  This experimental PhD work deals with the generation and transformation
processes of point defects induced by UV laser irradiation on amorphous silicon
dioxide (silica). The investigation relies on several spectroscopic techniques
used to probe the microscopic damage induced by UV laser on the material. The
most innovative approach, however, is the measurement in situ of the absorption
signals related to laser-induced point defects. This technique yields new
information on the kinetics of defect processes, inaccessible to previous works
based only on stationary measurements of laser-induced damage. The main result
is to show that a major damage mechanism of silica under 4.7eV laser radiation
is the generation of E' centers (silicon dangling bond defects) by photolysis
of pre-existing Si-H bonds. This process occurs by two-photon absorption of
laser light, and the produced E' centers are unstable in a wide temperature
range: in fact, rupture of Si-H produces hydrogen atoms H together with E';
hence, H atoms dimerize in H_2, which diffuses in the glass and reacts back
with the E' centers causing their decay. The generation and decay kinetics of
laser-induced E' centers are quantitatively modelled by considering the
competition between the laser-induced breakage of Si-H and the concurrent
reaction between E' and H_2. The latter reaction is studied in detail at
several temperatures: some of its features are found to be affected by the
disorder of the glass matrix; moreover, its kinetics is found to be
activation-limited rather than diffusion-limited, contrary to what is usually
found for the reactions of diffusing species with point defects.
