The chemical processes which led to the formation of molecules in the early universe are described. Molecular hydrogen is formed by two sequences. in one, radiative attachment to form H− is followed by associative detachment and in the other radiative association to form H2+ is followed by a chemical reaction with H. Trace amounts of HD and the molecular ion LiH+ are formed by the reaction of D+ with H2 and by the radiative association of Li+ and H. The H2 molecular fraction in the expanding universe is about 10−6.

Because they are cooling agents, hydrogen molecules are significant in the collapse of pre-galactic clouds. With increasing density the hydrogen gas is converted to molecular hydrogen by three-body recombination and emission from the rotational and vibrational levels cools the cloud and slows the rise in temperature of the gravitationally collapsing object. Ultimately though, the radiation is trapped and the temperature rises. The H2 molecules are then destroyed by collision-induced dissociation. At still higher temperatures collisional ionization occurs, initiated most probably by the process of associative ionization.

Molecules may be significant also in galaxy formation if the explosive amplification model is correct. Heavy elements may be present in the shell of gas swept up by the blast waves of the exploded seed galaxies and chemical processes leading to the formation of molecular hydrides may take place, in a scenario analogous to the aftermath of an interstellar dissociative shock.

Dynamical, chemical and thermal evolution of zero-metal gas clouds has been modeled to study conditions of star formation in the early universe. Numerical results are shown for collapse of a spherical cloud of mass 5 × 105 M⊙. Cooling by H2 lines and by photons emitted in H + e- → H- + hν maintains collapse until formation of an equilibrium protostellar core with mass 0.03 M⊙. the cooling by photons produced with H- is essential for low-mass star formation. If the cloud is fragmented, the evolution of each piece resembles that of the parent cloud.

Observations, which have been published since 1979, of molecular species in diffuse clouds will be discussed. Particular attention will be given to the ultraviolet measurements of CO with the Copernicus and IUE satellites and to ground-based optical measurements of CH, CH+, CN, and C2. These data encompass large enough samples to test the chemical schemes expected to occur in diffuse clouds. Upper limits for other species (e.g., H2O, H2O+, and C3) place restrictions on the pathways for molecular production. Moreover, analysis of the rotational distribution of the C2 molecule results in the determination of the physical conditions of the cloud. These parameters, including density, temperature, and the intensity of the radiation field, are necessary for modeling the chemistry.

We are actively engaged in observing and analysing molecules in diffuse clouds, in the directions of reddened stars (1). This paper presents some recent results. In the ultraviolet, using the IUE satellite, we have so far observed 60 stars in a continuing programme to study interstellar CO, with more than 30 detections of 12CO and 10 of 13CO; almost all of these are new detections (2). the results have been compared with microwave observations of CO in selected directions, and with our own and other optical observations of CH, CH+ and CN.

A 28kHz resolution acousto-optic spectrometer has been established. The modulator is a TeO2 cell with off-axis acoustic slow shear wave. The convolution between the profiles of the modulator and the photodiode array was considered. Its velocity resolution is 0.07 kms−1 for 115 GHz CO molecular lines. The test observations were made by association with the 4-m mm-wave telescope in Australia. Some features of the CO lines in dark clouds have been seen more in detail.

This paper reviews a number of ways in which the results of far-infrared and submillimeter (FIR/SMM) observations (50 μm ≤ λ ≤ 1 mm) have been used to address some of the important questions in astrochemistry. On scales of kiloparsecs these might include questions of abundance gradients; on scales of parsecs these might include questions about the chemical structure of molecular clouds; and on scales of tenths of parsecs or less, these might include questions about chemical processing in shocks.

Emission from vibrationally excited molecular hydrogen has been detected from a wide range of astronomical targets during the last decade (e.g. Shull and Beckwith 1982). in all cases the H2 has been shock excited, and we have grown accustomed to the idea that H2 is a useful tracer of interstellar shocks.

The problem of the formation of molecular hydrogen in interstellar clouds is revisited. the role played by cosmic ray bombardment under certain circumstances is considered mainly in the light of the low formation rate of H2 on grains due to the reduced mobility of adsorbed H atoms on their amorphous surfaces at interstellar temperatures.

