Assessment of the polycyclic aromatic hydrocarbon-diffuse interstellar band proposal

Closed-shell polycyclic aromatic hydrocarbon cations: a new category of interstellar polycyclic aromatic hydrocarbons

Density functional theory has been employed to calculate the harmonic frequencies and intensities of a range of polycyclic aromatic hydrocarbon (PAH) cations that explore both size and electronic structure effects on the infrared spectroscopic properties of these species. The sample extends the size range of PAH species considered to more than 50 carbon atoms and includes several representatives from each of two heretofore unexplored categories of PAH cations: (1) fully benzenoid PAH cations whose carbon skeleton is composed of an odd number of carbon atoms (C(odd) PAHs); and (2) protonated PAH cations (HPAH+). Unlike the radical electronic structures of the PAH cations that have been the subject of previous theoretical and experimental work, the species in these two classes have a 'closed'-shell electronic configuration. The calculated spectra of circumcoronene, C54H18, in both neutral and (radical) cationic form are also reported and compared with those of the other species. Overall, the C(odd) PAHs spectra are dominated by strong CC stretching modes near 1600 cm(-1) and display spectra that are remarkably insensitive to molecular size. The HPAH+ species evince a more complex spectrum consistent with the added contributions of aliphatic modes and their generally lower symmetry. Finally, for both classes of closed-shell cations, the intensity of the aromatic CH stretching modes is found to increase with molecular size far out of proportion with the number of CH groups, approaching a value more typical of neutral PAHs for the largest species studied.

Laboratory spectra of cold gas phase polycyclic aromatic hydrocarbon cations, and their possible relation to the diffuse interstellar bands

A novel laboratory technique is described, combining the use of supersonic expansion, laser excitation and small aromatic-rare gas van der Waals (vdW) clusters properties, which was developed to access the electronic absorption spectra of the polycyclic aromatic hydrocarbon (PAH) cations in the visible. It consists in preparing vdW complexes of the PAH molecule with a rare gas in a molecular beam, to photoionize it by resonant selective two-photon ionization, then to photodissociate this ionic complex by means of a delayed laser pulse in a time-of-flight mass spectrometer. The method is illustrated by presenting the visible spectra of the Naphthalene, Phenanthrene, Fluorene and Phenylacetylene cations. Such spectra can be unambiguously compared to the astronomical spectra of reddened stars, which exhibit the so-called diffuse interstellar bands (DIBs) in absorption. An interesting feature of the technique is its ability to measure the absolute absorption cross-sections. The large values of the oscillator strengths of the transitions, which are derived, are discussed in the astrophysical context which consists in considering that the PAH cations could be carriers for some of the DIBs.

Aromatic hydrocarbons, diamonds, and fullerenes in interstellar space: puzzles to be solved by laboratory and theoretical astrochemistry

New research is presented, and previous research is reviewed, on the emission and absorption of interstellar aromatic hydrocarbons. Emission from aromatic hydrocarbons dominates the mid-infrared emission of many galaxies, including our own Milky Way galaxy. Only recently have aromatic hydrocarbons been observed in absorption in the interstellar medium, along lines of sight with high column densities of interstellar gas and dust. Much work on interstellar aromatics has been carried out, with astronomical observations and laboratory and theoretical astrochemistry. In many cases, the predictions of laboratory and theoretical work are confirmed by astronomical observations but, in other cases, clear discrepancies exist that provide problems to be solved by a combination of astronomical observations, laboratory studies, and theoretical studies. The emphasis of this paper will be on current outstanding puzzles concerning aromatic hydrocarbons that require further laboratory and theoretical astrochemistry to resolve. This paper will also touch on related topics where laboratory and theoretical astrochemistry studies are needed to explain astrophysical observations, such as a possible absorption feature due to interstellar 'diamonds' and the search for fullerenes in space.

Electronic spectra and ionization potentials of a stable class of closed shell polycyclic aromatic hydrocarbon cations

Due to their stability, closed shell polycyclic aromatic hydrocarbon (PAH) cations are possible candidates as carriers for some of the diffuse interstellar bands (DIBs). The electronic absorption spectra and ionization potentials of several closed shell PAH cations are determined in this study. We use density functional theory (DFT) at the BLYP/6-31G* level to determine the ionization potentials and thus confirm the stability of the PAH cations of interest. We use time-dependent density functional theory (TDDFT), again at the BLYP/6-31G* level, to calculate the vertical excitation energies and oscillator strengths of the PAH cations. We observe dominant single absorptions within the DIB spectral region of interest in all of the PAH cation spectra except for the smallest member of the series.

