Diffuse clouds are defined as low-density clouds permeated by the ultraviolet interstellar radiation field. We discuss the ensemble of atomic, ionic and molecular observations in the context of our Photon Dominated Region model. The comparison with all observational constraints is limited and implies that the diffuse environment is more complex than originally thought.

The presence of large organic species in interstellar gas has important implications for the origin of life and pre-biotic chemistry. Accurate identifications of such molecules, however, are problematic in molecular clouds. There are many reasons for such difficulties. One is that the spectral density is very high in objects where the chemistry is sufficiently complex to produce such species—at least 10 lines per 100 MHz in Sgr B2(N), for example, at 3 mm, at a sensitivity of 10 mK peak-to-peak. Hence, the possibility of chance coincidences is large. Another reason is the presence of many large organic molecules at relatively high temperatures; these large asymmetric tops have vast numbers of favorable transitions under these conditions, including those originating from low-lying vibrational states. Confusion and blending of transitions of one large molecule with those of another add to the risks of an inaccurate identification.

A case in point is that of glycolaldehyde, CH$_2$OHCHO. In order to confirm the identification of this molecule in Sgr B2(N), we searched for its most favorable transitions in the 2 and 3 mm windows, spanning the energy range of ${\sim}$10–100 K—a total of 43 individual transitions. Of all these lines, only seven were not heavily blended or contaminated by other molecules, i.e. so-called “clean” features. Emission, however, was visibly detected at 34 of the other transitions, but one transition was clearly absent. The “missing” line corresponds to a weak transition originating in the $K_a =$ 3 ladder, and its absence is consistent with the other detected features. Based on the clean features only, glycolaldehyde has a $V_{LSR} = 61.7 \pm 1.5$ kms$^{-1}$ and $\Delta V_{1/2} = 7.8 \pm 1.8$ kms$^{-1}$ with intensities consistently in the range $T_R^* \sim 20-70$ mK. Given these data, the identification of glycolaldehyde is 99.9% secure. A rotational diagram from this data set yields a column density of $N_{tot} \sim 6 \times 10^{13}$ cm$^{-2}$ for CH$_2$OHCHO—roughly a factor of 27 less than that of H$_2$CO. These data illustrate the problems and subtleties in identifying large organic molecules in space.

Rich molecular complexity has been shown to exist in star forming regions primarily through millimeter to submillimeter single-dish spectral line surveys or searches over the years. With a few case studies, we demonstrate that interferometric observations are a powerful tool for advancing our knowledge of the chemical processes related to the formation of complex organic molecules in these regions. In particular, interferometers are sensitive to compact structures, such as those of the so called “hot molecular core” sources where complex organics are almost exclusively found. The high angular resolutions achieved by interferometers allow us to better spatially discriminate chemical differentiation and, possibly, evolutionary effects.

Interstellar aldehydes have been called the “sugars of space” ever since the discoveries of formaldehyde (H2CO) in 1969 and acetaldehyde (CH3CHO) in 1973. At present, more than 135 interstellar molecular species have been identified. Excluding diatomic species, 30% of all interstellar molecules have isomeric counterparts. The newest instrument in the interstellar molecule search arsenal is the Green Bank Telescope (GBT), which is credited with the discovery of the large aldehydes propenal (CH2CHCHO) and propanal (CH3CH<2CHO). In addition, the GBT has been used to observe interstellar glycolaldehyde (CH2OHCHO), the simplest possible aldehyde sugar, and interstellar ethylene glycol (HOCH2CH2OH), the sugar alcohol of glycolaldehyde. These new GBT observations that suggest a universal prebiotic chemistry are presented and discussed. While there is no consensus regarding how large complex interstellar molecules are formed, it may be that the first step in the polymerization of interstellar formaldehyde (H2CO) with its isomer trans hydroxy methylene (t-HCOH) is responsible for the formation of interstellar glycolaldehyde. We discuss this polymerization mechanism that can result in the generation of more complex sugars. We assess the likelihood of interstellar trans hydroxy methylene and suggest a search strategy for it using the GBT.

