  We have analyzed high resolution data of absorption lines of Si and C in the
absorption systems observed in the spectra of QSOs, in order to study the
ionization state and the overabundance of Si with respect to C in the absorbers
and also to study the change in these properties with redshift. No correlation
is found between column density ratios of Si IV to C IV of intervening systems
and redshift. The data do not provide any evidence for an abrupt change in the
values of the ratio at any particular redshift unlike that for Lyman alpha
forest clouds. We have compared the observed ratios of column densities of Si
II and Si IV and of Si IV and C IV in different classes of absorption systems
with predictions of photo ionization models for different shapes of the
background radiation field. Overabundance of Si over C can be ruled out in
several of the intervening systems for any shape of the background radiation.
For these systems we can also rule out any contribution from the stellar
sources to the background, which is then entirely from the AGNs. No
overabundance is needed in other intervening systems if the radiation field
from stellar sources contributes significantly to the UV background.
Overabundance is, however, present in Lyman alpha forest clouds at redshifts
larger than 3 and in systems associated with the QSOs. For all the intervening
systems a minimum of 10 % of the background is contributed by the AGNs.
