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At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Interstellar polycyclic aromatic hydrocarbons - The infrared emission bands, the excitation/emission mechanism, and the astrophysical implications

A comprehensive study of the PAH hypothesis is presented, including the interstellar, IR spectral features which have been attributed to emission from highly vibrationally excited PAHs. Spectroscopic and IR emission features are discussed in detail. A method for calculating the IR fluorescence spectrum from a vibrationally excited molecule is described. Analysis of interstellar spectrum suggests that the PAHs which dominate the IR spectra contain between 20 and 40 C atoms. The results are compared with results from a thermal approximation. It is found that, for high levels of vibrational excitation and emission from low-frequency modes, the two methods produce similar results. Also, consideration is given to the relationship between PAH molecules and amorphous C particles, the most likely interstellar PAH molecular structures, the spectroscopic structure produced by PAHs and PAH-related materials in the UV portion of the interstellar extinction curve, and the influence of PAH charge on the UV, visible, and IR regions.

Allamandola, L. J.↗
NASA NTRS1991

Electronic absorption spectroscopy of matrix-isolated polycyclic aromatic hydrocarbon cations. I - The naphthalene cation (C10H8/+/)

The ultraviolet, visible, and near-infrared absorption spectra of naphthalene (C10H8) and its radical ion (C10H8/+/), formed by vacuum ultraviolet irradiation, were measured in argon and neon matrices at 4.2 K. The associated vibronic band systems and their spectroscopic assignments are discussed together with the physical and chemical conditions governing ion production in the solid phase. The absorption coefficients were calculated for the ion and found lower than previous values, presumably due to the low polarizability of the neon matrix.

Salama, F.↗
NASA NTRS1991

On the state of the emitter of the 3.3 micron unidentified infrared band - Absorption spectroscopy of polycyclic aromatic hydrocarbon species

Results of absorption measurements indicate that the PAH species responsible for the UIR (unidentified infrared) emission probably exist in a condensed form rather than as isolated molecules. It is shown that the peak absorption of the C-H stretch feature of vapor-phase PAHs occurs at a higher frequency than that of the condensed-phase PAHs and does not match the 3.289-micron interstellar feature. The vapor-phase experiments duplicate the phenomenon of the 3.3-micron profile simplification of PAH in KBr at elevated temperature. This confirms that the change of the profile with temperature is an intrinsic molecular effect, and is not a consequence of matrix (KBr) or condensed state interactions.

Flickinger, Gregory C.↗
NASA NTRS1992

The ultraviolet and visible spectrum of the polycyclic aromatic hydrocarbon C10H8(+) - Possible contributions to the diffuse interstellar bands and to the ultraviolet-visible extinction

The properties of the cation of the PAH naphthalene (C10H8(+)) isolated in inert gas matrices under conditions relevant to astrophysical environments are described. The band at 6741 A is the strongest and falls close to the weak 6742 A diffuse interstellar bands (DIBs). Five other weaker bands also fall remarkably close to the positions of known DIBs. A very intense and broad continuum extended from the UV to the visible, which seems to be associated with the ion, is reported. The molar absorption coefficient at the peak of the continuum is 2.0 x 10 exp 6 cu dm/mol cm. If a continuum is a general property of PAH cations, this characteristic will have a strong impact on the understanding of how PAHs convert interstellar UV and visible radiation into IR radiation.

Salama, F.↗
NASA NTRS1993

Theoretical infrared spectra of some model polycyclic aromatic hydrocarbons - Effect of ionization

In order to test the hypothesis of ionized PAHs as possible carriers of the UIR bands, we realized a computational exploration on selected PAHs of small dimension in order to identify which changes ionization would induce on their IR spectra. In this study we performed ab initio calculations of the spectra of neutral and positively ionized naphthalene, anthracene, and pyrene. The results are significantly important. The frequencies in the cations are slightly shifted with respect to the neutral species, but no general conclusion can be reached from the three molecules considered. By contrast, the relative intensities of most vibrations are strongly affected by ionization, leading to a much better agreement between the calculated CH/CC vibration intensity ratios and those deduced from observations.