Pathways leading to the formation of complex organic molecules are described. Gas-phase processes that may build large carbon-chain species in cold molecular clouds are summarised. Catalytic reactions on grain surfaces can lead to a large variety of organic species, and model calculations of mantle formation by atom additions to multiply-bonded molecules are presented. The subsequent desorption of these mixed molecular ices can initiate a distinctive organic chemistry via ion-molecule pathways. The predictions of this theory are briefly compared with observations to show how possible organic formation pathways in the interstellar medium may be constrained.

The inventory of interstellar and solar system molecules now numbers well over 100 species, including both ions and neutrals. Gas-phase formation routes to many of the observed organics are still uncertain, so that solid-phase syntheses are of interest. Low-temperature reactions are thought to occur within interstellar ices, on ice and grain surfaces in the interstellar medium (ISM), and on icy surfaces of solar system objects (e.g., Europa, Pluto). Ionizing radiation, such as cosmic rays, and far-UV photons are two possible initiators of such chemistry.

In the Cosmic Ice Lab at NASA's Goddard Space Flight Center, we can study both the photo-and radiation-induced chemistries of ices at 8–300 K. Our most-recent work has been motivated by the detections of ethylene glycol, HOCH2CH2OH, in the ISM (Hollis et al. 2002) and in comet Hale-Bopp (Crovisier et al. 2002). Ethylene glycol is currently the largest firmly-identified cometary molecule as well as one of the larger interstellar organics. It's formation and accompanying chemistry provide challenges and tests for current astrochemical thought.

Here we discuss laboratory experiments on ethylene glycol's solid-phase formation and destruction. Using infrared spectroscopy, we have identified low-temperature radiation-chemical pathways that lead from known interstellar ices, such as either CH3OH or H<2O and CO, to ethylene glycol. We also have identified a role for ethylene glycol in the formation of glycolaldehyde, HOCH2C(O)H, a simple sugar (Hollis, Lovas, & Jewell 2000). Related to these results are the 6-and 7-atom molecules formed by oxidation and reduction processes in irradiated H2O + C2H<2 ices. Analogous experiments have been conducted on two other triply-bonded molecules in H<2O-rich ice, HCN and N2. An important focus of our work is the development of reaction schemes for the formation of complex molecules within astrochemically-relevant ices, and the use of such schemes to predict new molecules awaiting detection.

Understanding the molecular phase of the ISM in starburst and active galaxies is important for the modelling of the onset and evolution of their nuclear activity. Observations of high density gas tracers such as HCN, HNC, HC3N, HCO+ and CN are essential for probing physical and chemical conditions of the dense, star-forming gas. These tracers show great potential as indicators of the evolution of star formation as well as probes of X-ray illuminated molecular gas around an active galactic nucleus (AGN). In particular, towards the inner kpc of luminous and ultra luminous galaxies will molecular line ratios prove useful as diagnostic tools, since optical and even near infrared starburst tracers are difficult to apply in these highly obscured regions.

Observations of CO can provide constraints on the density and temperature of the dense star-forming gas in galaxies, while observations of [CI] trace photon-dominated regions and provide information on the amount of atomic carbon in molecular clouds. In this paper, I review CO and [CI] observations of nearby galaxies, with an emphasis on galaxies for which high-resolution observations of multiple transitions and isotopologues are available. I also briefly discuss upper limits on water emission from nearby starburst galaxies and on O2 emission in the SMC obtained with the Odin satellite.

We report on our first results from a mid-infrared spectroscopic study of ISM features in a sample of deeply obscured ULIRG nuclei using the InfraRed Spectrograph (IRS) on the Spitzer Space Telescope. The spectra are extremely rich and complex, revealing absorption features of both amorphous and crystalline silicates, aliphatic hydrocarbons, water ice and gas-phase bands of hot CO and warm C2H2, HCN and CO2. PAH emission bands were found to be generally weak and in some cases absent. The features probe a dense and warm environment, in which crystalline silicates and water ice are able to survive but volatile ices, commonly detected in Galactic dense molecular clouds, cannot. If powered largely by star formation, the stellar density and conditions of the gas and dust have to be extreme not to give rise to the commonly detected emission features associated with starbursts.

