A review of the most recent experimental results concerning reaction kinetics at low temperatures is presented, most of them having been obtained using the CRESU technique. Some astrochemical consequences are also highlighted.

During the last 5 years, laboratory experiments relevant to the formation of carbon-bearing molecules in extraterrestrial environments have been performed employing the crossed molecular beam technique and a high intensity source of ground state atomic carbon, C(3Pj). These investigations unraveled for the first time detailed information on the chemical reaction dynamics, involved collision complexes and intermediates, and – most important – reaction products of neutral-neutral reactions. Here, we extend these studies even further, and report on very recent crossed beam experiments of cyano radical, CN(2Σ+), reactions with unsaturated hydrocarbons to form nitriles in extraterrestrial environments and Saturn's moon Titan. Further, preliminary results on reactions of small carbon clusters and with acetylene, ethylene, and methylacetylene to synthesize hydrogen-deficient carbon-bearing molecules are presented.

The branching ratios for the dissociative recombination of various vibrationally cold polyatomic molecular ions have been measured using the ASTRID ion storage ring. The results show that many particles are ejected during the recombination process, and that isotopic effects exist when hydrogen atoms are replaced by deuterium.

Discoveries of large new organic molecules in space help provide a foundation for understanding the role of the chemical bond and organic chemistry on a cosmic scale, and by extension, an opportunity to begin to address the interesting but far more complicated questions of chemical and biological origins and the issues that relate to them. Most of the organic molecules so far detected in the interstellar gas and circumstellar shells by radio telescopes are highly unsaturated carbon chains, a configuration of linear carbon which is unstable at high density, and therefore unfamiliar on Earth. For this reason laboratory detection until quite recently has lagged behind astronomical discovery of many new carbon molecules. The application of Fourier transform microwave spectroscopy to supersonic molecular beams has now largely overcome this obstacle, yielding in just over three years the laboratory detection of forty-four new carbon chains and eight new carbon ring-chains and rings, including two rhomboidal isomers of SiC3. The set consists of 14 polyynes, 20 carbon chain radicals, and 18 free carbenes. For four other triplet chains HC7H, HC9H, HC11H, and HC13H, which by symmetry are nonpolar, strong electronic spectra have been measured in the gas-phase by cavity ringdown absorption spectroscopy. Almost all of these molecules are good candidates for detection in space, and six in fact have now been detected in at least one astronomical source with large radio telescopes during the past two years. The laboratory astrophysics of the entire set is complete for the time being, in the sense that nearly all the rotational transitions of interest to radio astronomy can be calculated to a small fraction of 1 km sec–1 in equivalent radial velocity. With powerful new radio facilities planned or under construction it would be surprising if many more could not eventually be found. For each of the four new symmetric chains detected in the visible, the wavelength of the origin band has been measured to high accuracy, permitting deliberate searches for these molecules in diffuse and translucent molecular clouds.

The reactions of H atoms with solid thin films at 10 K were studied by using thermal desorption mass spectrometry and FT-IR spectroscopy. The N, C, and O atoms trapped in solid matrices were converted efficiently to fully hydrogenated compounds. In the reaction of H atoms with a solid CO film, the formation of formaldehyde and methanol were confirmed. The relatively low yield of the reaction products suggests either the smaller rate constants of the H atom addition reactions to CO and/or the occurrence of the hydrogen abstraction reaction H + HCO → H2 + CO. The reactions of H atoms with thin films of acetone and 2-propanol were also studied. The major products from acetone were found to be methane and alcohols but 2-propanol was not detected as a reaction product. The reaction of H with 2-propanol led to the formation of methane, alcohols, and acetone as major products.

In the reaction of H with C2H2, C2H6 was found to be the major product but C2H4 could not be detected. This is due to the fact that the first-step addition reaction H + C2H2 → C2H3 is the rate-controlling process and the following reactions to form the final product C2H6 proceed much faster than the initial one. This finding is in accord with the observation of comets Hyakutake and Hale-Bopp, i.e., C2H2 and C2H6 but not C2H4 were detected in the coma of these comets. In the reactions of H with C2H2 and C2H4, the C2H6 product yields increased drastically with decrease of temperature from 50 to 10 K. This is most likely due to the increase of the sticking probability of H atoms on the solid films at lower temperature. These findings led us to conclude that the chemical evolution taking place on the dust grains via H-atom tunneling reactions becomes efficient only at cryogenic temperatures, i.e., ~ 10 K.