Electronic spectra of 1-methyl and 2-methyl phenanthrenes and their radical cations

Electronic spectra of phenanthrene (P), 1-methyl phenanthrene (1-MeP), 2-methyl phenanthrene (2-MeP) and their monopositive ions are investigated experimentally as well as theoretically. The ions were produced by photo-oxidation of the hydrocarbons in boric acid matrix. The electronic absorption spectrum of 2-methyl phenanthrene cation (2-MeP+) is entirely new. For the interpretation of the electronic spectra of neutral and ionized MePs, semi-empirical AM1 (Austin Model 1) calculations are carried out for the first time. The bathochromic shifts in the spectral bands of the neutral and ionized MePs are attributed to 'conjugative' effect. The present experiments reveal that the 448 nm band of 1-methyl phenanthrene cation (1-MeP+) and the 486 nm band of 2-MeP+ show close matching with the respective 450 nm and 488 nm 'diffuse interstellar bands'. This suggests the possibility of the existence of such ionic species in the interstellar matter.

Polycyclic aromatic hydrocarbons and the diffuse interstellar bands: a survey

We discuss the proposal relating the origin of some of the diffuse interstellar bands (DIBs) to neutral and ionized polycyclic aromatic hydrocarbons (PAHs) present in interstellar clouds. Laboratory spectra of several PAHs, isolated at low temperature in inert gas matrices, are compared with the spectra of five reddened early-type stars selected from an extensive set of astronomical spectra. From this comparison, it is concluded that PAH ions are good candidates to explain some of the DIBS. Unambiguous assignments are difficult, however, because of the shift in wavelengths and the band broadening induced in the laboratory spectra by the solid matrix. This situation is illustrated by a comparison with the gas-phase spectra made available recently for two PAH ions. Definitive band assignments and, ultimately, the test of the proposal that PAH ions carry some of the DIBs must await the availability of a larger set of gas-phase measurements in the laboratory. The present assessment offers a guideline for future laboratory experiments by allowing the preselection of promising PAH molecules to be studied in jet expansions.

Jet-discharge cavity ring-down spectroscopy of ionized polycyclic aromatic hydrocarbons: progress in testing the PAH hypothesis for the diffuse interstellar band problem

Naphthalene cations (C10H+8) were produced in a slit jet coupled with an electronic discharge, and cavity ring down was used to obtain its absorption spectrum in the region 645-680 nm. Two of the strongest C10H+8 bands previously characterized by matrix isolation spectroscopy were found, both with a fractional blue shift of about 0.5%. This is the first gas-phase electronic absorption spectrum of an ionized polycyclic aromatic hydrocarbon (PAH). This work opens the way for a direct comparison of laboratory PAH spectra with the diffuse interstellar bands (DIB), the origin of which still constitutes an open problem in astrophysics.

New estimates of ionization potentials of four DIB molecular carriers

We present a study of the behaviour and ionization properties of four Diffuse Interstellar Bands (DIBs) at lambda lambda 5780, 5797, 6379 and 6613 angstroms. In the lambda lambda 5797, 6379 and 6613 angstrom DIBs, substructures have recently been detected, indicating large gaseous molecular carriers. Studying DIBs in regions with different physical properties in terms of UV flux and density enables us to monitor the behaviour of the carriers and hence to constrain their nature. As a follow-up of Sonnentrucker et al. (1997), we add new lines of sight and generalize the results for lines of sight with 2 or 3 clouds. This refines the Ionization Potential estimates which are between 10 and 13 eV, hence reminiscent of PAH or fullerene cations for those DIBs.

Carbon in the universe

Carbon is a major player in the evolutionary scheme of the universe because of its abundance and its ability to form complex species. It is also a key element in the evolution of prebiotic molecules. The different forms of cosmic carbon are reviewed ranging from carbon atoms and carbon-bearing molecules to complex, solid-state, carbonaceous structures. The current state of knowledge is assessed on the observational and laboratory fronts. Fundamental astrophysical implications are examined as well as the impact of these studies on the hitherto poorly understood physical and chemical properties of carbon materials in space.