The study of the molecular gas in quasars and submillimeter galaxies at high redshift has significantly progressed during the last few years. From the current detection of CO emission in 37 sources spanning a range in redshift from $1<z<6.4$ with, in some cases, the measurement of a series of CO rotational transitions, it is possible to constrain the physical conditions of the massive ($\ge 10^{10} \, M_\odot$) reservoirs of gas in these objects. This review will present the current status of the studies of molecular gas in high-$z$ sources, detail the physical conditions which pertain in these systems, which are scaled-up versions of the local ULIRGS, and discuss the searches in high-$z$ sources for species other than CO, including the fine structure lines of neutral carbon and the recent detection of the redshifted [CII] emission line in the $z=6.4$ quasar J 1148+5251. These results hold great promise for the study of galaxy formation and their evolution with redshift. This review will conclude by outlining the expected progress in the field, in particular when future instruments such as ALMA will be operational, which will enable to study the astrochemistry and its evolution in the early universe.

We present the detection of molecular oxygen with Odin toward the dense molecular core pOph A, which is part of a region of active star formation. The observed spectral line is the $(N_J=1_1-1_0)$ ground state transition of O2 at 119GHz ($\lambda$=2.5 mm). The line center is at the LSR velocity of a number of optically thin lines from other species in the region. The O2 line also has a very similar, narrow, line width. Within the $10^{\prime}$ beam, the line intensity is $\int\!T_{\rm A}\,{\rm d}v = 28$mK kms−1, which corresponds to $5\,\sigma$ of the rms noise. A standard LTE analysis results in an O2 abundance of $5 \times 10^{-8}$, with an uncertainty of at least a factor of two. We show that standard methods, however, do not apply in this case, as the coupling of the Odin beam to the source structure needs to be accounted for. Preliminary model results indicate O2 abundances to be higher by one order of magnitude than suggested by the standard case. This model predicts the 487GHz line of O2 to be easily detectable by the future Herschel-HIFI facility, but to be out of reach for observations on a shorter time scale with the Odin space observatory.

The Submillimeter Wave Astronomy Satellite (SWAS) has recently completed 5.5 years of successful operation. Among the legacies of the mission has been a greater understanding of the abundance and spatial distribution of H2O and O2 within molecular clouds. We summarize SWAS results and discuss how the measured low abundance of water vapor and non-detections of molecular oxygen are suggestive of a general lack of atomic oxygen in the dense centers of molecular clouds. We also present a new more comprehensive model for the oxygen chemistry in the interstellar medium.

We present results from an ongoing program to map the spatial distribution of ices in dark cloud cores with the Spitzer Space Telescope and VLT-ISAAC. The ice maps are used to directly trace the freeze-out of CO and the formation of H2O, CH3OH and CO2.

The early results on astrochemistry from the Cores to Disks (c2d) Legacy program are summarized. The c2d program focuses on the formation of low-mass stars in nearby (within about 300 pc) clouds. Spectroscopy with IRS includes the following topics: ices seen against background stars, in protostellar envelopes, and in disks; grain growth and mineralogy, PAHs, and gas in circumstellar disks.

The joint U.S. and German SOFIA project to develop and operate a 2.5-meter infrared airborne telescope in a Boeing 747-SP is now in the final stages of development. First science flights will begin in 2008. The observatory is expected to operate for over 20 years. The sensitivity, characteristics, science instrument complement, and examples of 1-st light spectroscopic astrochemistry science are discussed.