Progress in laboratory and astronomical instrumentation has renewed the already large interest for simple astrophysical molecules. On the laboratory side, one of the most notable advances has been the spectroscopic observation of an increasing number of small reactive molecular species. On the astronomical side, the access to submillimetre wavelengths and the completion of millimetric interferometers and large single-dish telescopes, have allowed the detection of many new molecular species and open the way for detailed studies of the distribution of molecules in interstellar and circumstellar clouds.

We studied excitation of interstellar molecules and chemical environment in Orion KL by analyzing more than 200 spectral lines detected by the 45-m telescope at Nobeyama Radio Observatory (NRO).

Formaldehyde absorption has been observed with the Very Large Array in both the 6 cm and 2 cm transitions towards a number of ultracompact HII regions which are embedded in the dense cores of molecular clouds. Such data have been compared with the results of radiative transfer calculations to derive the distributions of the molecular hydrogen density and of the abundance of formaldehyde relative to molecular hydrogen. Results are presented for the sources DR 21 and W 3(OH).

The number of known interstellar molecules has increased steadily since 1970 and stands presently at 68. of the molecules discovered in recent years more than half contain three or more heavy elements.

The number of sources where HC5N or HC3N has been detected now includes 21 dark clouds, four circumstellar shells, two bipolar nebulae, the line of sight to the Cas A supernova remnant, and possibly a comet. the more abundant large molecules are useful diagnostic probes, and preliminary statistical equilibrium calculations of the widespread new ring, C3H2, are presented which indicate its usefulness as an indicator of H2 density.

We have observed transitions of methanol, methyl acetylene(1),(2) and acetaldehyde(3) in dark clouds. in methanol the 20−10 A+, 10−00 A+, 2-1−1−1 E lines and tentatively the 20−10 E line have been observed. The 10−00 E, 21−11 E and the 00−1−1 E lines were searched for but not detected. Maps of TMC1, L134N, and B335 in the 20−10 A+ and 2−1−1−1 E lines show that the methanol emission is very extended and follows the extent of the C180 J=1−0 emission well.

Both observational and theoretical studies of molecular cloud chemistry are hindered by many difficulties which are partially circumvented by studying the relative abundances of isomers synthesized from a common set of chemical precursors. Theoretical studies aimed at understanding interstellar abundance ratios for the isomers HCN/HNC, HCO+/HOC+, CH3CN/CH3NC, and C3H2 have been performed.

The three existing spectral surveys in the 3 mm window are compared. the NRAO and BTL surveys of SgrB2 are quite consistent, the NRAO and Onsala surveys of Ori(KL) less so. Lists of U-lines are assembled from the three surveys, and total 86 in Ori(KL), 66 in SgrB2, of which only 12 are common. Difficulties in selecting reliable lists of U lines are emphasized. Methods of identifying such lines include “direct,” “hybrid,” and “Shotgun” approaches; the probability of misidentification is analyzed for each. the “Shotgun” method is least reliable, although a few candidates should be followed up.

Some recent work on diffuse cloud chemistry is summarized. Distinctions between gas phase chemistry in steady state and alternative mechanisms are discussed.

Using most recent observational data as well as new experimental and theoretical determinations of various reaction rate coefficients, we present a model of the ζ θphiuchus cloud. the radiative transfer equation is solved in a plane parallel geometry taking into account absorptions by both the gas and by dust. A part certain atypical molecules (CH+, CN) and neutral iron, we are able to reproduce the observed column densities of neutral atoms, and molecular species, including the rotational populations of molecular Hydrogen with a two shell model. the concentrations of other simple molecules are predicted.

Based on analyses by a variety of investigators, it has become understood that gas phase reactions can account for much of the chemistry observed in dense interstellar clouds. However, quantitative calculations of molecular abundances utilizing gas phase reactions are beset with difficulties. These difficulties include uncertainties in needed rate coefficients at the low temperatures of interstellar clouds, uncertainties in the dynamics of physical processes such as cloud collapse and clumping, and uncertainties in our understanding of gasgrain interactions. New work in some of these areas and its impact on modelling is emphasized.

In recent years the nearby cold, dark clouds have been shown to possess a rich chemistry, with interesting differences with respect to warmer massive-star-forming regions and also among the cold clouds themselves. 39 molecular species are now known in these regions. Recent molecular detections and upper limits in dark clouds are discussed, with particular emphasis on the tri-carbon species C3O, C3H, and C3H2.