The current status of laboratory measurements of the rate coefficients for ionic reactions involved in interstellar molecular synthesis is discussed and the experimental techniques used to acquire such data are briefly described. Examples are given of laboratory data which are being obtained at temperatures close to those of interstellar clouds. Particular attention is given to the results of recent theoretical and experimental work which show that the rate coefficients for the binary reactions of ions with polar molecules at low temperatures are much larger than previously assumed. It is shown how these new developments in experiment and theory are reconciling the differences between predicted and observed abundances for some interstellar molecules. Also briefly discussed are: - the phenomenon of isotope exchange in ion/neutral reactions which explains the apparent enrichment of heavy isotopes in some interstellar molecules, the role of atoms in molecular synthesis, some studies of ion/neutral reactions pertaining to shocked regions of interstellar clouds, ternary association reactions and the analogous radiative association reactions, and recent new laboratory measurements of dissociative recombination coefficients. Finally, some guidance is offered in the proper choice of critical kinetic data for use in interstellar chemical modelling and some further requirements and likely future developments are mentioned.

The basic principles of the CRESU technique (Cinétique de Réactions en Ecoulement Supersonique Uniforme) are presented. This technique allows ion-molecule reaction rate coefficients under true thermal conditions at interstellar temperatures. Various behaviors of both third-body association and binary reactions with temperature have been observed, including ion-polar molecule reactions whose rate coefficients sharply increase at very low temperatures.

The rate coefficient for the charge transfer reaction C+2po + H → C 3p+ H+ is calculated with the introduction of the radial coupling between the two 3π states arising from both asymptotic atomic states. the derived rate coefficient at a temperature of 104K is 2 10−15 cm3s−1 which is two orders of magnitude larger than the value previously estimated by Butler and Dalgarno (1980) from a weak spin orbit coupling between the 3Σ− and 3Σ+ molecular states of CH+.

A significant reservoir of potential energy in hot astrophysical plasmas exists in multiply charged positive ions. Inelastic collisional processes involving such ions govern the ionization and energy balance in such plasmas. Although inelastic processes such as, charge transfer, have been widely investigated, there remains a paucity of knowledge about charge changing processes where both reactions and products are state-diagnosed. We have applied high-resolution translational energy gain/loss spectroscopy to investigate state-diagnosed collisions between Kr2+ and H2 leading to single electron capture into specific electronic states of Kr+ at collision energies in the range 1–6 keV.

A new experimental method has been developed which allows determination of the products nature in dissociative recombination (D.R.) of molecular ions. Results are presented on H2O+ D.R. which show that there is a large yield of oxygen atoms in this reaction. the measurements give a total yield for the two channel O + H + H and O + H2 of 0.45 and therefore a yield of 0.55 for OH + H. H2O+ ions are formed by charge exchange from N+ ions and are probably vibrationally excited in this experiment.

Among the low-energy nonreactive molecular collisions, the rotational and vibrational transitions are the most important inelastic processes. the collision partner is an electron, an ion, or a neutral particle. Depending on the process and the collision system concerned, the magnitude and the energy dependence of the relevant cross sections are widely different. the present status of our knowledge is briefly summarized and some sample cross section data for H2 and CO are shown. Useful relations among the rotational cross sections are indicated. the importance of the long-range intermolecular forces in determining some chemical reaction rates is also pointed out.

Microwave pulse techniques have proved to be very useful to obtain informations about coherence decay rates and population transfer rates in gas phase. the coherence decay rates have been measured by a technique which involves the detection of the transient emission signal of the molecular sample following an intense microwave pulse. the (π-τ-π/2) pulse sequence technique has been used to extract informations about population transfer rates. the experimental results on 1-doublet transitions of HCN and HCCCN perturbed by H2,D2 and He are given. the theoretical results obtained from the perturbative approach to collision dynamics have also been computed, and compared with the experimental results. for the 1-doublet transitions of HCN, the population decay rates have found to be considerably less than the corresponding coherence decay rates.

An Effective Straight-line Trajectory (EST) approach by introducing a parameter RX has been proposed for computations of collision-induced rotational line widths (HWHM) and excitation rates in case of atom-molecule systems under the frame work of Smith, Giraud and Cooper (1976) and molecule-molecule systems under the frame work of normalized semi-classical perturbative approach. An optimised parameter RX, which is a measure of significance of the curved trajectories of the colliding molecules, can be determined from the temperature dependence of collision-induced line widths. the EST approach has been tested for HCl-Ar system and further applied to X-He and X-H2 systems of interstellar interest, where X represents interstellar molecules CO, OCS and HCN. The results are given in Table I and II.

While sharply different rotational temperature for the lower and higher rotational states of H2 have been observed in astrophysical situations, studies on heavier molecules are less encountered. We report here results of plasma spectroscopic studies on the nitrogen system where the FNS bands (0,0) and (0,1) show much distinctive ‘two-temperature’ phenomena, the departure from the high rotational temperature part (840 ± 50°K and 920 ± 30°K respectively) being perceptible at N' (N' + 1) ≅ 160. the experimental system is an electron beam sustained magnetoplasma at 5 × 10−3 torr, with kTe ≅ 1.5 eV and ne ≅ 4 × 1010 cm−3 obtained from in situ measurements of plasma parameters. Relative excitation-deexcitation rates of the N2+ B2Σu+ rotational states and also in relation to those of H2 are discussed.

