR spectra of a number of planetary nebulae (PNe) and proto-planetary nebulae (PPNe). However, the interstellar radiation field (ISRF) is not strong enough to heat this solid-state dust to the equilibrium temperatures required for emission observed from the galactic plane and reflection nebulae. Even stochastic heating is insufficient in these cases because nanometric grains are too small to retain the desirable optical properties of the bulk. One way out of this deadlock is to look for other excitation mechanisms. Noting that spatial maxima of UIR emission usually occur in photodissociation regions between ionized and molecular gas maxima, where atomic H is most abundant, we have set up to study the interactions of atomic hydrogen with solid-state carbon surfaces. We consider, in particular, a mechanism in which, upon collision with a grain, the potential energy of recombination carried by an atom is delivered to one of the functional groups which will emit a UIR band. Such an IR chemiluminescence is found to be possible, in principle. A laboratory experiment dedicated to the quest of IR chemiluminescence has been built and is described.

Which carriers for the unidentified IR emission bands? Observations and laboratory simulations

The spectra of samples of pyrolysed polycyclic aromatic hydrocarbons (PAHs) are shown to present some striking similarities to the interstellar IR emission bands observed at 3.3, 6.2, 7.6, 7.8, 8.6, 11.3 µm and to the related features at 3.5 and 12.7 µm. The spectral variations which are observed during the pyrolysis of several PAHs: phenanthrene C14H10, coronene C24H12 and ovalene C32H14, are discussed in relation to the spectral diversity observed in the astronomical spectra. In the laboratory, the spectral changes are found to be related to the polymerization of PAHs, which induces the formation of very large PAHs and leads to various reactions between hydrogen and PAHs: H-transfer, H-loss upon aromatization and possibly H-capture by other species. These processes give characteristic features which seem to have their counterpart in astronomical spectra. This further constrains the identity of the carriers of the emission bands and the physico-chemical conditions which prevail in the regions where they are observed. A tentative identification is given, which attributes the classical emission bands to large (possibly ionised) PAHs, the anomalous 3.51(3.53) µm band to superhydrogenated PAHs, the broad 8.7 µm band observed in HII regions to very large PAH systems, and the 12.7 µm band to (slightly) dehydrogenated PAHs. The 11.0 µm band might be indicative of more severe dehydrogenation. The proposed identifications have yet to be further tested by dedicated observational and laboratory work.

The coal model applied to the Red Rectangle and the Galactic IR emission

Thanks to the knowledge of the spectral absorptivity of coal over a wide wavelength range, the coal model for interstellar dust is now in a favourable position to allow many quantitative comparisons with astronomical data. In this paper, we present two examples of such comparisons which are particularly relevant to the issue of the excitation mechanism of the UIBs (unidentified IR bands). First, we compare the predicted spatial distribution of the UIB emission in the Red Rectangle with the highly spatially resolved observations performed by Bregman et al. (Astrophys. J., 411, 794–796, 1993). It is shown that the two different spatial distributions observed at 3.3 and 11.3 μm are easily understandable in the frame of coal grain emission at thermal equilibrium in a decreasing radiation field. Next, we synthesize the average emission spectrum of the Galaxy from 2 to 400 μm, taking into account all the available observations. It is found that the near-IR Galactic emission can be attributed to a very small amount of warm coal-like grains (300 K) in the immediate vicinity of excited nebulae, while far-IR radiation is emitted by cold coal- and silicate-like grains in the diffuse ISM (interstellar medium). In-both cases considered here, steady-state heating of coal-like grains is compatible with all the observations.