De Frees, D. J.↗
NASA NTRS1993

3.3 and 11.3 micron images of HD 44179 - Evidence for an optically thick polycyclic aromatic hydrocarbon disk

Images of HD 44179 (the Red Rectangle) obtained in the 3.3 and 11.3 micron emission bands show two different spatial distributions. The 3.3 micron band image is centrally peaked and slightly extended N-S while the 11.3 micron image shows a N-S bipolar shape with no central peak. If the 3.3 micron band image shows the intrinsic emission of the 11.3 micron band, then the data suggest absorption of the 11.3 micron emission near the center of HD 44179 by a disk with an optical depth of about one, making HD 44179 the first object in which the IR emission bands have been observed to be optically thick. Since there is no evidence of absorption of the 3.3 micron emission band by the disk, the absorption cross section of the 3.3 micron band must be substantially less than for the 11.3 micron band. Since the 3.3 and 11.3 micron bands are thought to arise from different size PAHs, the similar N-S extents of the two images implies that the ratio of small to large PAHs does not change substantially with distance from the center.

Bregman, Jesse D.↗
NASA NTRS1993

Laboratory evidence for ionized polycyclic aromatic hydrocarbons in the interstellar medium

The infrared absorption from neutrals and cations of four PAHs - naphthalene, anthracene, pyrene, and perylene - integrated over the spectral regions corresponding to the interstellar bands are compared with astronomical observations. It is found that the interstellar bands cannot be explained solely on the basis of neutral PAH species, but that cations must be a significant, and in some cases dominant, component.

Szczepanski, Jan↗
NASA NTRS1993

Polycyclic aromatic hydrocarbons in the atmospheres of Titan and Jupiter

PAHs are important components of the interstellar medium and carbonaceous chondrites, but have never been identified in the reducing atmospheres of the outer solar system. Incompletely characterized complex organic solids (tholins) produced by irradiating simulated Titan atmospheres reproduce well the observed UV/visible/IR optical constants of the Titan stratospheric haze. Titan tholin and a tholin generated in a crude simulation of the atmosphere of Jupiter are examined by two-step laser desorption/multiphoton ionization mass spectrometry. A range of two- to four-ring PAHs, some with one to four alkylation sites, are identified, with a net abundance of about 0.0001 g/g (grams per gram) of tholins produced. Synchronous fluorescence techniques confirm this detection. Titan tholins have proportionately more one- and two-ring PAHs than do Jupiter tholins, which in turn have more four-ring and larger PAHs. The four-ringed PAH chrysene, prominent in some discussions of interstellar grains, is found in Jupiter tholins.

Sagan, Carl↗
NASA NTRS1946

The synthesis and purification of aromatic hydrocarbons IV : 1,2,3-trimethylbenzene

A six gallon quantity of 1,2,3-trimethylbenezene was prepared and purified in a four-step synthesis involving the condensation of 1,3-pentadiene with crotonaldehyde. The dimethylcyclohexencarboxaldehydes formed were hydrogenated to give the corresponding isomeric dimethylcyclohexylcarbinols. The dehydration of the carbinols and the subsequent dehydrogenation of the trimethylcyclohexenes yielded the 1,2,3-trimethylbenzene. The overall yield was 24 percent; the physical properties of the materials are given.

Lamberti, J M↗
NASA NTRS1946

The synthesis and purification of aromatic hydrocarbons V : 1-ethyl-3-methylbenzene

The method used for the synthesis and purification of an 8-gallon quantity of 1-ethyl-3-methylbenzene from m-creosol consists in obtaining m-methylcyclohexanone from m-creosol by hydrogenation followed by oxidation, condensation of the ketone with ethylmagnesium bromide, dehydration of the tertiary alcohol obtained, and the dehydration of the olefins to 1-ethyl-3-methylbenzene. A yield of 28 percent of the theoretical was obtained from 98 percent commercial m-creosol. The physical properties of the 1-ethyl-3-methylbenzene are compared with selected values from the literature.

Ebersole, Earl R↗