Molecules play a unique role in solar spectroscopy in allowing to derive, with high accuracy, the temperature structure of the photospheric layers from where the solar spectrum in the near ultraviolet, visible and infrared emerges. They also allow to observe the heterogeneous structure of the outer layers as well as the subtle convective motions in the layers just above the solar convection zone. Molecules play also a unique role in defining the solar chemical composition, at least as far as the very important elements like carbon, nitrogen and oxygen are concerned. They also offer the unique opportunity to derive meaningful isotopic ratios for C and O. Although we shall not deal with sunspots in the present review, it has to be recalled that molecules are the only tracers to derive the solar abundances of CI and F from the rotation-vibration lines of HC1 and HF present in the infrared sunspot spectra (Hall & Noyes 1972; Hall & Noyes 1969).
On the other hand, the Sun itself offers a unique opportunity to produce molecular transitions, in local thermodynamic equilibrium (LTE), at rather high temperatures (T ∼ 5000 K), higher in any case than in the laboratory, often in non LTE (see review by Johnson in this volume). Therefore, new lines and new transitions of higher excitation appear in the solar spectrum allowing to use the Sun as a “permanent laboratory source” in order to refine the molecular constants. Furthermore, the solar photosphere, where we believe the physical conditions and physical processes are better known than in any other stars, offers a unique possiblity to test basic molecular data like dissociation energies and transition probabilities.