6533b7d1fe1ef96bd125c197

RESEARCH PRODUCT

In situobservation of the formation, diffusion, and reactions of hydrogenous species inF2-laser-irradiatedSiO2glass using a pump-and-probe technique

subject

description

We quantitatively studied the formation, diffusion, and reactions of mobile interstitial hydrogen atoms $({\mathrm{H}}^{0})$ and molecules $({\mathrm{H}}_{2})$ in ${\mathrm{F}}_{2}$-laser-irradiated silica $(\mathrm{Si}{\mathrm{O}}_{2})$ glass between 10 and $330\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Two key techniques were used: single-pulse ${\mathrm{F}}_{2}$ laser photolysis of silanol (SiOH) groups to selectively create pairs of ${\mathrm{H}}^{0}$ and oxygen dangling bonds (nonbridging oxygen hole centers, NBOHC), and in situ photoluminescence measurements of NBOHCs to monitor their reactions with ${\mathrm{H}}^{0}$ and ${\mathrm{H}}_{2}$ as a function of time and temperature. A smaller quantum yield of the photolysis of the $\mathrm{Si}\mathrm{O}\mathrm{H}$ bond $(0.15\ifmmode\pm\else\textpm\fi{}0.05)$ compared with values reported for gas molecules containing $\mathrm{O}\mathrm{H}$ bonds $(\ensuremath{\sim}1)$ suggests that the separation of photogenerated ${\mathrm{H}}^{0}$ from NBOHC is hindered by the cage effect of the $\mathrm{Si}{\mathrm{O}}_{2}$ glass network. Distribution functions for the diffusion coefficients of ${\mathrm{H}}^{0}$ and ${\mathrm{H}}_{2}$ in the structurally disordered $\mathrm{Si}{\mathrm{O}}_{2}$ glass were evaluated by numerical analysis of the concentration changes of NBOHC based on diffusion-limited reaction theory. The average diffusion coefficient of ${\mathrm{H}}_{2}$ obtained by integrating the distribution agrees well with the values measured by the permeation of ${\mathrm{H}}_{2}$ through $\mathrm{Si}{\mathrm{O}}_{2}$ glass plates. In contrast, the average diffusion coefficient of ${\mathrm{H}}^{0}$ significantly decreases with time because the distribution of the diffusion coefficient of ${\mathrm{H}}^{0}$ is broad and ${\mathrm{H}}^{0}\mathrm{s}$ with greater mobility disappear at a faster rate. We suggest that the efficient conversion of ${\mathrm{H}}^{0}$ into ${\mathrm{H}}_{2}$ in $\mathrm{Si}{\mathrm{O}}_{2}$ glass is due to dissipation of the excess energy of the reaction intermediate via inelastic collisions with the glass network. The fraction of ${\mathrm{H}}^{0}$ that forms ${\mathrm{H}}_{2}$ is determined by the ratio of the capture radii of ${\mathrm{H}}^{0}$ and NBOHC, and it is independent of the diffusion coefficient and the initial concentration of ${\mathrm{H}}^{0}$.