One of the aims of our work at the National Research Council (Ottawa) during the last thirty years has been to contribute laboratory data that are of possible astronomical interest especially for the study of the interstellar medium.

Within the last few years, laser magnetic resonance (LMR) has emerged as a powerful technique for the study of pure rotational spectra of transient species in the gas phase. The sensitivity of this method is considerably higher than that of competing techniques, such as conventional optical and microwave spectroscopy and gas-phase EPR, while the resolution attainable is comparable with that of the latter two methods. Its domain of applicability presently includes atoms, ground states of molecules with up to 5 atoms, and most recently, metastable molecular electronic states in the millisecond lifetime range, and molecular ions. The only rigorous constraint on this applicability is that the species of interest must be paramagnetic.

Recent observational data are summarized on molecular species in diffuse interstellar clouds, and a comprehensive list of all species detected or sought is included. The discussion is focussed on the use of molecular observations to determine physical conditions in clouds, and on important species which provide constraints on cloud chemistry models. Directions for new research are suggested, including intensive work on the unidentified diffuse bands, which may have a molecular origin.

The ultraviolet spectra of 25 early-type stars have been obtained with the International Ultraviolet Explorer observatory. Bands of the 4th-positive system of interstellar C0 are seen towards 12 of these stars. Spectra of HD46223 have been examined for interstellar lines of CH, C2, CH2, OH, HC1, and H2O.

Spectroscopic observations of the dense interstellar cloud which obscures the VI Cygni association have yielded an optical map of the atomic and molecular constituents in a region which may represent a transition between the diffuse and radio clouds. The velocity of the optical components found is consistent with that of a CO cloud seen in front of No. 12.

We have detected 9 of the rotational fine structure lines of the 2-0 Phillips band of interstellar C2 toward ζ Persei using the Tull spectrograph and Reticon detector on the 2.7 m telescope at the McDonald Observatory. These data yield a total C2 column density of 1.2 × 1013 cm-2 and a rotational temperature of 97 K compared to 1.4 × 1013 cm-2 and 45 K predicted by the detailed model of the cloud by Black, Hartquist and Dalgarno. We suggest that radiative pumping through the Mulliken and Phillips systems has modified the C2 level populations in such a way as to produce an observed rotational temperature which exceeds that arising in pure thermal equilibrium.

The oscillator strengths of two ultraviolet molecular lines expected to be seen in diffuse interstellar clouds have been measured. For the 1290.257 A° line of HC1, f=0.16±0.06. For the 1114.225 A° line of H2O, f=(5.0±2.0)x10−3. These results have been used in conjunction with observational data to compare measured and predicted column densities of these molecules in the ζ Óphiuchi cloud.

The results of recent observations of the column densities of chlorine species in diffuse interstellar clouds are inconsistent with the column densities predicted by model calculations. The use of new atomic and molecular data fails to decrease the discrepancy. We speculate that the exothermic reaction C1++H2→HC1++H may not be rapid at the low temperatures prevailing in diffuse interstellar clouds.

A description is given of the processes of molecular formation and destruction in diffuse interstellar clouds and detailed models of the clouds lying towards ζ Ophiuchi, ζ Persei and o Persei are used to assess the validity of gas phase chemistry. Modifications that may arise from shock-heated regions are discussed.

The reaction between C+ ions and OH- ions on the surface of oxide or silicate grains in diffuse clouds is shown to be a source of CH, OH, CO, HCO and H2CO molecules.

Three new excited states of CH+ which can be reached by a dipole transition from the ground state with photon energies less than 13.6 eV have been obtained from accurate theoretical calculations. As these states are unbound with respect to nuclear motion, photon absorption into them results in dissociation of the molecule. Photodissociation cross sections have been calculated, and the CH+ photodissociation rates in the interstellar radiation field as a function of optical depth have been determined. The rates are approximately 500 times larger than previously assumed. We have compared these new photodissociation rates with the rates of other CH+ destruction mechanisms in three different models of the interstellar medium. Photodissociation appears to be a significant destruction mechanism in any model requiring large interstellar radiation fields.

The charge - transfer reaction between protons and oxygen atoms is critical to the chemistry of the oxygen family; the corresponding rate was evaluated by Field and Steigman (1971) on the assumption of orbiting collisions and statistical probability distribution among the levels. We re-examine this reaction, including the fine-structure excitation process, basing our analysis on a careful description of the different potential curves arising from the O-H+ and 0+-H systems and on the evaluation of the coupling responsible for the transitions.

On the evidence of some recent laboratory data for the association reaction H + OH + He → H2O + He, and following previously proposed ion-neutral association reaction schemes, we tentatively suggest that the radiative association reaction of H and OH can be a significant mechanism for the production of H2O in interstellar clouds, when compared with the ion-neutral reaction sequence. The ‘H-atom + radical’ association process may be of some general importance in the production of other small molecules in interstellar clouds.

The applicability of ion-molecule reaction rate constants measured at room temperature to simulation of interstellar cloud chemistry is discussed. Three-body association-rate constants of C+ and NH+3 have been measured between 100 K and 300 K. These results give information about the same associations by radiative processes. Possible implications for interstellar molecule production by radiative association and free radical reaction are discussed.

This paper summarizes results of a chemical model of SO, CS, and OCS chemistry in dense clouds.

Rate constants obtained by the ICR technique are reported for reaction (1), C2H2+ + HCN and reaction (2) HCN+ + C2H2 such that k1 = 3.6 × 10−10 and k2 = 6.9 × 10−10 cm3 molecule−1 s−1, respectively. Differences between these results and other measurements of reaction (1) are discussed. The relevance of reaction (1) to the formation of HC3N in interstellar clouds is also briefly assessed.

An ICR investigation of the association reaction

CH3+ + HCN CH3.HCN+

has shown the reaction follows second order kinetics over the pressure range 1 × 10-6 to 3 × 10-4 Torr with a rate coefficient of 2 × 10-10 cm3s-1. These results can be interpreted in terms of a saturated 3-body or radiative association mechanism.

Laboratory measurements are reported for ion-molecule reactions involving CN+, HCN+, C2N+ and HCN, and their implications for interstellar synthesis for C/N compounds are discussed. The reaction of C+ with HCN was found not to constitute a major loss process for HCN, which is regenerated by reactions of C2N+ with NH3, H20, CH4, C2H2 and H2S. These latter reactions lead to C addition rather than C-N bond formation. Rapid association reactions were observed for CH3+ and C2H2+ with HCN. These suggest efficient radiative association reactions under interstellar conditions to form ions which may form larger C/N compounds upon neutralization.

From our laboratory data relating to several hundreds of ion-atom and ion-molecule reactions at thermal energies, we qualitatively describe probable chemical paths to the synthesis of amino-, cyano-and nitroso-compounds in interstellar clouds.

A new statistical theory of ion-molecule association reaction rate coefficients has been formulated and found to give good agreement with three-body association rate coefficients studied in the laboratory in the temperature range 100-300 K (Herbst 1979a). The theory indicates that certain radiative association reactions proceed rapidly at low interstellar temperatures to produce complex interstellar molecules, as suggested by Smith and Adams (1978).