A NEW MID-INFRARED CAMERA FOR GROUND-BASED ASTRONOMY AND AN INFRARED STUDY OF PLANETARY NEBULAE

ABSTRACT The dissertation is composed of two parts. The first part is a description of the Mid-Infrared Array Camera (MIRAC), a new camera for ground-based astronomy. The second part of the dissertation is an infrared study of planetary nebulae utilizing observations with the new camera. MIRAC is a collaborative effort between the University of Arizona, Smithsonian Astrophysical Observatory, and Naval Research Laboratory. It currently utilizes a Hughes 20 X 64 Si:As impurity-band-conduction (IBC) detector array with a capacitive transimpedance amplifier (CTIA) readout, which is sensitive to IR radiation from 2 to 26 microns. The camera is equipped with 10% bandiwidth filters at 2.2, 3.8, 4.6, 8.8, 9.8, 11.7, and 12.5 microns, and a wide-band 8.0-12.8 micron (N-band) filter. There is also a 20% filter at 20.5 microns and an 8-14 micron CVF with a resolution of 1.8%. An adjustable slide with several pupil stops allows the camera to be configured for telescopes with focal ratios ranging from 15 to 45. Two externally selectable magnifications are provided by the fully reflective optics, giving plate scales of 0.35" and 0.7" pixel-1 on the NASA IRTF. The MIRAC electronics provides timing signals and coadds successive frames at a maximum rate of 10 kHz for the full array, and higher rates for a partial array readout. the data are transferred via a serial interface to a PC for real-time display, storage, and further processing. MIRAC recently achieved a NEFD of 0.010 Jy arcsec-2 at 8.8, 11.7, and 12.5 microns for a 900-s on-source integration on the Steward Observatory 1.5-m telescope. Planetary nebulae (PNe) are formed from circumstellar material ejected during the late stages of stellar evolution. These nebulae are an important source of enrichment for the interstellar medium. In many PNe, there is an excess of emission in the IR, indicating the presence of dust. There are several different components seen in the IR emission, including a family of unidentified IR (UIR) emission features at 3.3, 6.2, 7.7, and 11.3 microns. The source morphology provides information on the structure of the nebula as well as the carriers of the emission features. Images in the near- and/or mid-IR are presented for the following PNe: IC 418, BD+30degree3639, J900, NGC 2392, NGC 6543, AFGL 2688, and M 2-9. The mid-IR images were obtained using MIRAC, and the images at J, H, and K were obtained using the Steward Observatory 64 X 64 Hg:Cd:Te array camera. In BD+30degree3639, the UIR emission is seen to be spatially distinct from the IR continuum and the ionized gas as traced by the 12.8 micron [Ne II] line emission. The average temperature across the nebula of the dust responsible for the mid-IR continuum emission is 172 K. In IC 418, the size of the nebula increases as a function of wavelength in the near-IR images at J, H, and K. Emission from the halo is detected in the H and K images. In NGC 2392 and NGC 6543, evidence for excess emission is seen in the distribution of the near-IR flux. The distribution of IR emission differs significantly from the emission observed at optical wavelengths. In the bipolar nebulae AFGL 2688 and M 2-9, structures in the IR emission are seen that could be related to the equatorial density enhancements that have caused the bipolar morphology.

Contribution of polycyclic aromatic hydrocarbon molecules to the interstellar extinction curve

Absorption cross sections of terrestrial mixtures of gaseous polycyclic aromatic hydrocarbons (PAHs) have been measured in the visible to far-UV range. The spectral signatures that we have obtained are compatible with those of the extinction curve. These results support the PAH model which attributes the unidentified IR emission bands to these molecules. Interstellar PAHs can give an important contribution to the 2175 A bump, and do absorb in the far-UV rise. We derive that they contain about 18% of the cosmic carbon

Synthesis, reactions and catalytic activities of cationic iridium(I) complexes of cycloocta-1,5-diene

New cationic iridium(I) complexes, [Ir(cod)(PPh3)L]ClO41[cod = cycloocta-1,5-diene; L = PhCN, PhCHCHCN, CH2CHCN, CH2C(Me)CN, MeCHCHCN or CH2CHCH2CN co-ordinated through the nitrogen atom], have been prepared by the reactions of [IrCl(cod)(PPh3)] with AgClO4 in the presence of L. Reaction of [Ir(cod)(PPh3)(PhCN)]ClO41a with H2 gives the cis-dihydridoiridium(III) complex [IrH2(cod)(PPh3)(PhCN)]ClO42a where the two hydrides are trans to PhCN and an olefinic group of cod, respectively. Complex 2a is stable both in solution and in the solid state at low temperature and decomposes at 15 °C to give cyclooctane and unidentified Ir–cod complex(es). The nitrile (L) in 1 is readily replaced by both PPh3 and CO, while cod is replaced only by CO. In the presence of complexes 1, unsaturated alcohols such as CH2CHCH2OH, CH2CHCH(Me)OH and CH2CHCH(Ph)OH rapidly undergo isomerization to the corresponding saturated carbonyl compounds at room temperature. Complex 1a catalyses the hydrogenation of unsaturated aldehydes, PhCHCRCHO (R = H, Me or Cl) to give PhCHCRCH2OH, PhCH2CHRCHO and PhCH2CHRCH2OH under H2(pH2= 6 atm) at 50 °C.

