Laboratory studies on the interaction of atomic hydrogen with aliphatic and aromatichydrogenated carbon grains are discussed. When exposed to atomic hydrogen, both types ofhydrogenated carbon grains act as catalysts for molecular hydrogen formation. In the firstcase, an exchange reaction with hydrogen chemisorbed in aliphatic carbon sites is theformation route to H2 formation. For aromatic carbon grains, the formation ofmolecular hydrogen takes place through a two-step reaction sequence: 1) superhydrogenation of the aromatic carbon islands of grains 2) exchange reactions on theseislands. This mechanism represents a good approximation of molecular hydrogen formation onlarge neutral PAHs.

Near infrared observations of reflection nebulae have set the historical ground for thediscovery of interstellar PAHs, but since, space observations have focused on their mid-IRfeatures, and data shortward of 5   μm have remained scarce. TheSpitzer/IRAC images in the 3.6 and 4.5   μm channels do show that thenear-IR emission from small dust particles is ubiquitous across the Galaxy, but provide nospectroscopic information. To investigate the nature of this near-IR dust emission, wehave obtained AKARI spectroscopic observations, over the 2.5−5   μmspectral range, for a set of archetype PDRs mapped with the Spitzer spectrometer at mid-IRwavelengths. These AKARI data supplement earlier observations with the SWS ISOspectrometer, in providing the gain in sensitivity needed to observe low excitationsources, and the spatial information required to spatially correlate near-IR spectroscopicsignatures with physical conditions and observed changes in mid-IR spectra. This paperpresents the first results of the data analysis, in relation to two open questions oninterstellar PAHs. (1) Is there an evolutionary link from aliphatic carbon dust to PAHs?(2) What is the origin of the near-IR dust continuum? The AKARI spectra display featureslongward of the main 3.29   μm PAH feature, and continuum emission. Theintensity ratio between the features ascribed to aliphatic CH bonds and the3.29   μm aromatic band, varies spatially in a way that may beinterpreted as evidence for aromatization of the smallest dust particles byphoto-processing. The continuum displays a striking step-increase across the3.29   μm feature. We also present a spectrum of a photodissociationregion with a feature at 4.65   μm, which has been speculated to berelated to the CD stretch in aliphatic hydrocarbon side-groups on PAHs.

A classical nova outburst arises from a thermonuclear runaway in the hydrogen-richmaterial accreted onto the surface of a white dwarf in a binary system. These explosionscan produce copious amounts of heavy element enriched material that are ejected violentlyinto the surrounding interstellar medium. In some novae, conditions in the ejecta aresuitable for the formation of dust of various compositions, including silicates, amorphouscarbon, silicon carbide, and hydrocarbons. Multiple dust grain types are sometimesproduced in the same system. CO formation in novae may not reach saturation, thusinvalidating the usual paradigm in which the C:O ratio determines the dust species. A fewnovae, such as V705 Cas and DZ Cru, have exhibited emission features near 6, 8, and 11μmthat are similar to “Unidentified Infrared” (UIR) features, but withsignificant differences in position and band structure. Here, we present SpitzerIRS spectra of two recent dusty novae, V2361 Cyg and V2362 Cyg, that harborsimilar peculiar emission structures superimposed on features arising from carbonaceousgrains. In other astronomical objects, such as star forming regions and young stellarobjects, emission peaks at 6.2, 7.7, and 11.3 μmhave been associated withpolycyclic aromatic hydrocarbon (PAH) complexes. We suggest that hydrogenated amorphouscarbon (HAC) may be the source of these features in novae based upon the spectral behaviorof the emission features and the conditions under which the dust formed.

In the interstellar medium (ISM), PAHs are abundant and also carry most of the dustsurface. They are thus privileged sites for surface reactions such as the formation ofH2. In regions penetrated by UV photons, PAHs loose electrons by thephotoelectric effect and efficiently heat the gas. In more shielded regions, PAH recombineand may carry an important fraction of the cloud electronic charge which plays animportant role in the gas dynamics and chemistry. We review here processes involving PAHswhich control key aspects of the physics of the ISM. We also discuss the correspondingobservational constraints. Most of these processes involve a detailed knowledge of thecharge of PAHs and we therefore review current models in this area. We argue that morelaboratory measurements of the rate of electronic capture on large PAH cations areneeded.

