Abstract
In this work we perform theoretical calculations of the
population ratios of fine structure levels of C0, C+ and
Si+ considering in detail the effect of the various excitation
mechanisms: collisions, fluorescence and the cosmic background radiation. We
employ recent atomic data, gathered in the literature, to solve the related
statistical equilibrium equations.
The comparison of the calculated population ratios with the
observed column densities ratios available in the literature allows us to obtain
informations regarding the physical conditions (volume densities, intensity of a
UV radiation field present, temperature of the cosmic background radiation) and
properties (characteristic size and mass) of damped Lyman
and Lyman Limit systems seen in absorption in spectra of QSOs.
We also investigate the temperature law of the cosmic
background radiation, which bears a tight relationship with this work.