Molecular hydrogen is the most abundant molecule in the Universe and dominates the mass budget of the gas, particularly in regions of star formation. H2 is also an important chemical intermediate in the formation of larger species and can be an important gas coolant when the medium lacks metals. Because of the inefficiency of gas-phase reactions to form H2, this molecule is generally thought to form on grain surfaces. Observations of H2 in a wide variety of objects showed that this molecule could form efficiently over a wide range of physical conditions. To understand the mechanism responsible for such an efficient formation, we developed a model for molecular hydrogen formation on grain surfaces. This model considers the interaction between atom and surface as beeing either weak (Van der Waals interaction—physisorption) or strong (covalent bound—chemisorption), as well as the mobility of the atom on a surface due to

quantum mechanical diffusion and thermal hopping. This model solves the time-dependent kinetic rate equation for the formation of molecular hydrogen and its deuterated forms. Our results have been benchmarked with laboratory experiments on silicates, carbonaceous and graphitic surfaces. This comparison allowed us to derive some characteristics of the considered surfaces. An extension of our model to astrophysical conditions gives an estimate of H2 formation efficiency for a wide range of physical conditions. One of our main results is the efficient formation of molecular hydrogen for gas and grain temperatures up to several hundreds of kelvins. We also compared our predictions to observations in astrophysical objects such as photodissociation regions (PDRs). The addition of deuterium in our model for the formation of HD and D2 molecules is also discussed.

We present new experimental data and review previous experimental results on molecular hydrogen formation from atomic recombination on porous amorphous solid water (ASW) surfaces at temperatures from 10 K to 30 K, i.e. under conditions of relevance to cold dense interstellar clouds. We show that the desorption of molecular hydrogen formed on porous ASW surfaces is well described by a model, in which the molecules are assumed to be completely thermalized to the surface temperature and to be evenly distributed throughout the porous network of the ASW films. These results emphasize that, not only the chemical properties, but also the physical morphology of dust grain surfaces must be considered to obtain a full understanding of molecular hydrogen formation on interstellar dust grain surfaces.

The formation of H2 and HD molecules on interstellar dust grains is studied using rate equation and master equation models. Rate equations are used in the analysis of laboratory experiments, which examine the formation of molecular hydrogen on astrophysically relevant surfaces. However, under interstellar conditions, rate equations are not suitable for the calculation of reaction rates on dust-grain surfaces. Due to the low flux and the sub-micron size of the grains, the populations of H and D atoms on a single grain are likely to be small. In this case the reaction rates are dominated by fluctuations and should be calculated using stochastic methods. The rate of molecular hydrogen formation in interstellar clouds is evaluated using the master equation, taking into account the distribution of grain sizes.

We review the main laboratory results of investigations of processes of molecular hydrogen formation on surfaces. The problem of the formation of molecular hydrogen is a fundamental issue in astrophysics/astrochemistry, because of the great importance that molecular hydrogen has for the structure and evolution of our Universe. Such experiments are done using ultra-high vacuum, low temperature, and atomic/molecular beam techniques to study the formation of molecular hydrogen on dust grain analogues in conditions as close as technically feasible to the ones present in relevant ISM environments. In experiments conducted at Syracuse University, we studied H2 formation on the three most ISM-relevant classes of surfaces: silicates, carbonaceous materials and amorphous water ice. Our experimental investigations range from the evaluation of the catalytic efficiency of the studied surfaces to the energetics of the reaction, i.e. the partition of the formation energy between the grain and the nascent molecule. Such measurements have been done by changing various parameters such as: the temperature of the interstellar dust analogue, the kinetic temperature of the atoms, the morphology of the surface and, to be completed soon, the composition of the solid. Quantitative and qualitative information on the processes of H2 formation is then fed in theoretical models to extract results that pertain to desired ISM environments.

The role of high spectral and spatial resolution spectroscopy in understanding the evolution of the gaseous component of circumstellar accretion disks is described. Millimeter-wave emission lines from trace constituents such as CO, CN, HCO+, and HCN can be used to probe the kinematic and physico-chemical properties in the near-surface regions of disks beyond 50–100 AU, but, thanks to extensive depletion in the midplane, they are not a reliable proxy for the disk mass. For the special case of nearly edge-on circumstellar disks, the resulting ices can be directly observed through mid-infrared spectroscopy, using ground based facilities or spacecraft (cf. Najita, this volume). Emerging and planned millimeter-wave → THz arrays will possess sufficient sensitivity and resolution to probe much closer to the central star and to search for prebiotic compounds such as those detected in comets, meteorites and interplanetary dust particles.