In situ observation of the formation, diffusion, and reactions of hydrogenous species in F2 -laser-irradiated Si O2 glass using a pump-and-probe technique

We quantitatively studied the formation, diffusion, and reactions of mobile interstitial hydrogen atoms (H0) and molecules (H2) in F2 -laser-irradiated silica (Si O2) glass between 10 and 330 K. Two key techniques were used: single-pulse F2 laser photolysis of silanol (SiOH) groups to selectively create pairs of H0 and oxygen dangling bonds (nonbridging oxygen hole centers, NBOHC), and in situ photoluminescence measurements of NBOHCs to monitor their reactions with H0 and H2 as a function of time and temperature. A smaller quantum yield of the photolysis of the SiOH bond (0.15±0.05) compared with values reported for gas molecules containing OH bonds (∼1) suggests that the separation of photogenerated H0 from NBOHC is hindered by the cage effect of the Si O2 glass network. Distribution functions for the diffusion coefficients of H0 and H2 in the structurally disordered Si O2 glass were evaluated by numerical analysis of the concentration changes of NBOHC based on diffusion-limited reaction theory. The average diffusion coefficient of H2 obtained by integrating the distribution agrees well with the values measured by the permeation of H2 through Si O2 glass plates. In contrast, the average diffusion coefficient of H0 significantly decreases with time because the distribution of the diffusion coefficient of H0 is broad and H0 s with greater mobility disappear at a faster rate. We suggest that the efficient conversion of H0 into H2 in Si O2 glass is due to dissipation of the excess energy of the reaction intermediate via inelastic collisions with the glass network. The fraction of H0 that forms H2 is determined by the ratio of the capture radii of H0 and NBOHC, and it is independent of the diffusion coefficient and the initial concentration of H0.