Extraterrestrial organic matter: a review

We review the nature of the widespread organic material present in the Milky Way Galaxy and in the Solar System. Attention is given to the links between these environments and between primitive Solar System objects and the early Earth, indicating the preservation of organic material as an interstellar cloud collapsed to form the Solar System and as the Earth accreted such material from asteroids, comets and interplanetary dust particles. In the interstellar medium of the Milky Way Galaxy more than 100 molecular species, the bulk of them organic, have been securely identified, primarily through spectroscopy at the highest radio frequencies. There is considerable evidence for significantly heavier organic molecules, particularly polycyclic aromatics, although precise identification of individual species has not yet been obtained. The so-called diffuse interstellar bands are probably important in this context. The low temperature kinetics in interstellar clouds leads to very large isotopic fractionation, particularly for hydrogen, and this signature is present in organic components preserved in carbonaceous chondritic meteorites. Outer belt asteroids are the probable parent bodies of the carbonaceous chondrites, which may contain as much as 5% organic material, including a rich variety of amino acids, purines, pyrimidines, and other species of potential prebiotic interest. Richer in volatiles and hence less thermally processed are the comets, whose organic matter is abundant and poorly characterized. Cometary volatiles, observed after sublimation into the coma, include many species also present in the interstellar medium. There is evidence that most of the Earth's volatiles may have been supplied by a 'late' bombardment of comets and carbonaceous meteorites, scattered into the inner Solar System following the formation of the giant planets. How much in the way of intact organic molecules of potential prebiotic interest survived delivery to the Earth has become an increasingly debated topic over the last several years. The principal source for such intact organics was probably accretion of interplanetary dust particles of cometary origin.

The interstellar chemistry of PAH cations

Diffuse interstellar bands (DIBs) are mysterious absorption lines in the optical spectra of stars, and have been known for 75 years. Although it is widely believed that they arise from gas-phase organic molecules (rather than from dust grains) in the interstellar medium, no consensus has been reached regarding their precise cause. The realization that many emission features in astronomical infrared spectra probably arise from polycyclic aromatic hydrocarbons (PAHs), which may themselves be very abundant in the interstellar medium, has led to the suggestion that ionized PAHs might be the source of the DIBs. Laboratory investigations have revealed that small, positively charged PAHs in matrices have absorption features that bear some resemblance to DIBs, but no clear identification of any DIB with any specific PAH cation has yet been made. Here we report a laboratory study of the chemical reactivity of PAH cations (C6H6+, C10H8+ and C16H10+) in the gas phase. We find that these PAH cations are very reactive, and are therefore unlikely to survive in high abundances in the interstellar medium. Rather, such molecules will react rapidly with hydrogen, and we therefore suggest that the resulting protonated PAH cations (and species derived from them) should become the focus of future searches for a correspondence between molecular absorption features and the DIBs.

Fullerenes in space

The discovery and synthesis of fullerenes led to the hypothesis that they may be present and stable in interstellar space. Fullerenes have been reported in an impact crater on the LDEF spacecraft. Investigations of fullerenes in carbonaceous meteorites have yielded only small upper limits. Fullerene compounds and their ions could be interesting carrier molecules for some of the "diffuse interstellar bands" (DIBs), a long standing mystery in astronomy. We have detected two new diffuse bands that are consistent with laboratory measurements of the C60+, as first evidence for the largest molecule ever detected in space. Criteria for this identification are discussed. The inferred abundance (up to 0.9 % of cosmic carbon locked in C60+) suggests that fullerenes may play an important role in interstellar chemistry. We present new observations on DIB substructures consistent with fullerene compounds, and the search for neutral C60 in the diffuse medium.

Spectroscopic properties of polycyclic aromatic hydrocarbons (PAHs) and astrophysical implications

PAHs (polycyclic aromatic hydrocarbons) are probably present as a mixture of neutral and ionized species and are responsible for the set of infrared emission bands in the 2-15 microns regions, which are observed in many different objects like reflection and planetary nebulae and external galaxies. PAHs are suggested to be the most abundant free organic molecules and ubiquitous in space. PAHs might also exist in the solid phase, included in interstellar ices in dense clouds. A complex aromatic network is expected on interstellar grains in the diffuse interstellar medium. The existence of an aromatic kerogen-like structure in carbonaceous meteorites and its similarity with interstellar spectra suggests a link between interstellar matter and primitive Solar System bodies.