Recent advances in theoretical simulations of grain-surface processes are reviewed. Classical molecular dynamics (MD) computer simulations were performed to investigate the whole process of H2 formation on icy mantles of interstellar dust grains within a single model. Amorphous water ice slabs were generated at 10 K and 70 K as a realistic model surface of dust grains, and then two incident H atoms were successively thrown onto the surface to reproduce the H2 formation process via H + H → H2 on the dust surface. The following fundamental processes were studied in detail; 1) the sticking of H atom onto the grain surface, 2) the diffusion of H atom on the surface, 3) the reaction of two H atoms on the surface, 4) the ejection of H2 from the surface. Then, the formation pumping mechanism of H2 and the chemical desorption mechanism of frozen CO molecules in the vicinity of H2 forming sites on dust grains were also studied.

The development of astrochemistry is inextricably linked to the generation of fundamental data. In this report, we discuss data needs in terms of astrochemical models, gas-phase kinetics, molecular excitation, optical and UV spectroscopy, solid-state chemistry, and millimeter and submillimeter wave spectroscopy.

The properties of interstellar grains can now be defined by a rapidly growing wealth of observational data. We rely upon models to combine these data with unobserved properties such as the size distribution of grains, their structure and shape, and their detailed chemical composition, in a self-consistent manner. I will review the observational constraints which define the boundaries for possible models and will then summarize the features of the three quasi-comprehensive grain models most commonly referred to in the current literature. I will conclude by discussing recent work dealing with single aspects of interstellar grains and the phenomena attributed to them.

A cyclic evolutionary picture is presented which follows the sources and nature of organics from interstellar space to comets. The three major organic components discussed are the grain mantles, the carbonaceous particles responsible for the 216 nm hump in the extinction, and the large molecules/small particles polycyclic aromatic hydrocarbons (PAHs). The variability in the oxygen and hydrogen abundances relative to carbon is followed.

The unidentified diffuse interstellar bands are observed in near-UV, visible and near-IR spectra recorded towards stars which are partially obscured by interstellar dust. Their origin is the longest standing problem in astronomical spectroscopy and dates back to the 1930s when systematic study of the bands first started. Proposals for the carriers range from molecular hydrogen to porphyrins and from colour centres to species adsorbed on grain surfaces. This paper contains a short review of the problem and a discussion of recent possible assignments of some of the bands to transitions of the H2, and molecules. Observations of ultra-high resolution spectra of diffuse absorption bands, optical diffuse emission bands from the Red Rectangle, and complementary studies of the 3.3 μm ‘unidentified’ infrared (UIR) emission band are described.

ISO has shown that the mid–IR spectra of almost all sources are dominated by narrow emission features collectively known as the UIR bands. These bands are carried by a family of polycyclic aromatic hydrocarbon molecules containing about 50 C-atoms. Other classes of molecules such as C-chains as well as non-aromatic groups attached to the PAHs are much less abundant. ISO has revealed that the UIR spectrum is incredibly rich with a large number of weaker features, subfeatures, and shoulders; many of which were not known before. Some examples and the new insights they can provide are described. A systematic study of this spectral structure may well allow us to considerably narrow down the composition of the family of PAHs responsible for the observed emission.

The envelopes surrounding AGB stars are similar to interstellar dark clouds in many respects; opaque to the visible light, they are transparent to mm wavelengths. Several envelopes, close enough to be spatially resolved with mm-wave interferometers, exhibit a remarkably rich molecular content, which can then be studied in detail as a function of radius. Because the envelopes are in expansion, their outer layers are older and more evolved than the inner ones, making it possible, thanks to a relatively simple geometry, to probe the time-dependence of the ongoing chemical processes.

The recent results of the ISO satellite in the field of molecular spectrocopy of AGB stars are reviewed. For the fist time, the two spectrometers onboard ISO have provided the opportunity to observe the pure rotational lines of several molecules in the far infrared and the ro-vibrational bands of the most abundant molecular species in the near and mid-infrared. These data allow to carry out a systematic study of the circumstellar envelopes of AGB stars and Planetary Nebulae. I analyze in this paper the role of resonant scatterring in the pumping of the ro-vibrational molecular levels in CSEs.

A clear understanding of the chemical processing of matter as it is transferred from a molecular cloud to a planetary system depends heavily on the physical conditions endured by gas and dust as these accrete onto a disk and are incorporated into planetary bodies. Reviewed here are astrophysical observations of circumstellar disks which trace their evolving properties. Accretion disks that are massive enough to produce a solar system like our own are typically larger than 100 AU. This suggests that the chemistry of a large fraction of the infalling material is not radically altered upon contact with a vigorous accretion shock. The mechanisms of accretion onto the star and eventual dispersal are not yet well understood, but timescales for the removal of gas and optically thick dust appear to be a few times 106 yrs. At later times, tenuous “debris disks” of dust remain around stars as old as a few times 108 yrs. Features in the morphology of the latter, such as inner holes, warps, and azimuthal asymmetries, are likely to be the result of the dynamical influence of large planetary bodies. Future observations will enlighten our understanding of chemical evolution and will focus on the search for disks in transition from a viscous accretion stage to one represented by a gas-free assemblage of colliding planetesimals. In the near future, comparative analysis of circumstellar dust and gas properties within a statistically significant sample of young stars at various ages will be possible with instrumentation such as SIRTF and SOFIA. Well-designed surveys will help place solar system analogs in a general context of a diversity of possible pathways for circumstellar evolution, one which encompasses the formation of stellar and brown-dwarf companions as well as planetary systems.

