Molecular bands have been used as a diagnostic of spectral type since the 1860's. TiO was first identified as the dominant feature in optical spectra of cool giants in 1904 (Fowler 1904). Many diatomic molecules have now been included (e.g. Jørgensen & Larsson 1990) in detailed calculations of stellar opacity and attention is starting to focus on the incorporation of accurate data for triatomics. Triatomic molecules have a much greater density of states than diatomics; bending vibrational bands - generally lower in frequency than bond-stretching modes - may contribute considerably to “filling in” spectral gaps.

With the advent of high-speed computing and improved algorithms, computational chemistry techniques have become competitive with the best experimental techniques for determining line strengths for rovibrational and electronic transitions in molecular systems (Bauschlicher k Langhoff 1991). This is particularly the case at high temperatures where the molecules are highly rotationally excited. While it is difficult to measure line strengths at stellar temperatures, the theoretical values can be very reliable if a globally accurate transition moment function (TMF) is used in conjunction with high-quality experimental or theoretical potentials.

Quantum chemical methods have become a powerful tool for investigating the electronic structure of small molecules in ground and excited states. This is particularly true for species which are not easily accessible by experiment, either because they are short-lived, for example, or unstable under laboratory conditions or difficult to detect since their signals are hidden by other, prevailing molecules. In this sense theoretical methods are of special importance for molecules in atmospheric, stellar and interstellar environments.

To a quantum chemist with no particular background in astrophysics or astronomy, a brief glance at journals and textbooks in these fields shows at least three areas where computational quantum chemistry has had a valuable impact: Interstellar cloud chemistry; stellar atmosphere modelling; and chemistry in extreme conditions, such as at the surface of a neutron star. The first two uses are particularly suitable, since standard methods are directly applicable.

For such problems, good calculations of potential energy as well as expectation values and matrix elements of dipole and other operators appears to be in demand. Many electronic states may be involved, at least a broad range of problems involve fairly small molecules, often radicals, and conformation regions far from equilibrium. Such problems are addressed by three methods originated in our laboratory, and known by the acronyms RASSCF (Restricted Active Space Self-Consistent Field, Malmqvist et al. 1990), RASSI (RAS State Interaction) and CASPT2 (Complete Active Space Perturbation Theory to Second Order-Complete Active Space Perturbation Theory to Second Order, Andersson et al. 1990; Andersson et al. 1992).

The present communication is concerned with the problem of calculating the rotation-vibration energies and the absorption intensities of a triatomic molecule from the effective nuclear potential energy function and the dipole moment surfaces within the Born-Oppenheimer approximation (Born & Oppenheimer 1927). There are at least two astrophysical applications of such calculations:

— Detailed spectra of triatomic molecules can be computed under the conditions prevailing in stellar atmospheres and compared with observed spectra.

— Such spectra can also be used for constructing a wavenumber-dependent absorption coefficient which can be employed for opacity calculations as outlined, for example, by Jørgensen & Jensen (1993).

In modeling the opacity of astronomical objects, such as stellar atmospheres, the most important transitions are those with absorbance = αL ∼ 1, where α is the absorption coefficient of the atmosphere and L is the pathlength that light travels in leaving the atmosphere. Transitions with much smaller absorbance are not important, while those with much larger absorbance are effectively opaque. The relevant absorption lengths L are typically orders of magnitude larger than can be realized in the laboratory, and thus obtaining the needed laboratory data is a significant experimental challenge. This volume contains several examples of the power of modern ab initio methods to predict spectra of simple triatomic molecules, such as H2O, but it is still necessary to at least calibrate the predictions against laboratory bands of similar oscillator strength. The theoretical treatment of molecules with four or more atoms lags far behind the theoretical treatment of triatomics and present experimental capabilities.

It is really impressive to see the amount of information astrophysicists have been able to extract from the analysis of that faint glimmering light that stars se,nd to tell us about them. The main piece of this analysis consists in dispersing the light to get the spectrum, that is to say intensity versus wavenumbers. From the measurements of wavenumbers it is possible to identify the atomic and molecular species from comparison with high-resolution laboratory spectroscopic data. Once the constituents have been identified, models can be constructed in order to simulate the metabolism of circumstellar atmospheres. In such models a crucial point is the abundance of the identified atomic and molecular constituents.

A systematic program of laboratory analyses of selected molecular spectra of astrophysical interest started in 1958 and continues to the present time. The program includes production of spectral atlases, tabulations of spectral lines, analyses, calculations of excitation energies and molecular parameters, measurements of radiative lifetimes, and determinations of transition strengths. Work has been completed or is in progress on the spectra of ArH+, C2, carbon clusters, CN, CS, CaCl, CaH, CaS, FeD, FeH, HgH, HgD, InI, LaO, LaS, OD, OH, SH, Si2, SiC2, TiCl, TiO, TiO+, VO, YS, ZrCl, ZrO, and ZrS. The basic needs for astronomically useful data have not changed, but laboratory and analysis methods have become more sophisticated in order to cope with ever greater demands for consistency, accuracy, and breadth of information. The Fourier transform spectrometer and computer codes for analyses have enhanced our ability to satisfy some of these demands.

This subject is very wide. I shall present, here, a summary of our works related to radicals created in electric discharges. I shall review the methods and technical means used for studying these species in the infrared. This review will be accompanied with a detailed bibliography in which one will find the description of these methods and the most typical results obtained in our group in Orsay since 1986.

The discovery of a large variety of molecules by radio astronomy has been a very strong motivation for the development of laboratory millimeter wave spectroscopy. Among them, the reactive species, neutral and/or ionic, have been early recognized as playing a very important role in the chemistry of the interstellar and circumstellar medium. While the laboratory spectroscopy of free radicals started relatively early, with the observation of the OH radical by the group of Townes (Dousmanis et al. 1955), the detection of molecular ions proved to be a much more difficult task, and the first millimeter line due to an ion was actually detected by radio astronomy (Buhl & Snyder 1970). It was called “U89.2” until it was tentatively attributed to HCO+ by Klemperer (1970) on the basis of both considerations on the chemistry of the interstellar medium, and of ab initio calculations for the prediction of the expected line frequency. This identification was later confirmed by more elaborated ab initio calculations (Wahlgren et al. 1973, Kraemers & Diercksen 1976), and by the observation of a transition attributed to H13CO+ (Snyder et al. 1976), but the definite confirmation was the observation of the same transition in a laboratory glow discharge by the group of Woods (Woods et al. 1975).