Spectrocopy is the classical diagnostic tool of astrophysics. Intensities and line shapes of well identified emission and/or absorption atomic and molecular features are used to provide information on species concentrations, and degree of excitation, from which gas kinetic, rotational, vibrational, electronic and excitation “temperatures” can be inferred when LTE conditions exist. Departures from LTE can also be determined spectroscopically. Diagnostic interpretation of spectra in optically thin circumstances is fairly straightforward. However, in optically thick conditions when the photon mean free path is very much less than the geometrical path, the emission spectrum is controlled by the absorption coefficient (Armstrong and Nicholls, 1972), (see equation 4a).

Developments in the measurements of oscillator strengths, that are of current and future interest to astrophysics, have taken place in the past few years:

1. – there is increased emphasis on methods which are independent of level populations (i.e., of number densities and thus equilibrium in the vapour or plasma under investigation),

2. – clear progress has been made towards higher precision (and accuracy) and

3. – the dynamic range of astrophysically relevant oscillator strengths was further extended towards weaker lines.

In many low-charge ions of astrophysically abundant light elements, the spins of the ground and first-excited terms are different. Because of spin-orbit interactions, the states of these terms are mixtures of LS basis states, and as a consequence, transitions with ∆S = 1, i.e., ‘intersystem transitions’, can occur. The_transition probabilities (A-values) for such lines in low-Z ions are about 104 sec -1 to 102 sec-1. These values are of the same order of magnitude as the collisional de-excitation rates in many low-density astronomical objects. Consequently, intersystem lines, which are not readily seen in the laboratory, are significant features in many astronomical spectra. IUE spectra that show such lines in novae, hot stars, cool stars, symbiotic stars, binary stars, variable stars, Herbig-Haro objects, H II regions, planetary nebulae, quasars, and galaxies are discussed in Kondo et al. (1982). Moreover, because of the commensurate radiative and collisional de-excitation rates for the upper levels, the ratios of intersystem-line intensities to allowed-line intensities are density sensitive in many objects.

Dielectronic recombination (DR) is an electron-ion process particularly effective in high temperature plasmas such as those observed in the Solar Corona, Supernovae remnants in fusion plasmas (Tokamak and laser produced plasmas). This process is a resonant capture process of projectile electrons by a target ion as one of the target electron is excited, thereby forming an intermediate autionising state which can decay radiatively to a singly excited state. DR plays an important role on the establishment of ionisation equilibrium in the plasma and is also responsible for spectral lines appearing as satellites of the resonance lines of the target ion. The analysis and interpretation of such satellite lines in terms of plasma diagnostics has been widely used in soft X-ray spectroscopy during the last decade, and has given reliable estimates of the physical parameters of the plasma (electron and ion temperatures and densities). In the case of H-like and He-like resonance lines, high resolution spectra have been obtained in Tokamak for Z = 14 - 28 and most of the satellites have been clearly identified. To help the reader to go further we give some references of solar studies 2, 3, 4, 0, Tokamak 6, 7, laser plasma 8, 9.