The most likely processes responsible for the removal of circumstellar disks around young stars are reviewed with emphasis on the physical state of the disk during the period of destruction and the timescale for disk removal. Four disk dispersal mechanisms are discussed in detail: 1) viscous accretion of material onto the central source, 2) close stellar encounters, 3) stellar winds, and 4) photoevaporation by ultraviolet radiation. While viscous accretion is shown to be efficient in the inner regions of disks (r < 10 AU), photoevaporation is the principal process of disk dispersal at large radii (r > 10 AU). The commonly held view that stellar winds removed the remnant Solar Nebula is seriously questioned.

Circumstellar disks around classical T Tauri stars have still large amounts of primary molecular gas and dust, which may evolve to form planetary systems. Although it is the remnant of the parental cloud, the gas component shows significant chemical evolution. Fundamental disk properties (kinematics, size, temperature profile) can be derived from observations of CO lines with mm arrays. Less abundant molecules have been discovered with the IRAM 30-m telescope, in the DM Tau and GG Tau disks. Despite the high depletion factors which are measured with respect to the nearby Taurus cloud, these molecules allow independent measurements of the disk density. These observations provide an overview of the properties of young (a few Myr) proto-planetary disks around low-mass stars as seen by current mm instruments.

We investigate the evolution of molecular abundances in circumstellar disks around young stars via numerical simulations. The results are compared with the composition of comets and radio observations of protoplanetary disks. First, we consider the molecular evolution in the midplane of the disk. Because of ionization by cosmic-rays or decay of radioactive nuclei, the molecular evolution in the region R ≳ 10 AU bears some similarity to that in molecular clouds. Considerable amounts of CO and N2, which come from the cloud core, are transformed, however, into CO2, CH4, NH3, and HCN. In regions where the temperature is low enough for these products to freeze onto grains, they accumulate in ice mantles. In the inner warmer regions of the disk, these molecules are desorbed from the mantles and transformed by gas-phase reactions into less volatile molecules, such as larger hydrocarbons, which then freeze out. Molecular abundances both in the gas phase and in ice mantles crucially depend on the temperature and thus vary significantly with the distance from the central star. Molecular abundances in ice mantles show reasonable agreement with the composition of comets. Our model suggests that comets formed in different regions of the disk having different molecular compositions.

Secondly, we report two-dimensional (R, Z) distributions of molecules in circumstellar disks. In the Z-direction, which is perpendicular to the midplane, there is a steep gradient of density and radiation field. Since the density is lower in the regions removed from the midplane, there are significant amounts of gas-phase species in these regions, while most species are adsorbed onto grains in the midplane. Chemistry in the surface regions is also affected by the X-rays and UV radiation from the central star, and UV radiation from the interstellar field. The molecular abundances in these regions differ significantly from those in the midplane. Radicals such as CN are especially abundant near the disk surface. We obtain radial distributions of molecular column densities and averaged molecular abundances, which show reasonable agreement with the observational data at millimeter wavelengths.

ISO has opened new infrared windows for spectroscopy, enabling detailed studies of the composition of the dust particles present in circumstellar disks. For oxygen-rich dust, and in particular for silicates, a forest of new features has been discovered, and comparison with laboratory data has enabled the identification of most of them. Of special relevance is the detection of crystalline silicates, which present themselves as a new diagnostic for studying the formation of comets and planetesimals in the disks surrounding young and, surprisingly, also evolved stars.

Our knowledge of the volatile composition of comets has advanced considerably since the last IAU Astrochemistry Symposium, in large part due to the apparition of comet Hale-Bopp and its study with both new ground-based instruments and from spacecraft. Some 23 or 24 coma molecules are now known which are probably, at least in part, volatile constituents of the nucleus. Relative abundances have been measured for rarer isotopomers of molecules containing 13C, 15N, 34S, and D, and significant isotopic fractionation is observed for D-containing species. There are striking similarities in both relative abundances of molecular constituents and in isotopic fractionation between material in dense interstellar clouds and that in cometary comae. Whether this indicates that cometary nuclei consist of relatively unprocessed interstellar matter is less clear, since the observed coma composition is not simply related to the nuclear composition, and since recent chemical models of the outer solar nebula mimic interstellar chemistry in important respects.