This review describes five categories of the reactions of polycyclic aromatichydrocarbons, including photochemistry, electron attachment/detachment, recombinationprocesses, radical reactions, and ion-neutral chemistry. For each class of reaction, anoverview of the studies and their general results are presented, as well as references tothe literature. The thermochemistry of PAHs and relevant species is described, includingbond dissociation energies, ionization energies, electron affinities, basicities,acidities, and the interrelationships of these quantities. Modeling of the chemistry ofPAHs and their ions is discussed for both diffuse and dark clouds. The role of PAH cationsin the catalytic formation of molecular hydrogen is considered. Finally, this reviewconcludes with a discussion of current challenges in the chemical characterization ofPAHs, and a perspective for future studies.

The relevance of organometallic complexes in the chemistry of the interstellar medium(ISM) was first suggested in the early 90s. This initial proposal has recently beenre-considered in the light of new astronomical data, benefiting from pioneeringexperimental techniques and theoretical methods. In this article, a review of recenttheoretical and experimental results obtained for PAHs coordinated to Fe atoms ispresented, focusing on: -(i)- the IR spectra of [FePAH]+ and[Fe(PAH)2]+ complexes, found to be in line with the AIBs, withadditional specific features in the far-IR range and -(ii)- the photo-evaporation of[FexPAHy]+ clustersas possible candidates for very small grains releasing PAHs in photodissociation regions.These results call for new astronomical data at high spatial resolution. Studies on largerclusters will require further experimental and theoretical developments.

An active carbon chemistry is observed at the border of photo-dissociation regions(PDRs), involving small hydrocarbons, polycyclic aromatic hydrocarbon (PAH) macromoleculesand evaporating very small grains (VSGs). In this context, we aim at quantifying thephysical and chemical evolution of PAHs (hydrogenation and charge states, aggregation, andcomplexation with heavy atoms) as a function of the local physical conditions (radiationfield, temperature, density, abundances of atomic and molecular hydrogen, electrons andheavy atoms). We have developed a numerical model that follows the time dependency of theabundance and internal energy of each species. In this paper, we use this model tocalculate the hydrogenation and charge states of coronene C24H12 asan interstellar PAH prototype. We take advantage of recent results on photodissociationand reaction rates and provide guidelines for future laboratory studies. Reaction rates ofcoronene-derived radical cations with H and H2 are found to be sufficientlyconstrained by experiments, whereas the absence of experimental data for neutral speciesis critical.

The possible role of neutral PAHs as catalysts for H2 formation in theinterstellar medium is investigated by a combined experimental and density function theorystudy of the superhydrogenation of coronene (C24H12). Thecalculations suggest efficient hydrogenation of both edge and centre sites, along withcompeting abstraction reactions to form H2 in a series of catalytic cycles.Scanning tunneling microscopy and thermal desorption measurements have been used toprovide direct evidence of the formation of superhydrogenated coronene as a result ofexposure to D atoms. Lower limit estimates for the cross-sections of1.8 × 10-17, 5.5 × 10-18 and 1.1 × 10-18 cm2for the formation of singly, doubly and triply hydrogenated coronene are derived. Theresults suggest that superhydrogenated PAHs may play an important role in H2formation in the ISM.

This symposium has shown that the field of astrophysical PAHs remains extremely activeand lively. Thanks to Spitzer Space Telescope, the number of PAH papershas spectacularly increased, including now up to the young Universe. Laboratory andtheoretical works have progressed in proportion. Salient features of the six sessions ofthe symposium are briefly reviewed. Comprehensive analyses of the rich and complexinfrared spectra of interstellar PAHs are now well established, based on a large databaseof observational data. PAHs are fully confirmed as excellent tracers of star formation,but their emission strongly depends on metallicity. Various observations, especially inharsh environments, have confirmed the complexity of the lifecycle of PAHs in space, andthe need for multiple formation modes. Electronic properties remain a major issue forastronomical PAHs, including their possible connection with the diffuse interstellarbands, and the possible importance of protonated PAHs. Progress in studying complexcarbonaceous compounds, such as those of various soots, and in synthesizing very largePAHs may give important clues for understanding interstellar PAHs. Significant progresswas also reported in modeling the important role of PAHs in the physics and chemistry ofthe interstellar medium.