Raman spectra of different carbonaceous materials of astrophysical interest

We present here the Raman spectra of 9 different carbonaceous materials of astronomical relevance, including single polycyclic aromatic hydrocarbons and mixtures. Eight of 9 samples show two main bands falling respectively at around 1600 and 1350 cm −1 which imply that all the materials are composed by randomly oriented structural units varying from 50 to 80 Å in size. Further bands and/or shoulders appear in some cases between 800 and 1700 cm −1 (6 and 12 μm) and are attributed to CC or CH n ( n = 1,2,3) functional groups. The coronene spectrum is the only one which does not present any significant band although it shows a very high level of fluorescence. These results provide information about candidate materials which may account for unidentified IR bands observed in astronomical sources.

Grain ultraviolet extinction properties of recently discovered POST asymptotic giant branch stars

The circumstellar grains of two hot evolved postasymptotic giant branch stars, HD 89353 and HD 213985, are examined, deriving the extinction around 2175 A. An unsuccessful attempt to detect 6614 A diffuse band absorption arising from the grains implies that the carrier of this diffuse band is not the carrier of the unidentified IR features reported by other investigators nor of the 2175 A bump. It is found that the linear UV extinction of the carbon-rich star HD 89353 continues across the 2175 A region with no sign of the bump; for HD 213985 the reverse is found: a strong, wide bump in the mid-UV. A wavelength of 2300 A is derived for the HD 213985 bump, which lies longward of its unusual position in the interstellar medium. Since HD 89353 is metal-poor yet rich in hydrogen and carbon, the linear extinction arises from hydrocarbon grains of radius less than 300 A.

Large aromatic molecules in the interstellar medium

We propose a consistent explanation of the so-called unidentified IR emission bands of the Interstellar Medium observed at 3.3 - 6.2 - 7.7 - 8.6- 11.3 μm. Following Sellgren (1984), we consider the transient heating of Very Small Grains (∼ 50 atoms) to a peak temperature of ∼ 1000 K by the absorption of a single UV photon and estimate the subsequent IR emission.The necessary stability of these clusters againts sublimation suggests that they are graphitic and specially polycyclic Aromatic Hydrocarbon (PAH) molecules. We estimate the emission spectrum of a typical PAH, coronene: C24H12, and find a suggestive spectroscopic agreement of the main spectral features with the astronomical ones. This explanation of the observed emission bands can account for their excitation mechanism which was not hitherto explained.New IR absorption spectra of a large PAH molecules are presented with the cross section of the different vibrational modes. The corresponding emissions are calculated and it is shown that the agreement found for coronene is not casual : a family of PAH molecules can explain the astronomical observations. We can even specify that compact PAHs give a better agreement than non-compact ones.Different implications of this identification are reviewed such as the abundance of these molecules. It is found that they could contain several percent of the cosmic carbon, which raises them to the level of the most abundant organic molecules in the Interstellar Medium. We also mention several open problems.

A mixture of hydrogenated amorphous carbon grains and PAH molecules - A candidate for the unidentified infrared bands?

Transmission spectra of hydrogenated amorphous carbon (HAC) and different PAH collections have been measured in the laboratory to study the question of whether or not they can account for unidentified IR bands. It is found that transmission spectra of mixtures composed by HAC and PAH collections match the astronomical observations, although they do not fit the entire family of unidentified IR bands. The laboratory data are used in emission to simulate the spectrum of the planetary nebula NGC 7027, showing good agreement. 36 references.