The photodissociation processes of molecules in various astrophysical regions are discussed, and the rates for photodissociation by the unattenuated interstellar radiation field are reviewed.

Most of the cometary radicals are thought to be formed in the coma by photodissociation processes. Successful modeling of the coma requires a detailed knowledge of the products that are formed from the various parent molecules, and the energy partitioning among these products. This information has to be obtained as a function of wavelength, because the Sun is not a monochromatic source. in this review, the status of the experimental knowledge of some key molecules will be discussed, along with the prospects of some new laboratory techniques that can fill in the gaps in our present knowledge.

Molecules in the interstellar space are exposed to interstellar radiation, so that they are destroyed by photodissociation processes, leading to the production of smaller molecules and ions, which in turn can react with other molecular species. Therefore, dissociation rates and dissociation products are important for the chemistry of interstellar molecules. This holds true also for relatively opaque and dense clouds (τ ≲ 10 in the visual), due to the multiple scattering of UV radiation by dust.

Refinements of the instrumentation and methods used for laboratory microwave spectroscopy of molecular ions in the last few years have led to successful observation of spectra of several new molecular ions of particular astrophysical interest. Notable among these enhancements have been the use of magnetic fields to increase the density of ions in discharge plasmas and the extension of upper frequency limits of the spectrometers towards and into the sub-millimeter regime. the number of ions now observed by laboratory microwave spectroscopy is more than a dozen, and about two thirds of these have also been detected, at least tentatively, by radioastronomy. Even where detections are uncertain or impossible, useful upper bounds on ion abundances can be gleaned from radioastronomical observations once exact transition frequencies and assignments are known from laboratory studies. Special attention will be given in this presentation to the status of laboratory and radioastronomical work on HOC+, H2D+, and SO+, molecular ions which have been the object of our own recent interest and effort in this area.

The rotational spectra of C3H2, C3H, and C2D, molecules not previously studied, were investigated in a continuing program of mm-wave spectroscopy in space and in the laboratory of hydrocarbons of astrophysical interest. Laboratory measurements producing reactive species in a DC glow discharge through organic gases have been essential to the identification of these species.

The ammonia molecule is known to be useful as a probe for studying conditions inside interstellar clouds and planetary atmospheres. Correct interpretation of interstellar and planetary spectra need to be supported by adequate laboratory measurements. in the present studies we report the high resolution Fourier transform spectra of ammonia recorded with a pathlength of 192m at the Kitt Peak National Observatory. Transitions with intensities that are two orders of magnitude weaker than those that have been reported earlier, have been observed and assigned. These include high J transitions, hot bands and forbidden transitions. These transitions are not saturated under long paths such as those available in planetary atmospheres and are therefore useful in the estimation of temperatures. The forbidden transitions have been processed with other relevant data to provide complete information on the energy levels. Such information is required for the calculation of equilibrium population of energy levels and partition functions, which go into the estimation of spectral intensities and abundances in terrestrial, interstellar and planetary atmospheres.

Laboratory studies have been made on molecules of astrophysical interest such as AlO, CO, CrO, SiS, NH+ and OH. Vibrational and rotational constants have been determined more accurately in the various electronic states.

The experimental study of atomic chlorine absorption spectra using a flash-pyrolysis system and a 2-m normal incidence spectrograph in the wavelength region 95 nm to 61 nm has produced photoabsorption cross sections, wavelength measurements, and line series identification involving the 3P, 1D and 1S limits. Absorption spectra originating from both levels ( and ) of the ground state have been produced by this method.

The accuracy of the information obtained from observations of optical-wavelength, molecular absorption features in interstellar clouds is directly limited by the accuracy of the rotational-line oscillator strengths used in the analysis of data. This paper briefly discusses optical observations of molecules in interstellar clouds; surveys molecular lifetime and oscillator strengths; and reviews laboratory techniques for obtaining these data.

We present a new calculation of intercombination transition probabilities between levels X1Σg+ and a 3Πu of the C2 molecule. Starting from experimental energy levels, we calculate RKR potential curves using Leroy's Near Dissociation Expansion (NDE) method; these curves give us wave functions for all levels of interest. We then compute the energy matrix for the four lowest states of C2, taking into account Spin-Orbit coupling between a 3Πu and A 1Πu on the one hand and X 1Σ+g and b 3Σg− on the other. First order wave functions are then derived by diagonalization. Einstein emission transition probabilities of the Intercombination lines are finally obtained.

The presence of a nuclear spin in a molecule splits the rotational levels and microwave lines into hyperfine components. A second nuclear spin (such as exists, for example, in the molecule DCN) further splits these into hyperhyperfine components.